MARINE AND FISHERY SCIENCES 36 (3): XXX-XXX (2023)

https://doi.org/10.47193/mafis.3632023010901

ABSTRACT. Understanding the causes that generate variability of recruitment in marine popu-

lations constitutes one of the greatest challenges in fishery science. Our predictive capacity to

explain these variations is relatively low, due to the interaction of exogenous and endogenous fac-

tors, which vary across time and space within populations. In order to gain information on recruit-

ment fluctuations of the Argentine hake (Merluccius hubbsi) from Patagonian stock, we reviewed the

results obtained analyzing the reproductive ecology, trophic and energetic dynamics during different

stages of development of this species, and its relationship with environmental variables. We

observed that the reproductive potential is strongly influenced by characteristics of the parental

stock, particularly females, in terms of their size, age and condition. This feature, called ‘maternal

effect’, suggests that the spawning stock biomass, commonly used as an index of productivity in

fishery assessment, is a poor predictor of recruitment. We also observed that survival during hake

early life is affected by the spatial coincidence with the North Patagonian Frontal System, character-

ized by a high concentration of nutrients, high productivity, and food availability. Physical condi-

tions and larval density in the nursery area affected the nutritional state and mortality of hake, mainly

by competition for food or predation. It was observed that the transition of juveniles from pelagic to

demersal habitat occurs over a longer period than previously recognized for this species, stressing

the importance of using acoustic information to complement data from bottom trawls. This is one of

the main topics to be further developed in order to estimate new recruitment indices for Argentine

hake, along with other research items proposed to improve stock assessment.

Key words: Southwest Atlantic Ocean, maternal effect, north Patagonian Frontal System, larval

survival.

Reclutamiento de la merluza argentina, Merluccius hubbsi, del stock patagónico: una revisión

de las principales características que afectan el potencial reproductivo y la supervivencia

durante las primeras etapas de la vida

RESUMEN. Comprender las causas que generan la variabilidad del reclutamiento en las pobla-

ciones marinas constituye uno de los mayores desafíos de la ciencia pesquera. Nuestra capacidad

*Correspondence:

gmacchi@inidep.edu.ar

Received: 3 marzo 2023

Accepted: 13 April 2023

ISSN 2683-7595 (print)

ISSN 2683-7951 (online)

https://ojs.inidep.edu.ar

Journal of the Instituto Nacional de

Investigación y Desarrollo Pesquero

(INIDEP)

This work is licensed under a Creative

Commons Attribution-

NonCommercial-ShareAlike 4.0

International License

Marine and

Fishery Sciences

MAFIS

REVIEW

Recruitment of the Argentine hake, Merluccius hubbsi, from Patagonian

stock: a review of main features affecting the reproductive potential and

survival during early life stages

GUSTAVO J. MACCHI1, 2,*, GUSTAVO ÁLVAREZ COLOMBO2, MAURO BELLEGGIA1, 2, PAOLA BETTI2, DANIEL BROWN2,

GEORGINA CEPEDA1, 2, CARLA DERISIO2, MARINA V. DIAZ1, 2, MARTÍN EHRLICH2, EZEQUIEL LEONARDUZZI2,

LAURA MACHINANDIARENA2, PATRICIA MARTOS3, BETINA SANTOS2, AGUSTÍN SCHIARITI1, 2, LUCILA SOBRERO1, 2 and

BRENDA TEMPERONI1, 2

1Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Instituto de Investigaciones Marinas y Costeras (IIMyC),

Rodríguez Peña 4002, B7602GSD - Mar del Plata, Argentina. 2Instituto Nacional de Investigación y Desarrollo Pesquero (INIDEP),

Paseo Victoria Ocampo Nº 1, B7602HSA - Mar del Plata, Argentina. 3Departamento de Ciencias Marinas, Facultad de Ciencias Exactas y

Naturales, Universidad Nacional de Mar del Plata (UNMdP), Rodríguez Peña 4002, B7602GSD - Mar del Plata, Argentina.

ORCID Gustavo J. Macchi https://orcid.org/0000-0003-1821-5491, Gustavo Álvarez Colombo https://orcid.org/0000-0003-1683-6307,

Mauro Belleggia https://orcid.org/0000-0003-1584-4743, Marina V. Diaz https://orcid.org/0000-0002-2912-5232,

Martín Ehrlich https://orcid.org/0000-0001-9129-6270, Agustín Schiariti https://orcid.org/0000-0002-9262-1829,

Brenda Temperoni https://orcid.org/0000-0003-1580-221X

INTRODUCTION

The reproductive strategy adopted by fish is

associated with the availability of energy

resources and with the physical conditions in the

environment. These are key factors that deter-

mine the survival of the progeny and affect the

incorporation of new individuals into the popula-

tion, a process known as recruitment. For this rea-

son, knowing and understanding the causes that

generate variability of this process in marine pop-

ulations, and identifying the factors that deter-

mine changes in the abundance of individuals

constitutes one of the greatest challenges in fish-

eries biology (Marshall et al. 1998). Recruitment

is affected both by exogenous environmental fac-

tors and by those endogenous or intrinsic to the

population. Among the first are physical variables

in the breeding area, such as temperature, salinity,

stratification and dissolved oxygen, as well as

food availability, intra- and interspecific competi-

tion and predation (Houde 2009). Endogenous

factors are mainly associated with parental stock

features including abundance, size/age composi-

tion, physiological condition, or genetic diversity

(Jakobsen et al. 2009).

Regardless of the characteristics of the life

cycle of the species, the relationship between

parental stock and recruitment (S-R) occupies a

central role in the study of population dynamics

and management of marine resources. The S-R

models developed by Beverton and Holt (1957)

and Ricker (1954) originally used the term fecun-

dity, but it was later replaced by Spawning Stock

Biomass (SSB) as a proxy of stock productivity

(Rothschild and Fogarty 1989). In such cases, it is

assumed that a given adult biomass has the same

probability to generate the same level of recruit-

ment, independent of the age or size composition

of the population (Marshall et al.2003). Howev-

er, during the last years, it was demonstrated that

there are few examples of fisheries showing good

fits in the S-R relationships (Vert-pre et al. 2013).

At the end of 90’s, an alternative term to SSB was

introduced, the Stock Reproductive Potential

(SRP), which more accurately represents the

stock’s ability to produce viable eggs and larvae

that may eventually be recruited to the population

(Trippel 1999). This term includes parental fac-

tors that influence the early life stages of fish

related to recruitment processes. Given that the

set of individual characteristics will determine the

reproductive success of the population, special

attention has been paid to the structure and age

2MARINE AND FISHERY SCIENCES 36 (3): XXX-XXX (2023)

predictiva para explicar estas variaciones es relativamente baja, debido a la interacción de factores exógenos y endógenos, que varían en

el tiempo y el espacio dentro de las poblaciones. Con el fin de obtener información sobre las fluctuaciones en el reclutamiento de la mer-

luza argentina (Merluccius hubbsi) del stock patagónico, revisamos los resultados obtenidos analizando la ecología reproductiva, la diná-

mica trófica y energética durante las diferentes etapas de desarrollo de esta especie y su relación con variables ambientales. Observamos

que el potencial reproductivo está fuertemente influenciado por las características parentales, particularmente de las hembras, en cuanto

a su tamaño, edad y condición. Esta característica, denominada “efecto materno”, sugiere que la biomasa de la población reproductora,

comúnmente utilizada como índice de productividad en la evaluación de pesquerías, es un predictor deficiente del reclutamiento. Tam-

bién observamos que la supervivencia durante los primeros años de vida de la merluza se ve afectada por la coincidencia espacial con

el Sistema Frontal Norpatagónico, caracterizado por una alta concentración de nutrientes, alta productividad y disponibilidad de alimen-

to. Las condiciones físicas y la densidad larvaria en la zona de crianza afectaron el estado nutricional y la mortalidad de la merluza, prin-

cipalmente por competencia por alimento o depredación. Se observó que la transición del hábitat pelágico al demersal en los juveniles

se produce durante un período más extenso que el reconocido anteriormente para esta especie, lo que destaca la importancia de utilizar

los registros acústicos para complementar la información de las redes de arrastre de fondo. Este es uno de los principales temas a des-

arrollar para estimar nuevos índices de reclutamiento de merluza argentina, junto con otras líneas de investigación propuestas para mejo-

rar la evaluación del stock.

Palabras clave: Océano Atlántico Sudoccidental, efecto maternal, Sistema Frontal Norpatagónico, supervivencia larval.

diversity of the stocks (Marteinsdottir and Begg

2002; Scott et al. 2005; Mehault et al. 2010) or

the proportion of primiparous spawners (Evans et

al. 1996; Trippel 1998), since they can influence

the resilience of an overexploited population

(Rijnsdorp et al. 2010).

The link between reproductive potential and

recruitment is established through the selective

survival of eggs and larvae based on parental

characteristics. In other words, egg production is

not a sufficient mechanism to ensure certain

recruitment. It has been shown that some factors,

especially maternal, greatly influence the survival

of offspring, particularly in their interaction with

the environment (Marteinsdottir and Steinarsson

1998; Lambert et al. 2003). For example, egg

quality as a function of maternal characteristics,

such as size/age and nutritional condition

(Marteinsdottir and Steinarsson 1998; Saborido-

Rey et al. 2003; Macchi et al. 2013), could affect

the rate of development or the larval size (Miller

et al. 1995; Pepin et al. 1997). In general, it is

assumed that larger larvae have a better chance of

getting food, so they would have better chances

of survival during this critical life phase and

could give rise to higher recruitments (Rijnsdorp

and Vingerhoed 1994; Trippel 1998). Many stud-

ies on the early life history of fish suggest that

larger females would have a positive effect on the

fecundity and quality of eggs produced, generat-

ing more significant recruitments, which has led

to the BOFFFF hypothesis (Big, Old, Fat, Fecund

Female Fish) proposed by Berkeley et al. (2004).

In species subjected to intense fishing exploita-

tion, the extractive activity is mainly focused on

larger individuals, so it can generate changes in

the size/age structure of stocks. Thus, the disap-

pearance or decrease of large spawners would

directly affect the reproductive potential and

therefore the population recruitment.

The nutritional condition of spawners is anoth-

er trait that influences the reproductive potential

of stocks, affecting both fecundity and the quality

of eggs produced. For this reason, it is important

to consider the strategy of energy allocation of

fish, particularly in those species that accumulate

reserves prior to spawning (capital breeding),

since this process can be decisive for reproduc-

tive success, and also act as a recruitment proxy

(Marshall et al. 1999; Wuenschel et al. 2013).

Since a higher egg production generated by the

spawning stock does not ensure good recruitment

in the future, it is important to consider also the

interaction of the early life stages with the envi-

ronment. The effect of oceanographic variables,

such as temperature, salinity, or the vertical strat-

ification of the water column, determine the

availability of nutrients, and therefore the produc-

tivity of the system (Bakun 1996). Frontal zones,

for example, can be advantageous as retention

areas for larvae or act negatively due to the accu-

mulation of predators (Bailey and Houde 1989).

A central point in the theory of recruitment is

occupied by various hypotheses about the critical

period in the early life of fish, which arose from

Hjort’s postulate (1914), such as ‘Match-Mis-

match’, ‘Stable Ocean’ and ‘Member/ Vagrant’

(Houde 2009). These hypotheses, in general, con-

verge on the idea that larval phases must coincide

in time and space with a series of optimal ecolog-

ical circumstances for their maximum survival

(retention, greater production and concentration

of food or little competition). At this point, it has

also been shown that the structure of the parental

stock and its demography determine the timing,

duration and place of spawning, and therefore

have a profound effect on larval survival (Low-

erre-Barbieri et al. 2009, 2011; Wright and Trip-

pel 2009). In summary, recruitment success

would depend on the result of complex physical

and trophodynamic processes acting on different

temporal and spatial scales throughout the pre-

recruit life stage (Houde 2008). New approaches

in recruitment theories point to complementing

the traditional S-R relationship, which mainly

considers the productivity of stocks determined

by the abundance of parents and their fecundity,

by an S-R system where different aspects of the

MACCHI ET AL.: RECRUITMENT OF MERLUCCIUS HUBBSI FROM PATAGONIAN STOCK

reproductive strategy of the species operate, and

that define the reproductive resilience of popula-

tions (Lowerre-Barbieri et al. 2016).

Merluccius hubbsi (Argentine hake) is one of

the main fishing resources of Argentina, with bio-

mass estimates close to 1,315,000 t (Irusta et al.

2022; Santos and Villarino 2022). This species is

distributed over the continental shelves of

Uruguay and Argentina, mainly at depths

between 50 and 400 m from 35° S to 54° S (Otero

et al. 1982), but the highest concentrations are

found up to 48° S. Merluccius hubbsi reaches

21° S in Brazilian waters in response to the

upwelling of sub-Antarctic waters, but abundance

in this region is very low (Vaz-dos-Santos et al.

2009). On the Argentine continental shelf, there

are two main fishing stocks separated by parallel

41° S, whose reproductive cycles are spatially

and temporally out of phase. The southern or

Patagonian stock, which is the object of the pres-

ent review, is the most abundant population of

this species and represents 90% of its total bio-

mass, with an annual catch of 260,000 t reported

in 2022 (MAGyP 2022). This stock reproduces

during the austral spring and summer, between

November and April, with the main peak in Janu-

ary (Macchi et al. 2004; Pájaro et al. 2005). Dur-

ing the 1990s, the intense fishing activity of dif-

ferent commercial fleets on the Argentine hake

produced a sharp drop in the abundance of this

resource, resulting in the implementation of a per-

manent closure area in waters of Patagonia since

1997 (Irusta et al. 2016). At present, this stock is

assessed annually through age-structured models

(VPA-XSA and ECE) using SSB as the most

important biological reference point (Santos and

Villarino 2022). The S-R model obtained during

the assessment of Patagonian population showed

high variability, which increased studies on the

recruitment process in this fishing stock. For this

reason, starting in 2008, the Instituto Nacional de

Investigación y Desarrollo Pesquero (INIDEP,

Argentina) generated a research project called

Recruitment of Patagonian Hake Stock (REC),

whose main objective was to determine the phys-

ical and biological variables affecting the abun-

dance of hake individuals recruited to the popula-

tion in the first year of life. The project focused

on the study of factors that influence the repro-

ductive potential of hake during the adult phase

and on the analysis of the processes that affect the

survival of eggs, larvae, and juveniles, until their

recruitment at the age-1 class.

This paper is an up-to-date review of the main

results obtained for the REC project to date,

addressing aspects of the Argentine hake life

cycle in Patagonian waters, from the spawning to

the juvenile phase, passing through larval stages.

The study includes the analysis of the reproduc-

tive ecology, trophic and energetic dynamics dur-

ing different stages of development, and its rela-

tionship with environmental variables. We also

include information on new lines of research that

are being developed in the project and its future

perspectives. The importance of an integrative

comprehension of the species’ life traits, consid-

ering the reproductive strategy and early develop-

ment, is highlighted in order to improve the

recruitment indices.

PHYSICAL CHARACTERISTICS OF THE

SPAWNING AND NURSERY AREAS

Characteristic large tide ranges from the Patag-

onian region lead to high-energy dissipation rates.

The interaction of this energy with the bottom

topography and the stratification generated by

surface heating favors the formation of frontal

zones (Simpson and Bowers 1981; Rivas and

Piola 2002). This type of hydrographic structure

is characteristic of the ‘North Patagonian Frontal

System’ (NPFS) in the north Patagonian region

between 41° S and 45° S (Sabatini and Martos

2002), where a seasonal thermocline between 30

and 50 m depth begins to develop in early spring

due to increasing solar radiation (Figure 1). This

4MARINE AND FISHERY SCIENCES 36 (3): XXX-XXX (2023)

thermocline defines a two-layered structure

where the lower layer is under the influence of

colder waters from the middle shelf. While in the

coastal region the effect of tides homogenizes the

water column, in the stratified zone conditions are

more stable and the mixture between water mass-

es is relatively weaker, prevailing an increase in

temperature at the surface (Simpson and Hunter

1974; Acha et al.2015). During this process,

three main sectors are defined: homogeneous,

frontal, and stratified. In this sense, the frontal

zone represents the transition between the homo-

geneous coastal region and the more deeply strat-

ified one. In the cold season, the middle shelf

stratification begins to break by convection and

wind-driven mixing, and the frontal structure dis-

appears. The position of the tidal front has been

determined by hydrographic observations that

allowed estimating the stability parameter (Simp-

son 1981), establishing a 40 J m-3 reference value

that separates homogeneous and stratified regions

(Martos and Sánchez 1997). From a series of in

situ data, the mean position of the NPFS was esti-

mated (Figure 1 A) approximately on the 70 m

isobath, with a NE-SW direction following the

bathymetry (Sabatini and Martos 2002). The front

MACCHI ET AL.: RECRUITMENT OF MERLUCCIUS HUBBSI FROM PATAGONIAN STOCK

Figure 1. A) Mean location of the North Patagonian Frontal System (black line) as determined from the Simpson parameter

(Value 40 J m-3). B) Vertical section of temperature along the transect located near the Valdés Peninsula (red dotted line),

showing the homogeneous (H) and stratified (ST) zone, and the surface (SF) and bottom thermal front (BF). The num-

bers on the abscissa in panel B represent the distances in km. Modified from Sabatini and Martos (2002).

100 m

43°

45°

4°2

4°4

4°6

4°7

0 40 80 120 160

100

Depth (m)

San Jorge Gulf

Camarones Bay

W67° 66° 65° 64° 63° 62° 61°

50 m

80 m

Study

area

Argentina

0 40 80 120 160

100

Depth (m)

8°C

80 m

Rawson

Valdés

Peninsula

is present both on the surface and on the bottom,

showing the intersection of the thermocline in

both places with different thermal gradients

depending on the region (Figure 1 B). The physi-

cal structure and location of the front show latitu-

dinal differences related to variations of main

hydrometeorological forces, tide and wind, being

located further from the coast in front of Valdés

Peninsula and closer to it further south.

Circulation in frontal areas is affected by dif-

ferences in water masses density, which generates

convergence zones both at surface and bottom

(Largier 1993; Mann and Lazier 1996). High

amount of nitrates produced by phytoplankton

blooms and large copepod aggregations charac-

terize tidal fronts, which are typical of regions

with high biological productivity (Derisio et al.

2014; Temperoni et al. 2014). A clear transition of

nitrate concentrations has been observed through

the thermocline in the front stratified sector, and

relatively high values throughout the water col-

umn in the homogeneous zone (Carreto et al.

1985). Another characteristic of this type of front,

generated by the mix of turbulent tides, is that

variability in their position and structure is par-

tially associated with the transition between syzy-

gy and square tides, which may contribute to a

greater flow of nutrients through the front (Pisoni

et al. 2015). This scenario creates an ideal habitat

for spawning and development of early life stages

of many species (Sánchez and Ciechomski 1995;

Acha et al. 2015). This is partly due to the physi-

cal conditions generated by the differential strati-

fication of water masses, in addition to nutrient

enrichment, which has led to postulating one of

the best-known hypotheses on the variability of

recruitment in marine populations, such as the

‘fundamental Bakun triad’ (1996). The hypothe-

sis postulates that some frontal areas are essential

for fish survival during their first life stages since

they have three main characteristics: 1) the nutri-

ent-rich upwelling of deeper waters that enrich

superficial layers; 2) the concentration of plank-

tonic organisms that constitute the main food

source for fish larvae, and 3) the retention of fish

eggs and larvae in these favorable areas. Ele-

ments of this triad have been described for the

NPFS during spring and summer; particularly,

high concentrations of chlorophyll and microzoo-

plankton have been reported in the transitional

zone of the front, which favors a higher biological

production (Viñas and Santos 2000; Temperoni et

al. 2014). Although little information is available

on direct current measurements in that region,

retention seems to be a key feature in the hake

spawning and nursery area of the Patagonian

stock (Piola and Rivas 1997). Recently, as part of

the Cassis-Malvinas Project (http://www.cima.

fcen.uba.ar/malvinascurrent/es/), a current meter

deployment was carried out in a transect located

at the latitude of Camarones Bay (Figure 1 A)

from the coast to greater depths, whose informa-

tion has been partially analyzed by Lago (2022).

In any case, numerical models suggest the exis-

tence of circulation patterns in the north Patagon-

ian region that varies with depth (Palma et al.

2008), and that could support the existence of

retention mechanisms in the nearness of the ther-

mocline during the summer (Álvarez Colombo et

al. 2011). This area is coincident with the main

spawning site of hake, where the highest densities

of herbivorous calanoid copepods, the preferred

larval food, occur (Derisio 2012).

South of this region, the San Jorge Gulf (45° S-

47° S) is considered to be the main nursery area

for young-of-the-year juvenile hake (YOY or

age-0 group) of Patagonian stock (Álvarez

Colombo et al. 2014; Irusta et al. 2016). Accord-

ing to studies carried out in this area, the influ-

ence of tidal fronts led to high chlorophyll-a

(Chl-a) concentrations south of the gulf’s mouth

(Romero et al. 2006). On the other hand, various

authors (Tonini et al. 2006; Torres et al. 2018;

Pisoni et al. 2020) have observed that coastal

upwelling events in the SW of the gulf generate

changes in nutrient availability (especially

nitrates), boosting primary productivity (Pappa-

razzo et al. 2021). In the NE sector of the gulf,

6MARINE AND FISHERY SCIENCES 36 (3): XXX-XXX (2023)

there is a high biodiversity associated with the

mesoscale circulation generated by the interac-

tion of tidal currents with the islands and topo-

graphic features of the coastal zone (Gagliardini

et al. 2004). In addition to influencing the avail-

ability of nutrients and primary productivity,

these circulation patterns generate a series of rel-

evant processes for the development of commu-

nities, particularly those affecting the dispersal

and transport of planktonic larvae.

REPRODUCTIVE CYCLE AND SPAWNING

The first papers on the biology of the Argentine

hake suggested the existence of two reproductive

periods for the species in waters of the Argentine

continental shelf: the main one during spring-

summer, and a secondary one in autumn-winter

(Angelescu and Gneri 1958; Christiansen and

Cousseau 1971). The spring-summer spawning

corresponds to the Patagonian hake stock, which

takes place mainly in coastal waters of Chubut

Province, south of Valdés Peninsula (Ciechomski

et al. 1983). More recent papers based on the

macroscopic and histological analysis of gonads

and on the evaluation of the spatial distribution

and abundance of ichthyoplankton, allowed

adjusting the extension of the reproductive season

and determining variations in spawning intensity

during this period (Macchi et al. 2004; Ehrlich et

al. 2019). In addition, monitoring the hake repro-

duction from research surveys carried out

throughout the spawning period has made it pos-

sible to determine movements of pre- and post-

spawning aggregations, providing additional

information on the behavior and migrations of

hake during reproduction (Macchi et al. 2007).

At the beginning of the austral spring (Octo-

ber), adult fish at latitudes 43° S-44° S move

from shelf waters (>100 m depths) towards the

coast in order to reproduce (Macchi et al. 2007).

This analysis showed that hake spawning intensi-

fies in December, mainly in the south of Valdés

Peninsula, reaching the peak in January when the

spawning area extends between 43° S and 45° S,

from the 50 m isobath up to approximately 80 m

depth (Figure 2). Dense hake shoals have been

observed in December, mainly in the coastal area

called Isla Escondida, near Rawson city (Macchi

et al. 2005), coinciding with the first reproductive

aggregation. In February, the abundance of hake

in the spawning area decreases as a consequence

of offshore displacements of the species after

spawning (Figure 2). By the end of summer, the

abundance of spawning-capable-individuals de-

creases even more, and the presence of females in

regression or post-spawning phase increases,

reaching the maximum of this stage at the begin-

ning of autumn, when very few specimens are in

reproductive activity (Macchi et al. 2004). Both

direct trawl captures and acoustic methods have

MACCHI ET AL.: RECRUITMENT OF MERLUCCIUS HUBBSI FROM PATAGONIAN STOCK

Figure 2. Spawning cycle of Merluccius hubbsi in the north

Patagonian area. Arrows represent movements of

hake individuals in different periods: October-

November (green), December-January (orange) and

February-March (violet). The oval shows the main

spawning site during summer and the regressing or

post-spawning area offshore near the 100 m isobath.

Modified from Macchi et al (2007).

100 m

50 m

Spawning

San Jorge Gulf

Argentina

Camarones Bay

43°

45°

47°

4°2

4°4

4°6

100 m

50 m

67° 65° 63° 61°62°64°66°

Regressing

Rawson

Valdés

Peninsula

been used to determine variations in the abun-

dance of parental stock during the reproductive

period. The comparative analysis has shown that

trends obtained with both methods are very simi-

lar, giving greater certainty to the estimations

(Álvarez Colombo et al. 2006).

AGE/SIZE COMPOSITION AND CONDITION

OF THE SPAWNING STOCK

Spatial variations observed in the parental

stock of hake during the reproductive season are

also reflected in the size composition of spawning

shoals. Analysis of length distributions made it

possible to determine that smallest females (<50

cm TL) finished spawning earlier, moving out-

side the main reproductive area, which was also

reflected in the age structure of the reproductive

stock (Macchi et al. 2004). On the contrary, larger

and older females (>5-year-old) showed longer

spawning periods, allowing them to stay longer in

the breeding area. Males, as discussed for other

species, also tend to remain active for longer,

even after the reproductive activity has ended

(Macchi et al. 2004). This is probably due to the

lower energy investment by males during repro-

duction compared to that derived for the develop-

ment of female gametes. This feature coupled

with the migration of females to deeper waters

after spawning, causes male-biased proportions

in coastal areas at the end of the reproductive sea-

son (Pájaro et al. 2005). Moreover, a high propor-

tion of males is recorded in areas where hake

females are ovulating, suggesting the possible

existence of courtship mechanisms or competi-

tion between males before egg fertilization

(Pájaro et al. 2005; Macchi et al. 2007).

During some years, it was possible to detect

the presence of small groups of females spawning

in deeper waters, near the 100 m isobath, far from

the main reproductive aggregations of hake

(Macchi et al. 2010). These shoals were mainly

composed of larger females (>50 cm TL) near

the regression stage, with evidence of having

made several spawning events during the repro-

ductive season. Some of these females, particu-

larly the larger ones, were still in spawning-capa-

ble condition, but migrated into deeper waters to

feed and recover energy reserves (Figure 2). This

suggests that Argentine hake may incorporate

energy during the reproductive season, respond-

ing to an energy allocation strategy typical of an

income breeding species (Macchi et al. 2013;

Leonarduzzi 2018), as has also been reported for

the European hake M. merluccius (Domínguez-

Petit and Saborido-Rey 2010).

Traditional indices (hepatosomatic and K fac-

tor) and analysis of proximal composition of dif-

ferent tissues (liver, gonad, and muscle) used to

study the nutritional condition of the Argentine

hake revealed that females primarily store energy

in the liver as lipids, but they would also use pro-

teins from the muscle as they matured (Leonar-

duzzi 2018). This author observed that larger

females (>50 cm TL) showed a better energetic

condition compared to smaller ones, character-

ized by larger livers and high lipid content, which

could be associated with both feeding frequency

and quality of the prey available for larger indi-

viduals, such as squid and anchovy.

FEEDING OF THE SPAWNING STOCK

The trophic ecology of the Argentine hake

from Patagonian stock has been extensively stud-

ied through the classical analysis of stomach con-

tents during the last years. The diet of this species

exhibits regional and seasonal variations, which

would be related to prey availability. During sum-

mer, it feeds on zooplanktonic crustaceans such

as the white shrimp Peisos petrunkevitchi in

Escondida Island (Ruiz and Fondacaro 1997),

squid (Illex argentinus) and Argentine anchovy

(Engraulis anchoita) in the reproductive area

8MARINE AND FISHERY SCIENCES 36 (3): XXX-XXX (2023)

south of Valdés Peninsula (Belleggia et al. 2014).

Conversely, during winter, Argentine hake feed

on euphausiids (mainly Euphausia lucens) and

amphipods (Themisto gaudichaudii) in San Jorge

Gulf and surrounding area, making vertical nicte-

meral migrations (Belleggia et al. 2014; 2019),

and on myctophids and squids in the shelf break

region (Angelescu and Cousseau 1969; Belleggia

et al. 2014). From zooplankton crustaceans dur-

ing early life stages of hake to fish and

cephalopods in the adult phase, growth-associat-

ed ontogenetic dietary changes were also ob-

served (Belleggia et al. 2014).

Since 2008, and more intensely in 2011, there

have been changes in the diet of hake from San

Jorge Gulf, with an increased consumption of the

lobster krill Munida gregaria (Belleggia et al.

2017). The decrease in the abundance of preda-

tors of M. gregaria, such as the pink cusk eel

Genypterus blacodes, Argentine seabass Acan-

thistius patachonicus and Rajidae skates (Zearaja

brevicaudata, Psammobatis spp. and Symptery-

gia bonapartii) can be considered among the bio-

logical factors explaining the expansion of the

lobster krill population and the increased biomass

in San Jorge Gulf (Belleggia et al. 2017).

The Argentine hake is considered a facultative

opportunist, because although the general propor-

tion of prey consumed and that present in the

environment is similar, the species is able to

select under certain circumstances high-energy-

density prey (i.e. lobster krill M. gregaria) to sat-

isfy their energetic and nutritional requirements,

disregarding less caloric content food items even

when they are abundant in the environment (Bel-

leggia et al.2019). However, despite that since

2011 it has fed mainly on this crustacean, prelim-

inary studies carried out with stable carbon and

nitrogen isotopes revealed that the ‘new’ prey is

not efficiently assimilated into hake’s muscle

(Belleggia et al. 2022a). These analyses showed

that euphausiids, particularly E. lucens, were the

main food item assimilated in the San Jorge Gulf

region (Belleggia et al. 2022a).

Cannibalism occurs in Argentine hake as a

denso-independent process (Belleggia et al.

2019), influenced by the proximity of YOY indi-

viduals to the demersal habitat (Belleggia et al.

2022b). During daylight hours, YOY hake in the

San Jorge Gulf nursery area distributes in the

pelagic layers, while the age-1+group is found

close to the bottom in the demersal layer (Álvarez-

Colombo et al. 2014; Belleggia et al.2022b). The

vertical spatial segregation of hake life stages in

different layers of the water column in San Jorge

Gulf may reduce trophic overlap, diminish

intraspecific competition and limit cannibalism

(Belleggia et al.2022b). Pelagic YOY hake are

more abundant and are located more distant from

the bottom during the cold season, thus, cannibal-

ism is less during winter (Belleggia et al.2022b).

REPRODUCTIVE POTENTIAL OF THE STOCK

The Argentine hake from Patagonian stock

reaches sexual maturity at 2-3 years old, showing

differences between sexes (Macchi et al. 2017).

The length at maturity (L50) has been estimated

close to 33-35 cm TL for females and between 26

and 28 cm TL for males (Macchi et al. 2007;

2021). This is an important parameter for the

assessment and management of fisheries, being

considered when establishing catch limits by size

for many species to reduce the mortality of juve-

niles in the population. The length at maturity

model, also estimated with age, is commonly

called maturity ogive, and is relevant for the stock

assessment, since it is used to estimate the SSB

(Mace and Sissenwine 1993). Seasonal variations

of L50 in the hake from Patagonian stock charac-

terized by higher values of this parameter at the

end of the reproductive period were also observed

(Pájaro et al. 2005). A similar pattern was regis-

tered when L50 was estimated with samples col-

lected in autumn-winter during the resting phase

of the maturity cycle (Macchi et al. 2017).

MACCHI ET AL.: RECRUITMENT OF MERLUCCIUS HUBBSI FROM PATAGONIAN STOCK

Regarding the interannual variations of L50, com-

parisons made using samples collected during the

reproductive peak (January) of this stock between

2001 and 2018, have not shown trends indicating

changes in the length or age at maturity during

that period (Macchi et al. 2021).

A recently recorded phenomenon in the Patag-

onian hake stock that affects the estimation of the

maturity ogive, is skipped spawning (Macchi et al.

2016). This is the interruption of the spawning

process in a fraction of the parental population

during the annual reproductive cycle. It has been

observed that during the spawning peak, a propor-

tion of adult females remain in the resting stage,

with no evidence of oocyte development or recent

ovulation (Macchi et al. 2016). These authors

observed that skipped spawning occurred mostly

in young adult females, and that it could be related

to a poorer nutritional condition of these individu-

als in comparison to older specimens as a strategy

of female hake to conserve energy reserves. This

‘irregularity’ in the reproductive cycle would have

consequences for the estimation of the SSB, since

not every adult fish would contribute to the annual

reproductive potential of the population. In fact, it

was estimated that skipped spawning could cause

a reduction in the annual egg production of hake

between 4% and 12% (Macchi et al. 2017).

The Argentine hake is a multiple spawner with

an indeterminate annual fecundity, meaning that

unyolked oocytes continuously mature and are

spawned throughout the reproductive season

(Hunter et al. 1992). The number of eggs pro-

duced by batch (batch fecundity, BF) increases

with total length, weight and age of females (Fig-

ure 3 A). Relative fecundity (RF), which repre-

sents the number of eggs per unit of female

weight, may also evidence positive relationships

with the size of spawners, although models

obtained in this case showed great variability

(Figure 3 B). A comparative analysis of these

variables between different months of the spawn-

ing season showed a decrease in the number of

oocytes produced towards the end of this period,

10 MARINE AND FISHERY SCIENCES 36 (3): XXX-XXX (2023)

which is possibly associated with a depletion of

energy reserves used for reproduction (Macchi et

al. 2004).

The spawning frequency (S) is another neces-

sary parameter to determine the reproductive

potential of a species, since it represents the time

elapsed between egg batches and is an estimator

of the number of spawning events carried out by

females during the reproductive season (Hunter

and Goldberg 1980). For the Patagonian stock, the

estimated value of S at the beginning of the

spawning season was close to 13 d, but during the

reproductive peak (January) the time elapsed

between spawning events was shorter, reaching

up to 7 d (Macchi et al. 2018). Spawning frequen-

cy is generally estimated as an average value for

the population, although differences associated

with the size-age of females have been reported

(Claramunt et al. 2007). In fact, for the Patagonian

stock, the number of days between spawning

events decreases as the size/age of females

increases (Figure 3 C), e.g. older spawners would

have a higher number of spawning events during

the reproductive season (Macchi et al. 2018). The

combination of batch fecundity, spawning fre-

quency and abundance of mature females by size

class is necessary to estimate the potential egg

production of the stock. This parameter, in the

case of the Patagonian hake population, is charac-

terized by a significant contribution of larger

females (Rodrigues et al. 2015).

The third component to consider when evaluat-

ing the reproductive potential of stocks is the

quality of oocytes produced, since it is a factor

that can influence the survival of first life stages.

The size and weight of oocytes are variables com-

monly used to assess egg quality in fish, since

both are associated with the amount of nutritive

substances available in the yolk (Brooks et al.

1997). The most accurate parameter for hake was

the estimated oocyte dry weight during the hydra-

tion phase, just before ovulation. Analysis of this

variable during the reproductive season of Patag-

onian stock has shown that the quality of eggs

produced decreases towards the end of this peri-

od, coupled with the reduction of batch and rela-

tive fecundity (Macchi et al. 2006).

Regarding the relationship between oocyte

quality and maternal characteristics in hake, stud-

ies showed that both the dry weight of hydrated

oocytes and the size of the oily droplet increase

with total length (Figure 3 D), weight and age of

females (Macchi et al. 2006, 2013; Rodrigues et

al. 2018). These results suggest that eggs pro-

duced by largest females would have a greater

amount of stored reserves, giving rise to larger

larvae, having a positive effect on survival rates

during this critical phase of early development

(Hinckley 1990; Rijnsdorp and Vingerhoed

1994). Female condition, as determined from the

hepatosomatic index (HSI) and gonadosomatic

index (GSI), has also shown positive relation-

ships with oocyte quality (Macchi and Leonar-

duzzi 2022). However, the analysis of proximal

composition carried out with female hake did not

show a direct relationship between the lipid con-

tent in the liver and muscle and the lipid content

in the ovaries (Leonarduzzi 2018). This is proba-

bly due to the energy allocation strategy of hake,

which shows a very dynamic exchange between

food intake and organs involved in energy storage

during reproduction, or that the level of oocyte

quality is independent of liver reserves available

at the time of spawning.

MACCHI ET AL.: RECRUITMENT OF MERLUCCIUS HUBBSI FROM PATAGONIAN STOCK

Figure 3. Influence of maternal size on the reproductive potential of Merluccius hubbsi from the Patagonian stock determined

by the relationships between batch fecundity (A), relative fecundity (B), spawning frequency in days (C), and oocyte dry

weight (n =100 hydrated oocytes) (D) with female total length. All data correspond to the spawning peak of the

Patagonian stock (January) and were obtained from Macchi et al. (2013, 2018).

BF = 0.92 TL3.31

R = 0.81

n = 1 031,

500

1 000,

1 500,

2 000,

2 500,

3 000,

3 500,

4 000,

25 30 35 40 45 50 55 60 65 70 75 80 85 90

Batch fecundity (thousands)

Total length (cm)

ARF = 4.53 TL + 283.69

R2= 0.10

n = 98

200

400

600

800

1 000,

1 200,

25 30 35 40 45 50 55 60 65 70 75 80 85 90

Relative fecundity

Total length (cm)

SF = 9.76 0.06 TL-

R = 0.46

n = 46

25 30 35 40 45 50 55 60 65 70 75 80 85 90

Spawning frequency (days)

Total length (cm)

CDW = 0.53 ln (TL) + 0.85

R = 0.15

n = 511

25 30 35 40 45 50 55 60 65 70 75 80 85 90

Oocyte dry weight (mg)

Total length (cm)

In summary, characteristics of the parental

stock, particularly size, age, and condition of

females, have a significant impact on the repro-

ductive potential of this population. This charac-

teristic, called ‘maternal effect’, suggests that

SSB, commonly used as an index of productivity

in fishery assessment, is a poor predictor of

recruitment for hake. Therefore, in order to pre-

serve a level of egg production that allows a high-

er rate of larval survival, it is convenient to pre-

serve a population structure that ensures the pres-

ence of large females, as they have longer repro-

ductive periods with higher fecundities and major

quality eggs. As a result, it was decided to include

a goal of reaching a SSB composed of at least 16-

18% of large individuals (age-5+) in the assess-

ment of the Patagonian stock since 2013 (Santos

and Villarino 2013). These percentages arose from

analyzing the population structure of hake

obtained from research surveys carried out since

the early 1990s, and from the information result-

ing from commercial catches carried out during

years prior to the overexploitation of this resource.

EARLY LIFE STAGES (EGGS, LARVAE AND

YOUNG-OF-THE-YEAR)

Abundance and spatial distribution

Studies of ichthyoplankton in the Argentine

hake were based on developmental stages

described by Ehrlich (1998) and Betti et al.

(2009). In general, highest densities of eggs and

larvae occur within the main spawning area of the

stock, between 42° S and 45° S from the coast to

100 m depth. During summer research surveys

conducted in this region from 2009 to 2018, a

wide distribution of hake ichthyoplankton was

observed. Eggs were found in 50-82% of stations

and larvae were found in 28-60% of plankton

trawls (Ehrlich et al. 2019). When evaluation the

presence of hake ichthyoplankton during the

whole reproductive season, it was discovered that

smaller larvae were typically found during the

first spawning events in December, while the size

range increased and abundance reached its high-

est values in January (Machinandiarena et al.

2004a, 2004b). Acoustic records obtained during

monthly cruises throughout the reproductive sea-

son also reflected this, showing that hake larvae

dispersed southwards by the end of summer

(Álvarez Colombo et al. 2011).

Analysis of hake ichthyoplankton during the

last 20 years showed inter-annual variations in the

spatial distribution and density of egg and larval

aggregations in the north Patagonian area, which

could be associated with movements made by

hake spawners during the reproductive season or

with environmental aspects (Macchi et al. 2013,

2021). In general, sampling stations with maxi-

mum egg and larval records were characterized

by a strong thermocline. The thermal variability

observed at surface and bottom allowed us to con-

firm that hake larvae are resilient to temperature

changes, at least within a range of 3-4 °C during

the larval phase (Ehrlich et al. 2019). Acoustic

records in the nursery area showed that larvae

with swim bladder (>4 mm TL) performed verti-

cal migrations during the daily cycle, locating

near the bottom during daylight hours and near

the thermocline during the night (Álvarez Colom-

bo et al. 2011). These authors suggest that the

migratory behavior could be associated with the

two-layered circulation pattern with an opposite

flow described for the area by Palma et al. (2008)

in summer, allowing the larvae to remain retained

in that area with optimal survival conditions.

As the reproductive season progresses the hake

larval nursery area in the north Patagonian region

provides recruits to the stock. This fact has been

verified by the displacement and size increase of

post-larvae and juveniles towards San Jorge Gulf,

which is the main aggregation area for YOY spec-

imens (Machinandiarena et al. 2006; Álvarez

Colombo et al. 2011). This group includes juve-

niles that span the developmental stage between

12 MARINE AND FISHERY SCIENCES 36 (3): XXX-XXX (2023)

larval metamorphosis and the first year of life,

generally overlapping with the settlement of the

species to the demersal environment. Settlement

refers to the acquisition of the habit of settling

near the bottom, which in the case of hake is

mostly observed during daylight hours. This habi-

tat change between larvae and juveniles can be

studied by analyzing the microstructure of

otoliths with the formation of accessory growth

cores (Campana 1984). In M. hubbsi from Patag-

onian stock it was observed that the transition to

demersal habit began at approximately 50 days of

life (15 mm LT) and ended at 80 days (30 mm

LT). The average age of settlement was estimated

at 66.7 ±8.6 days (Buratti and Santos 2010).

However, as detailed further in this review, new

evidence on the vertical distribution of YOY indi-

viduals obtained from acoustic records suggests

that the final settlement would occur later, possi-

bly at the end of age-0 stage (35-150 mm TL).

Studies on the characteristics of benthic commu-

nities associated with hake pre-recruits suggested

a preference for boulder and sponge bottoms dur-

ing this phase of the life cycle (Giberto et al.

2014, 2015).

Growth

Knowledge of the processes that act generating

mortality during early life stages is fundamental

to understanding inter-annual variability in the

recruitment (Houde 1987). As larvae grow, they

are less vulnerable to mortality due to predation,

so an accelerated growth increases larval survival

rate by decreasing the residence time of early

stages (Legget and Deblois 1994; Houde 2008).

In addition, the growth of fish larvae and juve-

niles is a metabolic indicator that provides infor-

mation on the potential of nursery areas, which

could be relevant for the management of a fishery

resource. The most commonly used method for

determining the age and daily growth of fish lar-

vae is the analysis of daily increments of sagitta

otoliths (Jones 1992; Houde 2008). The daily pat-

tern of otolith increments deposition in the

Argentine hake larvae was adopted according to

criteria established by other authors for larvae of

the Genus Merluccius such as M. productus (Bai-

ley 1982) and M. merluccius (Arneri and

Morales-Nin 2000; Morales-Nin et al. 2005).

Several research cruises were conducted along

the Patagonian coast in the summers of 2001,

2005, and 2009, with a wide spatiotemporal cov-

erage, including most of the reproductive season

of hake. In 2001, hake larvae were collected in a

size range between 2 and 11 mm TL, their ages

were determined and a linear growth model was

fitted (Brown et al. 2004). The growth rate (0.156

mm day-1) was quite similar to values obtained by

other authors for larvae of the same genus, such

as M. products: 0.16 mm day-1 (Bailey 1982) and

0.156 mm day-1 (Butler and Nishimoto 1997),

and M. bilinearis, 0.17 mm day-1 (Jeffrey and Tag-

gart 2000).

Different types of nets were used in subsequent

studies to better characterize the growth and size

range of the larvae caught (Betti et al.2014).

These authors analyzed hake larvae in a size

range from 2 to 24 mm TL, and established two

exponential growth models in 2005 and 2009,

which were statistically different from each other.

Mean daily growth values were 0.22 mm day-1 in

2005 and 0.31 mm day-1 in 2009, similar to val-

ues recorded for other hake larvae of the genus

(Brown et al. 2004; Álvarez and Cotano 2005;

Palomera et al. 2005; Grote et al. 2012). During

the 2009 spawning season, variations in the daily

growth of hake larvae were detected, which were

attributed to different availability of copepods

during that time (Betti et al. 2014).

The hatching dates distribution curve could

provide information about periods of high mortal-

ity or good survival of different cohorts of larvae.

Hatching dates of larvae born in 2001 ranged

from 11 December to 20 February, with a maxi-

mum between 21 and 31 January. The hatching

period for larvae caught in 2005 ranged from 11

November to 20 February, with a peak between 1

MACCHI ET AL.: RECRUITMENT OF MERLUCCIUS HUBBSI FROM PATAGONIAN STOCK

and 10 January, while in 2009 it ranged from 11

December to 31 March, with a peak between 21-

31 December. Although several cohorts of hake

larvae were observed in the same spawning sea-

son (Figure 4), as a general trend, all larvae

hatched within the spawning period reported for

this species in the north Patagonian area (Macchi

et al.2004, 2010).

With respect to juvenile hake, studies of daily

growth have been carried out on specimens cap-

tured in autumn and winter of 2001, ranging

between 26 and 190 mm TL (Santos et al. 2005).

Results from the analysis of back-calculated

spawning dates revealed that the highest number

of births would have occurred between January

and mid-February 2001. This result was condi-

tioned to the sampling time; therefore, the period

was not necessarily the moment of the greatest

spawning. Estimates of juvenile growth rates

showed values of ca. 0.60 mm d-1 for individuals

captured in May, born between December 2000

and April 2001 (ca. 150 and 70 days, respective-

ly). Highest growth rates (0.70 mm d-1) corre-

sponded to individuals born in January and Febru-

ary, while the lowest (0.40 mm d-1) to those born

at the beginning of spawning, between September

and October. These data are similar to those esti-

mated by Hollowed (1992) and lower than those

found by Woodbury et al. (1995) for M. produc-

tus. From these studies, it was possible to corrob-

orate that specimens born in late winter/spring

would have slower growth rates than those born

in summer, and that individuals captured in

Camarones Bay showed higher growth rates than

those from San Jorge Gulf and shelf waters.

Mortality

Most fish suffer high mortality rates during

early life stages, which in the case of marine

teleosts exceed 99% during the egg and larval

stages (Bailey and Houde 1989; Houde 2008).

The classic methodology used to determine fish

larval mortality is the construction of survival

curves (Houde 2002), which seek to rebuild the

decline in numbers of a cohort of larvae from the

age-specific abundances of individuals. For this

purpose, an exponential extinction model is fitted

to the averages of larval abundances by age class,

expressed as densities.

14 MARINE AND FISHERY SCIENCES 36 (3): XXX-XXX (2023)

Figure 4. Back-calculated hatching dates of Merluccius hubbsi larvae from the Patagonian stock captured during reproductive

seasons 2001, 2005 and 2009. Data from Brown et al. (2004, 2009, 2013).

11-20

Nov

21-30

Nov

1-10

Dec

11-20

Dec

21-31

Dec

1-10

Jan

11-20

Jan

21-31

Jan

1-10

Feb

11-20

Feb

21-28

Feb

01-10

Mar

11-20

Mar

21-31

Mar

Frequency (%)

Back calculated hatching-

2001 2005 2009

Estimates of daily mortality rates for hake lar-

vae (Table 1) were obtained from eight research

surveys conducted along the north Patagonian

area during different months of reproductive sea-

sons 2001, 2004-2005, 2009 and 2010 (Brown et

al. 2004, 2009, 2013). Except for the value of

0.27 in January 2001, which was associated with

a very high density of larvae at age-0, reflecting a

recent spawning pulse (Brown et al.2004), mor-

tality coefficients ranged from 0.062 to 0.12.

Although it has commonly been associated with

predation (Houde 2008) or with competition for

food, as recently suggested for hake larvae from

Patagonian stock by analyzing the relationship

between egg production, larval abundance in the

nursery area and their nutritional condition (Diaz

et al. 2020; Macchi et al. 2021), it is particularly

challenging to establish the causes of mortality in

fish larvae.

Feeding

The traditional gut content analysis was used to

conduct trophic studies on larvae and YOY indi-

viduals of hake from Patagonian stock (Ciechom-

ski and Weiss 1974; Viñas and Santos 2000;

Moriondo 2002). Coupled with these studies, a

large amount of information has been gathered on

the availability of their zooplanktonic prey in the

environment, which has allowed determining their

preference and/or selectivity and the implications

of such choices upon recruitment.

The larval diet was analyzed in the warm peri-

od covering the size range from the onset of

exogenous feeding (<5 mm TL) to early (5-10

mm TL) and advanced stages (10-25 mm TL)

(Temperoni and Viñas 2013). With increasing

length, a higher number of preys consumed was

observed, which highlights the ability of hake lar-

MACCHI ET AL.: RECRUITMENT OF MERLUCCIUS HUBBSI FROM PATAGONIAN STOCK

Table 1. Daily growth models and comparison of number of individuals at age-0 (N0) and natural mortality (M) of hake larvae

estimated during different months of the reproductive seasons 2001, 2004-2005, 2009 and 2010. N =number of sampled

larvae; DM =daily mortality percentage; R2=coefficient of determination. From Brown et al. (2004, 2009, 2013).

Reproductive Month N Size range Age range Growth model N0 M DM

seasons (mm) (days)

2001 January 2,343 2.00-6.50 1-35 L(t) =0.136 t +1.76 880.07 0.270 23.67

(R2=0.86)

February 305 2.50-11.00 3-59 L(t) =0.153 t +1.95 72.24 0.120 11.30

(R2=0.87)

2004-2005 December 190 1.92-5.68 0-36 L(t) = 2.08exp(0.04t) 12.077 0.092 8.83

(R2=0.923)

January 339 2.92-14.00 9-67 L(t) = 2.06exp(0.04t) 121.15 0.092 8.79

(R2=0.889)

February 516 1.92-16.17 0-40 L(t) = 2.06exp(0.03t) 32.394 0.087 8.31

(R2=0.916)

2009 January 454 2.00-12.00 1-44 L(t) = 2.147exp(0.04t) 79.34 0.062 6.01

(R2=0.71)

March 2,359 2.00-20.00 1-59 L(t) = 2.662exp(0.034t) 103.40 0.105 9.97

(R2=0.931)

2010 January 1,520 1.40-8.20 0-57 L(t) =1.95exp (0.042 t) 111.70 0.080 7.70

(R2=0.88)

vae to pursue, capture and handle prey items

along ontogeny, as well as the higher energetic

requirements as they grow and develop. Larvae

capture small prey at the onset of exogenous

feeding that are replaced with larger items as their

body increases in size, increasing the diversity of

their diet. While first feeding larvae selected adult

stages of Drepanopus forcipatus and calanoid

copepodites ranging in size 1-2 mm and < 1 mm

(despite being less abundant in the environment

than other available prey), early and advanced

larvae exclusively predated upon adult stages of

Calanoides carinatus and D. forcipatus, even

when their abundances were very low in the zoo-

planktonic community (Temperoni and Viñas

2013). Hake larvae probably preferred these

preys considering their intermediate size, reflect-

ing an optimal balance between the energetic

profit and the cost of their capture. On the other

hand, the high incidence of calanoid copepods in

the larval diet might relate to their nutritional

benefit in terms of the high amount of polyunsat-

urated fatty acids (Temperoni et al. 2019a).

The ‘fatty acid trophic markers (FATM)’

approach was also used to investigate the diet of

hake larvae (Temperoni et al. 2019a), identifying

prey specific signatures in their tissues. While the

gut content analysis provides a ‘snapshot’ of the

recent diet, FATM integrates the diet on longer

time scales by analyzing the assimilated food

items in the tissues. As expected, a high overlap

in the fatty acids profile between hake larvae and

calanoid copepods was observed, especially by

means of markers 22:1n9 and 22:1n11. Moreover,

signatures typical of bacteria (15:0, 17:0) were

identified, suggesting a microbial input at the

base of the food web in the hake spawning

ground, as well as those from dinoflagellates

(18:4n3, 22:6n3) that probably act as intermedi-

aries towards the larvae. FATM also suggested

possible direct predation of hake larvae upon pro-

tozoans, which cannot be easily detected in the

gut content analysis, thus broadening the infor-

mation gathered from the traditional approach.

Information on the available prey field for hake

larvae was improved in recent years (Derisio et

al. 2014, 2021; Temperoni et al. 2014; Cepeda et

al. 2019) by the incorporation of a finer sampling

mesh (e.g. 67 mm). This allowed expanding the

knowledge on the zooplankton community with

respect to older studies (e.g. Santos and Ramírez

1995) that only properly characterized meso (0.2-

20 mm) and the macro (2-20 cm) zooplanktonic

fractions. Since hake larvae consume mostly indi-

viduals from the microzooplankton (20-200 µm)

fraction, this information has been key to better

establishing their prey selectivity.

With respect to the YOY individuals, older

studies compiled by Temperoni (2015) were

mostly qualitative and included a low number of

samples. More recently, research was expanded to

include individuals from 100 to 150 mm TL that

are going through their first year of life (Temper-

oni et al. 2013, 2018, 2020b). The preferred prey

of this group were crustaceans from the macro-

zooplankton fraction, mainly hyperiid amphipods

(Themisto gaudichaudii) and euphausiids

(Euphausia spp.), and to a lesser extent decapods

(Peisos petrunkevitchi and pelagic stages of

Munida gregaria). The capture of these preys

involves a lower energetic cost associated with

searching, handling and ingestion compared to

more mobile prey such as fish, which probably

explains their great incidence in YOY individuals.

In the context of these trophic studies, and given

the advanced decomposition of some prey items

in the gut content, equations were developed to

estimate the size of T. gaudichaudii and Euphau-

sia spp. from their remains that resist digestion

(such as carapaces or eyes), to obtain an estimate

of their individual weight from length-weight

relationships (Temperoni et al. 2013).

It should be mentioned that, asides from the tra-

ditional gut content approach, the stable isotope

analysis has been applied to determine the trophic

niche and position of hake larvae (from the spawn-

ing ground) to juveniles until age-2 (in the nursery

ground) (Botto et al. 2019). Hake occupies differ-

16 MARINE AND FISHERY SCIENCES 36 (3): XXX-XXX (2023)

ent trophic niches along ontogeny. Larvae (8-34

mm TL) mostly ate copepods, while age-0 indi-

viduals (35-150 mm TL) exhibited a rather diverse

diet with consumption of amphipods and

euphausiids, as well as epibenthic prey (mysi-

daceans and P. petrunkevitchi). The older (150-

320 mm TL) and already settled individuals

showed an almost exclusively epibenthic diet.

Within the nursery ground in San Jorge Gulf,

high availability of prey items for YOY hake has

been described, most likely guaranteeing their

development and survival. First studies described

horizontal spatial and temporal patterns in the

distribution of the main groups, such as cope-

pods, euphausiids and hyperiid amphipods (Pérez

Seijas et al. 1987; Viñas et al. 1992; Fernández

Aráoz 1994). More recent analyses have shown

the importance of the macrozooplankton fraction

in terms of abundance within the gulf when com-

pared with adjacent areas further north (Derisio

and Martos 2018; Derisio 2020, 2021). Repre-

sentative species are euphausiids Euphausia

lucens and Nematoscelis megalops, hyperiid

amphipod Themisto gaudichaudii and squat lob-

ster Munida gregaria. Particularly for the

euphausiids, studies on their vertical distribution

pattern have been performed by means of net

samplings and video plankton recorder data in

summer (Nocera et al. 2021). A classical diel ver-

tical migratory pattern was observed, with indi-

viduals ascending in the water column during

dawn (~18:30 h) and descending at dusk (~

06:30 h), remaining as dense aggregates near the

bottom during the day. In agreement, studies per-

formed in the cold season combining acoustic

and net samplings allowed to describe a similar

migratory pattern for both euphausiids and cope-

pods (Menna et al. 2022).

Carbon sources and trophic structure in San

Jorge Gulf have been evaluated by means of sta-

ble isotope analysis (Giménez et al. 2018). Feed-

ing regimes indicated that zooplankton items

could be either herbivorous, carnivorous or

omnivorous, with appendicularians located at the

base of the food web, while copepods occupied

an intermediate position and chaetognaths at the

top. An enrichment in carbon isotopes was

observed from north to south of the gulf, indicat-

ing that the main carbon source would not be the

same in every sector of the nursery ground.

Nutritional condition

The study of nutritional condition allows the

evaluation of the individual physiological state

of fish larvae and juveniles, which is a reflection

of environmental features to which they have

been exposed. Nutritional condition indices have

been widely used to determine the importance of

starvation in the early stages of fish, and to esti-

mate their survival probabilities, thus making it

possible to detect high mortality events during

ontogenetic development or to characterize

breeding areas. Different criteria have been

developed to estimate larval condition based on

the effects of starvation on body shape, condition

factor, cell constituents, or characteristics of their

tissues (Ferron and Leggett 1994). Currently, the

standardized RNA/DNA (RDs) ratio is the most

widely used biochemical index to assess the

nutritional condition of fish larvae (Chícharo and

Chícharo 2008). Cohen et al. (2020) used mor-

phometric variables and RDs to compare the con-

dition of hake larvae from northern and southern

stocks. Both indicators showed a better condition

for larvae from the southern stock, but statistical-

ly significant differences could only be detected

using the RDs, indicating that RDs are more sen-

sitive than morphometrical indicators. However,

previous studies using morphometric variables

such as body height and weight have also shown

to be reliable indicators of condition for hake lar-

vae of the Patagonian stock (Diaz et al. 2014a).

Since the effect of fixation causes the larval body

to shrink, one of the drawbacks presented by

these techniques is the necessity of establishing

shrinkage indices prior to their application (Diaz

et al. 2015).

MACCHI ET AL.: RECRUITMENT OF MERLUCCIUS HUBBSI FROM PATAGONIAN STOCK

Studies based on the RDs of hake larvae and

early juveniles (2-72 mm SL) from the Patagon-

ian stock that were collected during the austral

summer season between 2010 and 2021 (Diaz et

al. 2014b, 2020; Rodriguez et al. 2023), have

shown variations in nucleic acid concentrations

during the early ontogeny of the species at differ-

ent developmental stages (Figure 5 A). The RDs

showed a significant decrease during the begin-

ning of ontogeny in the larval preflexion stage (3-

4 mm SL) and in the postflexion stage (15-16 mm

SL). The highest values of the RDs and the great-

est variability were observed at the beginning of

the transformation toward the juvenile stages.

After that, indices decreased toward the initial

juvenile stage (60-70 mm). The ontogenetic pat-

tern of the RDs was similar to that observed in M.

paradoxus and M. capensis larvae by Grote et al.

(2012). At least two stages of great vulnerability

are observed by reductions registered in the RDs:

one coinciding with the start of exogenous feed-

ing and another during the transition from the

postflexion stage to the larval transformation

stage, when settlement occurs and the trophic

niche abruptly changes. These periods represent

critical moments with a significant reduction in

the abundance of individuals and their condition.

The RDs indices showed spatial variations in

the nutritional condition of hake larvae. Cohen et

al. (2020) observed differences between larvae

from northern and southern stocks (Figure 5 B).

For the Patagonian hake stock, spatial changes in

adult reproductive activity caused an expansion

in the spawning area and variability in larval con-

dition outside the traditional breading area (Fig-

ure 5 C). Larvae hatched in disadvantageous

areas for development are less likely to survive

and therefore a reduction in subsequent recruit-

ment will probably be observed (Diaz et al.

2014a, 2020). Diaz et al. (2020) indicated that

variables regulating the larval condition of the

Patagonian hake stock are larval length, tempera-

ture, and larval density. Both temperature and lar-

val density presented negative coefficients, indi-

cating that lower temperatures favor larval condi-

tion and the existence of density-dependent

mechanisms at high larval densities. These results

coincide with those previously mentioned, in that

highest larval densities are associated with the

presence of marked water stratification. The pres-

ence of the NPFS would favor the aggregation of

larvae and their potential food, although at high

densities it could cause a detriment to the larval

condition due to competition for food. Future

studies integrating endogenous and environmen-

tal variables will provide in-depth knowledge of

parameters that determine the larval condition of

hake, influencing their survival and subsequent

recruitment.

Contamination by microplastics (MPs) has

recently been incorporated as a determining factor

of the nutritional condition of specimens, given

that MPs accumulate in areas with oceanographic

discontinuities and would have an overlap in size

with potential preys (Di Mauro et al. 2022).

Efforts are currently focused on the development

of models including all available variables that

allow explaining the nutritional condition of lar-

vae of this species of great fishing relevance.

First studies to assess the condition of YOY

hake were carried out by Prenski and Angelescu

(1993) using the Fulton index. In more recent

analyses, information on lipids stored in the liver

(hepatosomatic index HSI, and lipid percentage

%L) was incorporated as a proxy of condition

(Temperoni et al. 2018, 2020a). Both indices

showed significant seasonal and spatial varia-

tions, being maximum in spring and minimum in

winter, with intermediate values in summer, asso-

ciated with variations of Chl-aconcentration

from satellite measures. Hence, the existence of a

bottom-up type control upon YOY nutritional

condition mediated by the consumption of her-

bivorous euphausiids, which are the preferred

prey throughout the year was suggested. In this

sense, the simultaneous evaluation of lipid and

fatty acid composition of their main zooplankton-

ic prey (Temperoni et al. 2019b, 2020c, 2022;

18 MARINE AND FISHERY SCIENCES 36 (3): XXX-XXX (2023)

MACCHI ET AL.: RECRUITMENT OF MERLUCCIUS HUBBSI FROM PATAGONIAN STOCK

Figure 5. A) Box-plot diagram of standardized RNA/DNA ratio (RDs) obtained for each size class of Merluccius hubbsi larvae.

Developmental stages are indicated. B) Box-plot diagram of RDs obtained for each hake larvae developmental stage

from northern and southern stocks. Crosses indicate position of sampling stations; arrows indicate where hake larvae for

RDs ratio analysis were collected for stock comparisons. Different letters indicate significant mean value differences (p

<0.05) of RDs in Tukey’s post hoc comparisons by the two-way ANOVA (stock and stage of development). C) Box-

plot diagram of standardized RDs ratio obtained for each hake larvae developmental stage from different areas of the

southern stock. In light blue: schematic delimitation of the Traditional spawning grounds (from Álvarez Colombo et al.

2011). T: traditional spawning area. Ex-N: external northern area. Ex-C: external central area. Ex-S: external southern

area. Different letters indicate significant differences (p <0.05) in mean values of the RDs in Tukey’s post hoc compar-

isons of the two-way ANOVA (area and stage of development). Modified from Diaz et al. (2014a, 2020), Cohen et al.

(2020) and Rodriguez et al. (2023).

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 4850 52 54 56 58 60 62 64 66 68 70 72 74 76 7880

SL mm()

W68° 66° 64° 62° 60° 58° 56° 54° 52° 50°

36°

38°

40°

42°

48°

44°

46°

Mean

Median

Outlier

M ximaum

M nimium

75 epercentil

25 epercentil

Preﬂexion

Flexion

Postﬂexion

Transformation

Late larvae Juvenile

100 m

50 m

43°

45°

47°

67° 65° 63° 61°

Preflexi no

oFlexi n

oPostflexi n

tioTransforma n

Ex-N

Ex-S

Ex-C

4 °2

4 °4

62°64°66°

4 °8

Standardized sRNA/DNA (RD )

RDs

Northern Southern

Stock

RDs

Temperoni and Massa 2021), provides a funda-

mental perspective to understand the food prefer-

ences of YOY individuals in terms of their nutri-

tional quality and energy intake. For instance, in

summer, low condition values observed in the

central region of San Jorge Gulf were associated

with the consumption of T. gaudichaudii, that has

a lower nutritional value in comparison to

euphausiids, due to the lower amount of polyun-

saturated fatty acids (PUFA). In this area, it was

observed that YOY hake retain essential PUFAs

and mobilize monounsaturated ones, such as 16:0

palmitic and 18:1n-9 oleic acids to obtain energy

(Temperoni et al. 2018).

Predation and competition

In terms of predation pressure within the

spawning area, a high horizontal overlap between

hake eggs and larvae and adult specimens of the

ctenophore Mnemiopsis leidyi was observed

(Mianzan 1999; Mianzan and Guerrero 2000). It

has been suggested that ctenophores may affect

fish recruitment by direct predation of eggs and

larvae and/or through competition for food with

the larvae (Purcell and Arai 2001, and references

therein). In this context, the existence of a high

overlap and an inverse correlation between the

mean abundance of M. leidyi and hake larvae sug-

gests a potential risk for M. hubbsi recruitment in

the north Patagonian area. However, this

approach needs to be complemented as recent

studies describe the low clearance rate of

ctenophores on fish eggs and larvae (Jaspers et al.

2011). Furthermore, despite the horizontal over-

lap, they could be occupying different vertical

strata.

As a prey, YOY hake in San Jorge Gulf nursery

area constitutes an important food source for sev-

eral demersal fish such as Squalus acanthias,

Zearaja brevicaudata, Genypterus blacodes and

G. brasiliensis (Sánchez and Prenski 1996; Gar-

cía de la Rosa and Sánchez 1997). The energy

density deriving from proteins and lipids stored in

muscle and liver, as a food quality proxy for these

predators, show that the YOY hake has a low or

moderate quality compared to other available

prey in the nursery area, such as E. anchoita, mol-

lusks (e.g. squid) and some crustaceans (e.g.

shrimps) (Temperoni et al. 2020b).

ENVIRONMENTAL VARIABILITY AND

RECRUITMENT

There are few studies that analyze the influence

of the interannual variability of physical factors on

the reproduction and recruitment of the Argentine

hake. Temperature is generally considered among

the most important variables affecting the matura-

tion and spawning process of fish, also influenc-

ing growth rates during early life stages (Houde

2009). Salinity may be another significant vari-

able for fish reproduction; however, in the case of

hake from the Patagonian stock, values obtained

in the spawning area in general show little varia-

tion ranging between 33.40 and 33.60 near the

bottom (Macchi et al. 2010). In this area, matura-

tion of hake may occur in a wide temperature

range from 7 °C to 15 °C, but females in ovulation

were mainly recorded at temperatures between 9

°C and 13 °C (Macchi et al. 2021). Areas with a

higher abundance of eggs and larvae of hake are

coincident with the location of main spawning

groups of this species (Álvarez Colombo et al.

2011); thus, the thermal range is similar to that

observed for females in ovulation (Macchi et al.

2021). Based on this information, optimal areas

for the spawning and nursery of hake have been

defined, whose dimensions showed interannual

variations, depending on the position of the bot-

tom thermal front (Macchi et al. 2021). These

authors observed that years characterized by a

reduced optimal spawning area and higher larval

density resulted in lower recruitments, which

could be associated with higher mortality rates

due to density-dependent processes.

20 MARINE AND FISHERY SCIENCES 36 (3): XXX-XXX (2023)

Satellite analysis of Chl-aconcentration in the

north Patagonian area during spring and summer

of 1997 to 2015, showed that years with the high-

est values of Chl-ahad a positive influence on the

recruitment of hake one year later (Marrari et al.

2019). These results suggest that higher primary

productivity during the spawning season could

lead to higher larval survival rates, probably due

to greater availability of food in the nursery area.

In any case, the relationships obtained were

weaker than those reported for other species, such

as Engraulis anchoita from Buenos Aires area

(Marrari et al. 2013), possibly due to values of

Chl-aconcentration estimated in the north Patag-

onian region during November-January (average

1.84 mg m-3) remain relatively high compared to

other areas (Marrari et al. 2019).

Information on the variability of surface tem-

perature (ST) in the north Patagonian area for the

same period, showed that lower surface tempera-

tures by the end of the spawning season (April)

could be beneficial for larval survival, generating

higher recruitments one year later (Marrari et al.

2019). These authors also demonstrated that

physical variables analyzed (Chl-aand ST)

showed a positive trend in the area during the last

two decades. These findings are significant in the

current context of climate change because of their

potential impact on the reproductive success and

recruitment of many species, including the

Argentine hake.

ADVANCES IN THE ESTIMATION OF

RECRUITMENT INDICES

The assessment model used to diagnose the

state of the Patagonian hake stock (VPA-XSA and

ECE) allows obtaining estimates of recruitments

during the period under exploitation. The model

is based on catch-at-age data adjusted with catch

per unit effort (CPUE) indices and abundance

indices independent of fishing activity, derived

from research surveys (Santos and Villarino

2022). During the last years, attempts have been

made to include new indices derived from

advances in the knowledge of the life history of

hake, such as the abundance of YOY juveniles

per swept area and estimates of reproductive

potential during the spawning peak of the stock.

However, this new information did not evidence

a significant improvement in the S-R relationship

estimated by the assessment model.

The YOY juvenile abundance estimates were

obtained by bottom net trawls of reduced dimen-

sions (~4 m horizontal and ~0.8 m vertical open-

ing) called ‘pilot net’, assuming that hake juve-

niles adopt demersal habits during the settlement

process, shortly after the end of the metamorphosis

process from larval stages, thus becoming avail-

able for capture with bottom trawls. This hypothe-

sis on the vertical distribution of early juveniles in

the nursery area was mainly based on knowledge

of the size and timing of settlement of different

hake species (Papaconstantinou and Stergiou

1995; Payne and Punt 1995; Auster et al. 1997),

including M. hubbsi (Ehrlich 1998; Machinandi-

arena et al. 2006; Buratti and Santos 2010). Never-

theless, during research surveys carried out to

assess the abundance of juvenile hake of the Patag-

onian stock, it was observed that catches with the

Pilot net were inefficient for assessing smallest

juveniles (Castrucci et al. 2003). The acoustic

information demonstrated that the transition from

pelagic to demersal habitat of YOY hake takes

longer than previously recognized for this species,

which may explain their low catchability with bot-

tom trawls (Álvarez Colombo et al. 2014). These

last authors observed a diurnal pelagic behaviour

of hake juveniles through acoustic records, which

was later validated by the capture with pelagic

nets. The assessment of YOY juvenile (A-0) with

acoustic methods provided for the first time esti-

mates of abundance in orders of magnitude com-

patible with recruitments obtained one year later,

represented by the age-1 group (A-1). Information

from research surveys carried out in the winters of

MACCHI ET AL.: RECRUITMENT OF MERLUCCIUS HUBBSI FROM PATAGONIAN STOCK

2011, 2012 and 2016 showed a large difference

between the A-0 estimates obtained from acoustic

data with those derived from bottom trawling with

the Engel net (Figure 6 A). However, values

obtained in 2011 and 2012 from acoustic records

were compatible with the abundance of survivors

in the following year (A-1) estimated from the

swept area method (Figure 6 B). Acoustic esti-

mates of A-0 juveniles were combined with esti-

mates from annual modelling assessments of other

age groups to get a complete picture of the stock

structure in different years (Figure 7), revealing

that 2011 and 2012 cohorts turned out to be very

similar based on the acoustic data and model

results. Acoustics from 2016 indicated a strong

cohort during this year that was later confirmed by

the model information (Santos and Villarino

2022). While these results are promising, the pos-

sibility of using an acoustic index of YOY juve-

niles should rely on data from a larger number of

research surveys, allowing the evolution of a larger

number of cohorts to be analysed. In any case,

acoustics could provide a valid recruitment index

for the Patagonian hake stock based on direct field

observations, considering the expected level of

natural mortality of fully developed juveniles, as

opposed to the reduced survival of eggs and larvae

of the species. These results show the importance

of the information obtained with echo sounders,

which is complementary to estimations made by

swept area with bottom trawls. In other gadoid

fisheries, such as cod and haddock, acoustic and

swept area indices of abundance are combined in

an estimate of absolute abundance (Godø and

Wespestad 1993; Guttormsen et al. 2003; Hjellvik

et al. 2003; Fleischer et al. 2005), providing a

broader framework for understanding the sources

of variability associated with such estimates.

Recent advances from specific hake recruit-

ment surveys using an Engel net have shown that

A-0 juveniles vary their vertical distribution pat-

tern (Álvarez Colombo et al. 2021). These authors

observed that highest concentrations in San Jorge

Gulf remain pelagic during the day, but occasion-

ally approached to the bottom beyond the range of

echo sounders, effectively being captured by bot-

tom trawls. On-going studies on the effect of

marine circulation, as well as other environmental

and habitat structure factors and survey designs in

the nursery sector of the species, may explain part

of the variability detected in the respective abun-

dance estimates.

22 MARINE AND FISHERY SCIENCES 36 (3): XXX-XXX (2023)

Figure 6. A) Absolute abundance of Merluccius hubbsi age-0 group from Patagonian stock estimated using acoustic (A-0

acoustic) and bottom trawl swept area (A-0 bottom trawl) methods during 2011, 2012 and 2016. B) Abundance of age-

0 group estimated by acoustic (A-0 acoustic) in 2011, 2012 and 2016, and survivors in the following year estimated from

swept area method (A-1 bottom trawl) corresponding to 2011 and 2012 cohorts. Data from Álvarez Colombo (2021) and

Santos and Villarino (2022).

1 000,

2 000,

3 000,

4 000,

5 000,

6 000,

7 000,

8 000,

9 000,

10 000,

11 000,

A-0 acoustic A-0 bottom trawl

Number of ﬁsh (millions)

1 000,

2 000,

3 000,

4 000,

5 000,

6 000,

7 000,

8 000,

9 000,

10 000,

11 000,

A-0 acoustic A-1 bottom trawl

Number of ﬁsh (millions)

2011

2012

2016

2011

2012

2016

PERSPECTIVES AND FUTURE CHALLENGES

Several research lines may contribute to the

understanding of recruitment variability of the

Patagonian hake stock. Future studies may focus

on male characteristics and their influence on the

reproductive success and recruitment of hake,

which is the influence of the ‘parental effect’ on

the offspring. This topic has rarely been studied

in most species, since information on reproduc-

tive potential has generally focused on females.

However, the contribution of males to ensure the

reproductive success of fish, beyond eggs fertil-

ization, has reached greater relevance in recent

years (Domínguez-Petit et al. 2022). For exam-

ple, it is important to analyze the influence of

male characteristics (size, age and nutritional

condition) on sperm quality, or to establish how

the sex ratio and the size/age composition of

males may affect fertilization rates or larval sur-

vival. It is essential to plan experimental studies

in order to evaluate these aspects as well as to

corroborate the maternal effect hypothesis. With

the help of these studies, it may be possible to

establish whether there is a correlation between

parental characteristics and those of their off-

spring (larval size, condition or survival rates).

The selection of spawning sites and environ-

mental factors that trigger maturation and ovula-

tion of this species are currently the subject of

research. In this regard, reproductive ecophysiolo-

gy studies involving sex steroids (testosterone,

estradiol, progesterone) during the maturation

cycle are being developed in order to evaluate hor-

monal profiles of specimens depending on envi-

ronmental conditions (Elisio et al. 2021). Since

the Patagonian stock reproduces in a transition

zone within the NPFS, where temperature exhibits

strong horizontal and vertical gradients, the main

hypothesis suggests that temperature is the vari-

able that determines ovulation. However, mecha-

nisms underlying this process are unknown.

Further studies on the nutritional condition are

important to complement traditional analysis on

the feeding of the Argentine hake with the meas-

urement of stable C and N isotopes, in order to

build the trophic web and analyze the energy flow

and its role in survival and reproductive success.

MACCHI ET AL.: RECRUITMENT OF MERLUCCIUS HUBBSI FROM PATAGONIAN STOCK

Figure 7. Abundances of 2011, 2012 and 2016 cohorts of Merluccius hubbsi from Patagonian stock estimated by combining the

age-0 group (G0) obtained by using the acoustic method, with the rest of the age groups (G1-G7) obtained with the

assessment model. Data from Álvarez Colombo (2021) and Santos and Villarino (2022).

1 000,

2 000,

3 000,

4 000,

5 000,

6 000,

7 000,

8 000,

9000

10 000,

11 000,

G0 G1 G2 G3 G4 G5 G6 G7

Number of ﬁsh (millions)

Age group

2011

2012

2016

Stomach content studies provides a ‘snapshot’ of

an individual’s diet, while stable isotope analysis

use Bayesian mixing models to fit values of d13C

and d15N from tissues to estimates of prey assim-

ilated rather than ingested. This point is highly

relevant, since recently observed changes in the

diet of the Patagonian hake stock related to the

increase of Munida gregaria in that region, could

affect the condition and reproductive potential of

the species. In addition, further studies on the

composition of fatty acids, both in hake and their

prey, could serve not only to evaluate the condi-

tion of specimens but also as food web biomark-

ers. This is particularly relevant in the current

scenario of changes for the north Patagonian

ecosystem, since variations in the de novo pro-

duction of fatty acids at the base of the food web

could move towards YOY hake with implications

for recruitment success (Jónasdóttir 2019).

In the case of hake larvae, it should be high-

lighted the importance of carrying out laboratory

experiments keeping specimens under controlled

conditions, in order to compare with individuals

captured in the field. These studies would make it

possible to infer the survival potential of larvae

and eventually estimate the probability that these

individuals will survive the critical period and be

recruited to the fishery. In this sense, some

attempts have been made to keep specimens alive

in INIDEP aquaria (Betti et al. 2013; Radonic et

al. 2013; Suárez et al. 2013), but experiments did

not exceed 20 days of larval survival. Recently,

some studies showed the existence of density-

dependent mortality processes in the Argentine

hake larvae from the north Patagonian area,

which could affect the recruitment of this stock

(Diaz et al. 2020; Macchi et al. 2021). This would

be reflected in both the nutritional condition and

survival rates during early stages of larval life,

either due to competition for food between indi-

viduals of the same species or with other organ-

isms. For this reason, studies are currently being

carried out to evaluate the role of macrozooplank-

ton, particularly ctenophores, as a competitor of

hake larvae. The aim is to complement field

observations with experimental studies, which

allow corroborating whether the abundance of

ctenophores has a significant effect on the sur-

vival of hake eggs and larvae, acting either as a

predator or as competitor of this species.

One of the most relevant results obtained with-

in the REC project was the observation and

recording of pelagic aggregations of YOY hake

during daylight hours (Álvarez Colombo et al.

2014). This finding was first made acoustically

and later corroborated by the capture of speci-

mens with pelagic nets. The relevance of these

results lies in the fact that, until then, it was

thought that once the Argentine hake settled near

the bottom, it maintained an exclusively demersal

behavior during the day and only migrated

towards the pelagic region at night. For this rea-

son, to assess the abundance of the juvenile frac-

tion, demersal trawls have traditionally been used

during the day, whose vertical opening does not

exceed one meter from the bottom (Álvarez

Colombo et al. 2014). As was mentioned in the

previous item, this means that abundance values

obtained with this capture method actually repre-

sent a small fraction of juveniles to be evaluated.

This situation raises the need to generate new

methodologies that allow estimating abundance

indices for the age-0 group of hake in a more

appropriate way. A possibility with great potential

for the future is to use information from acoustic

transects to obtain abundance indices that esti-

mate the real magnitude of pelagic groups of

hake. This methodology would not only allow

obtaining more realistic estimates, but would also

provide a synoptic overview of the distribution of

juveniles of the species, both in horizontal and

vertical dimensions.

The distribution of the Argentine hake in the

north Patagonian area shows spatial changes dur-

ing its life cycle. As already mentioned, hatching

and development of early stages of this species

take place mostly in coastal and shelf waters

between 43° S and 45° S; however, highest abun-

24 MARINE AND FISHERY SCIENCES 36 (3): XXX-XXX (2023)

dances of the YOY individuals, particularly pelag-

ic aggregations, occur towards the south of this

region, in San Jorge Gulf (Álvarez Colombo et al.

2014). Preliminary results suggest that the early

larvae remain retained in the main spawning area,

and that later post-larvae and early juveniles (<9

cm TL) drift towards San Jorge Gulf at the end of

the reproductive period (Álvarez Colombo et al.

2011). To support this hypothesis, it is necessary

to model the current flow in the involved areas.

This will help to understand how the retention

process works in the breeding zone and how it

relates to the juvenile zone. At present, there are

little data on coastal currents from the north Patag-

onian area, so it is necessary to increase this infor-

mation in order to contribute to the development

of new models that allow explaining how circula-

tion at different depths affects hake recruitment.

Studies with satellite monitoring of drifting buoys

are also required to detect drift routes for hake

eggs and larvae. Such studies should be related to

the directionality and intensity of currents at dif-

ferent spatial and temporal scales.

Finally, a relevant aspect for the future is the

need to develop new recruitment models allowing

the incorporation of biological and environmental

information in order to generate greater reliability

in the stock assessment. This would be a funda-

mental advance compared to traditional models,

since it would make it possible to broaden the

spectrum of variables associated with the process

of incorporating new individuals into the popula-

tion, improving understanding of the dynamics of

the stock-recruitment system and, consequently,

increasing the predictive capacity of models

(Lowerre-Barbieri et al. 2016).

ACKNOWLEDGMENTS

We wish to thank the researchers, technical

staff and crew of the INIDEP research vessels

who participated in the numerous research sur-

veys carried out to assess the Patagonian hake

stock during its reproductive period between 2000

and 2021. We would like to thank the technical

staff of the Cabinets of Reproductive Ecology of

Marine Organisms, Age Reading, Oceanography,

Zooplankton and the Ichthyoplankton Laboratory

that are part of the REC Project, for processing of

the samples collected during the research surveys.

We would also like to thank the referees for their

suggestions, which helped us to improve the orig-

inal manuscript. It was supported by the INIDEP,

Consejo Nacional de Investigaciones Científicas

y Técnicas (CONICET 112 202001-01807CO;

PIP11 220200102831CO) and Fondo para la In-

vestigación Científica y Tecnológica (FONCYT-

PICT 2018-03872; PICT 2020-030 22). INIDEP

contribution no 2280.

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