Marine and Fishery sciences 38 (2): xxx-xxx (2025)

https://doi.org/10.47193/mas.3822025010105

ABSTRACT. In the 1980s, the Pacic oyster Magallana gigas (Thunberg, 1783) was deliberately

introduced in the southern region of the Province of Buenos Aires (Bahía Anegada, BA), Argentina.

In 2004, its presence expanded 80 km south of the Río Negro estuary along the coast of El Cóndor

(EC). Although oysters have demonstrated dispersal capability, there is limited data as regards the

EC population since 2011. This research focusses on the present M. gigas population encompassing

distribution, abundance, and size structure along a 180-km coastal line from EC to San Antonio Este

(SAE). Subsequently, we compared these data with those for the BA population. The presence of M.

gigas in the Province of Río Negro was detected in four sites: three of them near the Río Negro estuary

(EC, Piedras Verdes PV, and El Pescadero); and the last one in San Antonio Bay. Estimated average

abundances near the estuary were lower (range 1.8 10-3 ± 0.6 10-3 and 9 10-2 ± 3.4 10-2 ind. m-2) than

BA (105 ± 2 ind. m-2). Presence in SAE was only limited to one site and three adults M. gigas. The BA

oyster population exhibited a multimodal distribution, with a signicant number of recruits, whereas

the PV site displayed a trimodal structure dominated by large specimens. In EC, owing to the limited

number of individuals, modal components were less discernible, but small oysters predominated. The

current abundance of M. gigas in EC was considerably lower than that in 2011, indicating a popula-

tion decline. Despite this, the presence of juvenile oysters suggests recent recruitment, emphasizing

species resilience. These results show that M. gigas faces challenges when attempting to establish

itself in this specic region. Studying the underlying causes would help to understand the factors that

limit the expansion of a species considered to be a global invader.

Key words: Invasive species, population dynamics, intertidal, ecosystem engineer, recruitment.

Distribución, abundancia y estructura de tamaño de la ostra cóncava del Pacíco, Magallana

gigas, en el norte de la Patagonia

RESUMEN. En la década de 1980, la ostra del Pacíco, Magallana gigas (Thunberg, 1783), fue

introducida deliberadamente en la región sur de la Provincia de Buenos Aires (Bahía Anegada, BA),

Argentina. En 2004, su presencia se expandió 80 km al sur del estuario del Río Negro, a lo largo de

la costa de El Cóndor (EC). Si bien las ostras han demostrado capacidad de dispersión, existen datos

limitados sobre la población de EC desde 2011. Esta investigación se centra en la población actual de

M. gigas que abarca la distribución, abundancia y estructura de talla a lo largo de una línea costera

de 180 km desde EC hasta San Antonio Este (SAE). Posteriormente, comparamos estos datos con los

de la población de BA. La presencia de M. gigas en la Provincia de Río Negro se detectó en cuatro

ORIGINAL RESEARCH

Distribution, abundance, and size structure of the Pacic cupped oyster

Magallana gigas in northern Patagonia

Leandro a. hünicken1, *, raúL GonzáLez1, 2, dennis Landete1, Maité a. Barrena1, 2, Juan F. saad1, 2 and

Maite a. narvarte1, 2

1Centro de Investigación Aplicada y Transferencia Tecnológica en Recursos Marinos “Almirante Storni”, Güemes 1030, R8520CXV - San Antonio

Oeste, Argentina. 2Escuela Superior de Ciencias Marinas, Universidad Nacional del Comahue, San Martín 224, R8520BNF - San Antonio Oeste,

Argentina. ORCID Leandro A. Hünicken https://orcid.org/0000-0002-9763-2615, Raúl González https://orcid.org/0000-0003-4660-4662,

Dennis Landete https://orcid.org/0009-0001-1937-7882, Maité A. Barrena https://orcid.org/0000-0002-3412-3645,

Juan F. Saad https://orcid.org/0000-0002-4482-8892, Maite A. Narvarte https://orcid.org/0000-0002-6051-4842

Marine and

Fishery Sciences

MAFIS

*Correspondence:

leandrohunicken@gmail.com

Received: 10 November 2023

Accepted: 20 December 2024

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 38 (2): xxx-xxx (2025)

INTRODUCTION

Human actions facilitate certain species to

reach previously inaccessible regions, establish

self-sustaining populations, and spread to novel

environments, a phenomenon known as biologi-

cal invasions (Elton 1958). In marine and estua-

rine environments, oysters represent a signicant

example of invasive species (Carlton 1992; Reise

1998), acting as ecosystem engineers that modify

physical and chemical environments, inuencing

populations, communities, and food webs through

the creation of biogenic reefs in soft-sediment ma-

rine landscapes (Ruesink et al. 2005). Aquaculture

has been a key driver for oyster introductions since

the 1950s, often aimed at replacing declining na-

tive populations or developing new export products

(Shatkin et al. 1997; Ruesink et al. 2005).

The Pacic oyster, Magallana (= Crassostrea)

gigas (Thunberg, 1783), native to the northwest

Pacic coast, has been introduced to at least 45

ecoregions worldwide (Molnar et al. 2008). While

deliberate introductions account for many of these

occurrences, the species has also expanded its

range through unintentional transport and natural

dispersal from established populations, as evi-

denced in Scandinavian waters including Denmark,

Norway, and Sweden (Dolmer et al. 2014), as well

as in New Zealand (Dinamani 1991). Although M.

gigas has demonstrated considerable success in

colonizing new environments, its establishment

patterns vary signicantly across regions (Carras-

co 2012). In some areas, environmental constraints

have either precluded successful establishment or

resulted in intermittent recruitment dynamics, char-

acterized by periodic population pulses that cor-

respond with favorable environmental conditions

(Diederich et al. 2005).

In Argentina, this species was introduced for

commercial purposes in 1982 in Bahía Anegada,

in the southern area of the Province of Buenos

Aires (39° 00' S to 40° 40' S and 62° 10' W), where

it established a wild population (Orensanz et al.

2002). Since then, the species have spread north-

wards to the Bahía Blanca estuary (Dos Santos and

Fiori 2010) and southwards to El Cóndor, in the

area inuenced by the Río Negro estuary, which

constitutes the administrative boundary between

the provinces of Buenos Aires and Río Negro

(González et al. 2005) (Figure 1). The success of

the species in this region is attributed to several

factors, including suitable water temperatures for

gonad maturation and spawning (Castaños et al.

2009), availability of appropriate substrates, and its

ability to recruit on various surfaces (Carrasco et

al. 2018). These established populations have cre-

ated shallow intertidal reefs that have signicantly

altered local community structure by providing

habitat for invertebrates and affecting shorebird

feeding patterns (Escapa et al. 2004; Bazterrica et

al. 2022), demonstrating ecological implications of

oyster invasions in the region.

sitios: tres de ellos cerca del estuario del Río Negro (EC, Piedras Verdes PV y El Pescadero); y el último en la Bahía de San Antonio.

Las abundancias promedio estimadas cerca del estuario fueron menores (rango 1,8 10-3 ± 0,6 10-3 y 9 10-2 ± 3,4 10-2 ind. m-2) que en

BA (105 ± 2 ind. m-2). La presencia en SAE solo se limitó a un sitio y tres individuos adultos de M. gigas. La población de ostras de BA

exhibió una distribución multimodal, con un número signicativo de reclutas, mientras que el sitio PV mostró una estructura trimodal

dominada por especímenes grandes. En EC, debido al número limitado de individuos, los componentes modales fueron menos discernibles,

pero predominaron las ostras pequeñas. La abundancia actual de M. gigas en EC fue considerablemente menor que en 2011, lo que indica

una disminución de la población. A pesar de esto, la presencia de ostras juveniles sugiere un reclutamiento reciente, lo que enfatiza la

resiliencia de la especie. Estos resultados muestran que M. gigas enfrenta desafíos cuando intenta establecerse en esta región especíca.

Estudiar las causas subyacentes ayudaría a comprender los factores que limitan la expansión de una especie considerada invasora global.

Palabras clave: Especies invasoras, dinámica poblacional, intermareal, ingeniero ecosistémico, reclutamiento.

Hünicken et al.: Pacific cuPPed oyster in nortH Patagonia 3

A monitoring effort that began in 2008 tracked

M. gigas distribution along the northern coast of

the Province of Río Negro. Findings indicated

higher densities near El Cóndor’s central beach

and a 30-km western expansion, reaching near

the eastern limit of San Matías Gulf (Roche et al.

2010) (Figure 1). However, the program ceased

in 2011, leaving the current species distribution

unknown in an area that encompasses four protect-

ed natural zones and a crucial provincial shing

reserve (Provincial laws 1960, 2519, 3222, 2670,

2669; http://www.legisrn.gov.ar). Since impacts

of invasive species depend on distribution, abun-

dance, and other ecological factors (Markert et al.

2010), assessing these parameters for M. gigas

an initial step to evaluate its potential effects in

the area. Environmental conditions at the invasion

front that affect the survival and individual growth

of colonizing individuals can profoundly inuence

the establishment and subsequent dispersal of the

invasive species (Burton et al. 2010). The original

monitoring of the cupped oyster in the Province

of Río Negro (Roche et al. 2010) and a prelimi-

nary survey carried out in April 2019 in El Cón-

dor revealed the persistence of the species in the

area, although in relatively very low abundance

(approximately 0.1 ind. m-2) in comparison with

those reached in other natural environments in the

world and Argentina (Bahía Anegada: 90ind. m-2,

Escapa et al. 2004; Las Toninas: 131 ind. m-2, Gib-

erto et al. 2012; Sylt, Wadden Sea: 125.8 ind. m-2,

Diederich et al. 2005). In other locations where M.

gigas was introduced, lag phases were observed.

For example, in Sylt, Wadden Sea, the rst estab-

lished populations emerged 17 years post-commer-

cial introduction (Wehrmann et al. 2000), while in

Figure 1. Distribution of Magallana gigas on the northern coast of the Province of Río Negro, Argentina. Years in brackets indicate

rst report in the corresponding site.

Marine and Fishery sciences 38 (2): xxx-xxx (2025)

South Africa, a lag span of 51 years was observed

(Robinson et al. 2005). Typically, these lag phas-

es coincide with suboptimal temperatures and/or

salinities for larval development, or unfavorable

hydrological regimes that prevent postlarvae set-

tlement by carrying them to unsuitable substrates

(Guy and Roberts 2010). Until now, no studies

have explored environmental conditions in El

Cóndor and their connection to M. gigas inva-

sive patterns. Assessing physicochemical factors

(temperature, dissolved oxygen, salinity, turbidity,

wave energy) and food availability through a com-

parison with established population zones provides

insights into impediments to the establishment of

new population.

This study aimed to evaluate the distribution,

abundance and size structure of M. gigas at the

invasion forefront on the northern coast of the

Province of Río Negro. Additionally, it sought to

identify environmental factors inuencing its po-

tential expansion.

MATERIALS AND METHODS

Distribution

In October 2020, a one-time comprehensive sur-

vey was conducted between El Cóndor (41° 3.30' S,

62° 49.81' W) and Caleta de los Loros (41° 0.98' S,

64° 11.5' W), following recommendations for mon-

itoring frequency during early invasion stages

(Guy and Roberts 2010). At each site, two parallel

transects of 1,000 m each were surveyed (totaling

2,000 m per site) along the low-tide line in the

lower intertidal zone. To distinguish M. gigas from

native oysters (Ostrea spreta d’Orbigny, 1845 and

O. puelchana d’Orbigny, 1842), morphological

features were examined (Borges 2006). The most

evident diagnostic character between genera is the

absence of interlocking teeth and sockets in the

hinge of M. gigas, which is typical of the genus Os-

trea. Additionally, the upper or right valve is at in

native oysters (at oysters), whereas it is curved in

M. gigas (cupped oyster) (Borges 2006). Magalla-

na gigas valves are also distinctive by being robust,

irregularly shaped, with a nacreous interior; the

hinge presents a central ligament and deep folds

that are not observed in native species of the genus

Ostrea (Evseev et al. 1996; Borges 2006).

Transects involved daily low-tide surveys at sites

with previous M. gigas records (El Cóndor, Segun-

da Bajada del Faro, El Espigón, and La Lobería)

and in areas of potential ongoing invasion (Bajada

Echandi, Bahía Rosas, Bahía Creek, and Caleta de

los Loros). The latter sites were selected to provide

uniform coverage along the 120 km of coastline

extending from the Río Negro estuary, while ensur-

ing accessibility to sampling areas. Access to this

predominantly cliff-lined coast is limited to man-

made descents (Bajada Echandi and Bahía Creek)

and natural embayment (Bahía Rosas and Caleta

de los Loros). Caleta de los Loros, a protected area

characterized by reduced wave exposure, was of

particular interest since this environmental fea-

ture could potentially inuence oyster settlement.

Moreover, based on information from local sh-

ermen and divers about the presence of M. gigas

at the mouth of the Río Negro (El Pescadero) and

San Antonio Este port, two sampling surveys were

carried out in April and September 2022, respec-

tively. The objective was to map the distribution of

the species in this region (Figure 1).

Abundance

The abundance of M. gigas was assessed at Los

Pocitos (LP), a site within Bahía Anegada char-

acterized by dense oyster reefs, and two sites in

the Province of Río Negro (El Cóndor EC, and

Piedras Verdes PV), where oyster distribution pri-

marily consisted of dispersed solitary individuals

on abrasion platforms. In each scenario, despite

challenges in comparability across sites, distinct

sampling techniques were employed. While plot

sampling is recommended for high-density popu-

lations, the more efcient linear distance sampling

Hünicken et al.: Pacific cuPPed oyster in nortH Patagonia 5

approach was employed for populations with dis-

persed individuals (Buckland et al. 2005; Miller

et al. 2019).

In LP, the abundance estimation involved ran-

dom 0.25 m2 quadrats. The process began by out-

lining the study area, which comprised the rock

abrasion platform of the lower and middle intertid-

al zone near the shermen’s walkway (7,688 m2).

Google Earth was used to place 35 random spots.

At each point, a quadrat was positioned, and all in-

dividuals within it were collected and subsequent-

ly transported to the laboratory for counting. The

mean number of oysters per quadrat in the site

was estimated using a generalized linear model

performed with the package MASS in R (Venables

and Ripley 2002; Bolker et al. 2009; R Core Team

2020) using the number of oysters per quadrat as

the response variable.

In EC and PV, the density was estimated by

means of linear distance sampling (Buckland et

al. 2005). To achieve this, linear transects were

established along the lower and middle intertidal

zone using a rope with lengths ranging from 20

to 30 m, depending on the irregularities of terrain.

Under good visibility conditions, divers assisted

in covering 50-m subtidal transects (n = 6) in San

Antonio Este port, estimating a surveyed area of

roughly 900 m2. Oysters were searched for along

these transects and all perpendicular distances be-

tween detected individuals and the transects were

recorded. These distances were used to build a de-

tection probability model as a function of the dis-

tance to the observer (detection function). Finally,

with the detection function, the density of oysters

at each site was estimated. The analysis was per-

formed with the package Distance in R (Miller et

al. 2019; R Core Team 2020).

Size structure

Individuals collected during the abundance sur-

veys were brought to the Laboratorio de Biodiver-

sidad y Servicios Ecosistémicos (Escuela de Cien-

cias Marinas, Universidad Nacional del Comahue)

where they were weighed and their length, height,

and width were measured. In addition, the volume

of each individual was recorded using the water

displacement method (Lawrence and Scott 1982).

Given the great variability in the relationships be-

tween linear measurements, i.e. the variable mor-

phology between individuals, the volume was used

as a global measure of size to construct size-fre-

quency histograms (6 ml interval). Subsequently,

size-frequency distributions were decomposed into

modal components using the Bhattacharya method,

which was validated using the Normsep tool within

the Fisat II software application (Pauly and Caddy

1985; Gayanilo et al. 2002). Recruits were dened

as individuals with a height less than 30 mm (Fey

et al. 2010; Lagarde et al. 2017).

Environmental variables

Substrate availability in study sites

The mouth of the Río Negro extends along

12.5 km of the Atlantic coastline of the Argentine

Patagonia (del Río et al. 1991). Sediments from the

Río Negro cliff contribute to a littoral drift towards

the northeast, estimated at 900,000 m3 annually,

accumulating over intertidal sandbanks and result-

ing in an accretion of 50 m per year (del Río et al.

1991; Etcheverría et al. 2006; Vergara Dal Pont

et al. 2017). Thus, ne, well-sorted sands make

up the predominantly granulometry (del Río et al.

1991), resulting in a scarcity of hard substrates for

oyster settlement. The primary substrates available

for oyster settlement in the inner estuary include

gravel in areas of increased current speed and the

roots and stems of Sporobolus (Spartina) alterni-

ora and S. densiora (Alberti et al. 2007; Isaac et

al. 2014). On the western coast of the estuary, the

extensive sand beach of El Cóndor is interrupted

by abrasion platforms composed mainly of ne-

grained sandstones of the Río Negro Formation

(Etcheverría et al. 2006; Mendez et al. 2015). To

the southwest of the Río Negro mouth, erosion

features dominate, characterized by cliff proles

and wave abrasion platforms, primarily composed

Marine and Fishery sciences 38 (2): xxx-xxx (2025)

of ne-grained sandstones alternating with gray-

green sandstones, vulcanogenic deposits, earthy

limestones, and red claystone (Etcheverría et al.

2006; Vergara Dal Pont et al. 2017).

Los Pocitos is situated within Anegada Bay,

characterized by a vast tidal at with a gradual de-

cline towards the east (Etcheverría et al. 2006). Sal-

icornia and Sporobolus patches are scattered across

the ne-grained sandy marsh of the intertidal zone,

while valves and coarse sand buildup on the distal

beach show the effects of storms (Isla and Bertola

2003). The most recent sedimentary layers con-

sist of ne silt-clayey deposits of grayish-brown

hue, dominated by crab burrows (Etcheverría et al.

2006). Occasionally, abrasion platforms formed by

cohesive sediments of the Río Negro Formation are

found (Cuadrado and Gómez 2010).

Physicochemical and biological environment

Seawater environmental parameters that typi-

cally have relevance for the settlement of bivalve

larvae on substrates were selected. To survey these

environmental variables, monthly samples were

collected from three sites (LP, EC, PV) between

October 2020 and November 2021. Temperature

was measured in situ, while for the rest of the pa-

rameters, subsurface samples (~ 15 cm depth, 3 m

from the shore) were collected in plastic bottles

that were immediately transferred to the laborato-

ry in ice-cold coolers. Conductivity and pH were

taken from 200 ml samples with a multiparam-

eter sensor (AtlasScienticTM Hydroponics Kit).

Chlorophyll-a samples were collected by lter-

ing between 300 and 750 ml of water through

47 mm diameter glass ber lters (0.7 µm pore),

which were stored in a freezer at -20 °C and then

extraction with ethanol was performed for 12 h

(Lorenzen 1967). Chlorophyll-a concentration was

determined by spectrophotometry before and af-

ter acidication with 0.1 N hydrochloric acid to

correct for pheopigments. Absorbances were tak-

en with a UV-Vis spectrophotometer (Persee T7S),

while chlorophyll-a concentrations were calculated

following the equation of Marker et al. (1980).

For the estimation of suspended solids, a volume

of 250 ml of seawater was ltered through ber-

glass lters of 47 mm in diameter and 0.7 µm pore

previously mufed at 300 °C and weighed. Then,

they were dried in an oven at 60 °C and the concen-

tration was obtained by applying the gravimetric

method (APHA 2005). The inorganic fraction was

measured after burning the lters at 500 °C in a

mufe for 3 h.

Comparisons between environments were per-

formed by means of a principal component analysis

where all measured environmental variables were

included, previously eliminating variables that

presented a strong correlation (Pearson correlation

coefcient greater than 0.8 or between 0.5 and 0.8

with signicant correlation, α = 0.05).

RESULTS

Distribution

Magallana gigas individuals were found in four

of the ten surveyed sites on the north coast of the

Province of Río Negro. Sites with presence corre-

sponded mostly to the estuary and the area of in-

uence by the Río Negro (El Cóndor, El Pescadero

and Piedras Verdes) (Figure 1), where the species

had been previously reported. The nding in San

Antonio Bay (three individuals in Puerto del Este)

constitutes the rst record of the species within the

San Matías Gulf.

Abundance

Abundances in the Río Negro estuary and its

surrounding area were notably lower, spanning

between ve to six orders of magnitude less than

those observed in LP (Table 1). Variations were

also evident among sites along the invasion front,

with the highest abundance detected in the estuary

(El Pescadero) and the lowest in EC. In San Anto-

nio Bay, only three individuals were located within

Hünicken et al.: Pacific cuPPed oyster in nortH Patagonia 7

a single transect out of the six surveyed, rendering

the density calculation unfeasible.

Size structure

The population at LP exhibited a multimodal

size structure, notably featuring a substantial per-

centage of small individuals (25% of individuals

with a volume of 8.22 ± 1.60 ml, approximately

40 mm in height). Conversely, at PV, three modal

components were discerned, all representing larger

sizes (> 78 ml, approximately 90 mm in height).

In EC, modal components could not be identied

due to the limited number of individuals, but re-

cruits prevailed (74% of individuals with volumes

less than 12 ml, approximately 28 mm in height)

(Table 2).

Environmental variables

Temperatures ranged from 8.1 °C to 24.9 °C (Ta-

ble 3). In EC and PV, the highest temperatures were

recorded in February (EC: 22.9 °C, PV: 21.5 °C),

while the lowest occurred in August (EC: 9 °C, PV:

8.1 °C) (Appendix, Table A1). In LP, temperatures

exhibited a different pattern, with values exceeding

23 °C from November to February and the lowest

temperature occurring in July (9.1 °C). Salinity lev-

els differed as expected among the sites due to the

inuence of the Río Negro. The EC had the lowest

Table 1. Abundances of Magallana gigas in three sites on the northern coast of the Province of Río Negro (El Cóndor, El Pescadero,

Piedras Verdes) and Los Pocitos (Bahía Anegada, Province of Buenos Aires).

Density (ind. m-2)

Site Mean ± SD 95% CI

El Cóndor 1.8 10-3 ± 0.6 10-3 0.9 10-3-3.5 10-3

El Pescadero 9.2 10-2 ± 3.4 10-2 4.1 10-2-20.77 10-2

Piedras Verdes 1.35 10-2 ± 0.35 10-2 0.80 10-2-2.27 10-2

Los Pocitos 104.71 ± 1.35 61.22-201.05

Table 2. Mean volume, standard deviation (SD), number of individuals (N) and separation index (SI) of the modal components

(cohorts) identied for Magallana gigas populations in the studied sites.

Volume (ml)

Site Cohort Mean SD N SI

Los Pocitos C1 8.22 1.60 173 n. a.

C2 23.74 9.78 345 2.73

C3 41.38 13.51 315 2.01

C4 80.63 11.03 18 3.20

Piedras Verdes C1 78.46 17.25 19 n. a.

C2 114.64 7.83 11 2.89

C3 138.68 9.78 5 2.73

Marine and Fishery sciences 38 (2): xxx-xxx (2025)

salinity (25.6 ± 1.5), followed by PV (28.8 ± 1.0),

and LP (29.8 ± 1.2). In terms of suspended solids,

EC exhibited the highest values, with an average

of 400 mg l-1 (ranging from 170 to 1,103 mg l-1).

On the other hand, PV and LP displayed lower and

similar values, both in the range of approximately

200 mg l-1 (Table 3). Chlorophyll-a concentration

also followed this pattern of differences between

sites since they were signicantly correlated with

suspended solids (Pearson correlation coefcient =

0.81; p < 0.001).

Principal component analysis (for which the

variables chlorophyll-a and organic and inorganic

suspended solids were excluded due to their high

correlation with salinity and total suspended solids,

Appendix, Table A2) showed that the rst two com-

ponents together explained 69.2% of the data var-

iability (Table 4). The rst component was mainly

associated with the concentration of suspended

solids and salinity and allowed samples from EC

to be separated from those from the other two sites,

since these samples presented a higher concentra-

tion of total solids and a lower salinity (Figure 2).

The second component was mainly associated with

temperature and comprised the temporal (seasonal)

variability between samples.

DISCUSSION

Our study aimed to update essential population

data for the Pacic cupped oyster, M. gigas, along

the northern coast of Río Negro. Despite its initial

report 19 years ago (2004), its distribution con-

tinues to be restricted to the sector of the mouth

of the Río Negro and its vicinity. Its presence in

San Antonio Bay, 100 km west from the estuary,

suggests a separate introduction event, although

our data cannot denitively rule out natural dis-

persal. Current results show a notable abundance

difference between Bahía Anegada and Río Negro

populations. While BA maintains high densities

(105 ± 2 ind. m-2), RN populations exhibit signif-

Table 3. Annual means ± SD along with the number of monthly samples taken (in parentheses in the same line) of the measured environmental variables at

three locations along the Argentine maritime coast (Los Pocitos, El Cóndor, and Piedras Verdes) from October 2020 to November 2021. Maximum and

minimum values are displayed in parentheses below.

Suspended solids (mg l -1)

Site Temperature (°C) Salinity pH Total Inorganic Organic Chl-a (µg l-1)

Los Pocitos 18.0 ± 5.7 (13) 29.8 ± 1.2 (13) 8.21 ± 0.1 (13) 240 ± 180 (13) 210 ± 157 (13) 30.2 ± 22 (13) 2.25 ± 1.6 (13)

(9.1-24.9) (28.1-32.3) (7.9-8.4) (84-727) (72-633) (12-93) (0.18-5.38)

El Cóndor 16.7 ± 4.4 (13) 25.6 ± 1.5 (13) 8.20 ± 0.0 (13) 451 ± 231 (13) 400 ± 210 (13) 51.6 ± 22. (13) 8.87 ± 7.6 (14)

(9.0-22.9) (22.4-27.9) (8.1-8.4) (170-1,103) (152-1,003) (21-100) (1.27-28.67)

Piedras Verdes 16.4 ± 4.9 (12) 28.8 ± 1.0 (12) 8.21 ± 0.1 (12) 207 ± 70 (12) 182 ± 63 (12) 24.6 ± 8.9 (12) 3.70 ± 3.2 (13)

(8.1-23.3) (27.1-31.1) (7.9-8.4) (92-345) (84-305) (8-40) (0.73-10.07)

Hünicken et al.: Pacific cuPPed oyster in nortH Patagonia 9

icantly lower values (1.8 10-3 ± 0.6 10-3 ind. m-2).

The persistence of the species in the area, charac-

terized by low population densities and few modal

components in the size-frequency distribution, in-

dicates a pattern typical of peripheral populations.

Difculties in establishing the species may be due

to a combination of intrinsic population factors,

such as the Allee effect and the low gamete en-

counter rate, along with external factors such as

unfavorable environmental conditions. Even in

challenging environments, this latency stage has

resulted in signicant population growth in other

Table 4. Principal component analysis carried out from the environmental variables measured in the three study sites (Los Pocitos,

El Cóndor and Piedras Verdes). Correlation values between each independent variable and principal components (PC)

are shown. Additionally, descriptive statistical values for each principal component (standard deviation, proportion of the

variance explained, and accumulated variance) are included.

PC1 PC2 PC3 PC4

Temperature -0.01 0.83 0.30 0.46

pH -0.37 -0.31 0.87 -0.01

Salinity -0.64 0.40 -0.14 -0.64

Total suspended solids 0.67 0.22 0.36 -0.62

Standard deviation 1.25 1.10 0.93 0.60

Variance ratio 0.39 0.30 0.22 0.09

Variance accumulated 0.39 0.69 0.91 1.00

Figure 2. Biplot of principal components analysis carried out with environmental variables surveyed in the three study sites: Los

Pocitos, El Cóndor, and Piedras Verdes. TSS: total suspended solids.

Pc1 (38 9% explained variance).

Pc2 (30 3% explained variance).

-2

-1 Sites

El Cóndor

Los Pocitos

Piedras Verdes

Temperature

Salinity

TSS

Marine and Fishery sciences 38 (2): xxx-xxx (2025)

parts of the world. However, the unpredictable na-

ture of the population trajectory of M. gigas pre-

vents future projections. Therefore, annual moni-

toring of populations is suggested as a useful tool

for assessing population trends in this area.

The establishment of a new population of M. gi-

gas at a given location depends on the successful

completion of several stages: larval production,

larval transport, initial settlement, and subsequent

survival (Dolmer et al. 2014). Environmental con-

ditions, particularly salinity and temperature, play

a crucial role in these stages of establishment, as

does the availability of settlement substrata. Ga-

metogenesis initiates at 10 °C with salinities rang-

ing from 15 to 32, while gamete release occurs at

16 °C between salinities of 23 and 28 (Dolmer et al.

2014). Within this study area, salinity and temper-

ature fall within these ranges (Table 3). As a result,

individuals can complete their reproductive cycle

once established, as previously conrmed in the

area by Roche et al. (2010).

Additionally, food availability (indicated by

chlorophyll-a concentration and the composition

of algal community) inuences the entire process

(Brown 1988; Brown and Hartwick 1988). Chloro-

phyll-a concentrations along the coast of the Prov-

ince of Río Negro were similar or even exceeded

those in Bahía Anegada, and suspended organic

solids (a food source for suspension feeders like

oysters; Brown 1988; Mitchell 2001) were higher,

reecting that food availability is not a limiting

factor. However, high concentrations of suspended

inorganic solids reduce ltration efciency and can

damage the gills (Dutertre et al. 2009; Barille et

al. 2011). Laboratory experiments showed that M.

gigas ceases ltration activity above 196 mg l-1, but

there are wild populations in areas with even higher

solid concentrations, resulting in changes of oys-

ter´s internal organs (gills and palps) and reduced

ltering efciency (Dutertre et al. 2009). In France,

for instance, noticeable effects from suspended sol-

ids occur at concentrations of 156 mg l-1, while

environments with concentrations of 600 mg l-l

are considered highly turbid for oyster farming

(Dutertre et al. 2009). In El Cóndor, mean annual

values exceeding this threshold were found. This

suggests that the concentrations in El Cóndor could

indeed pose challenging conditions for the growth

of oysters.

Other factors such as substrate availability and

hydrological conditions impact oyster larval set-

tlement and growth (Pogoda et al. 2011; Graham

et al. 2020). For instance, Guy and Roberts (2010)

showed that strong currents and wave energy ac-

tion could potentially dilute and ush away oyster

larvae before they have the opportunity to settle on

substrates. Information about hydrodynamic con-

ditions in the area is limited, and this aspect has

generally received little attention concerning oyster

reefs formation. The scarce information available

indicates that the Río Negro estuary and nearby

zone experiences semidiurnal tides with average

amplitudes of 2.84 m and maximum amplitudes

of 4.31 m (SHN 2023), with tide currents rang-

ing from 3.7 to 9.3 km h-1, predominantly owing

in northeast or southwest directions (del Río et

al. 1991; Isla and Bertola 2003). In contrast, the

Bahía Anegada area, specically Los Pocitos site,

has comparatively smaller tidal amplitudes, wave

heights, and periods, measuring 1.61 m, 0.1 m,

and 5 s, respectively (Isla and Bertola 2003; SHN

2023). This suggests that hydrological conditions

in the estuary may pose greater challenges for oys-

ter establishment compared to Bahía Anegada, al-

though more targeted studies would be required to

verify this hypothesis.

The size structure at Los Pocitos displayed sever-

al modal components, with a signicant proportion

of juveniles, suggesting frequent recruitment events

and a periodically replenished population. In con-

trast, few modes were observed at sites close the

southern distribution limit within the Province of

Río Negro, such as in Piedras Verdes, or no modal

components could be distinguished due to a low

number of individuals, such as in El Cóndor. In

Piedras Verdes, collected individuals were mostly

large, indicating the absence of recent successful

recruitments. In contrast, El Cóndor showed a pre-

Hünicken et al.: Pacific cuPPed oyster in nortH Patagonia 11

dominance of small individuals, indicating recent

recruitment. Latency periods have been document-

ed in several regions worldwide (Wehrmann et al.

2000; Robinson et al. 2005; among others) marked

by sporadic recruitment, as seen in Germany with

over 18 years and 6 successful recruitment events

(Holm et al. 2015). When reproductive success is

inconsistent and population trends are unpredicta-

ble, with years of decline followed by occasional

increases, it is referred to peripheral population dy-

namics (Lewis et al. 2022). Outlying populations of

M. gigas at the southern edge of their range are sus-

ceptible to population decline and potential extirpa-

tion due to ongoing changes in climate and habitat,

coupled with a low recruitment rate. Nevertheless,

it is plausible that low detectability poses a common

challenge when researching these peripheral popu-

lations, rendering abundance estimates less precise.

Finally, it is worth noting the record of the pres-

ence of the species in San Antonio Bay, represent-

ing the rst report within San Matías Gulf. Given

the great distance between this site and the closest

ones with the presence of the oyster, and the pre-

vailing eastward currents along San Matías Gulf

northern coast, the arrival of larvae by drift seems

unlikely. This hypothesis is further supported by

multiple factors: the considerable geographical

distance (100 km west from the estuary), the pop-

ulation decline in the estuarine area (evidenced by

its absence from previously occupied sites like El

Espigón and La Lobería), and the low population

density near the river mouth. Sampling was con-

ducted in San Antonio Este port, identied as the

site with the highest likelihood of invasive species

occurrence in San Antonio Bay. This has been at-

tributed to the regional marine trafc, where nu-

merous invasive species have been detected in bal-

last water and through biofouling (Schwindt et al.

2014). Another potential source could be a former

at oyster (O. puelchana) farming operation that

was abandoned around 20 years ago, and where

bags containing illegally introduced M. gigas were

discovered (RG, MN pers observ.). This venture

was located 600 m of the site where oysters were

found, and it is unknown whether it allowed the

establishment of breeding individuals in a nearby

area that would eventually generate recruitment in

San Antonio Bay. In this specic area, six 50-m

transects were covered and only three adhered indi-

viduals were discovered. As a precautionary meas-

ure, it is recommended to conduct yearly moni-

toring to identify the existence of M. gigas and

assess its population trajectory, even if oysters are

not very abundant and their origin is unknown. A

study examining mollusk larvae currents and drift

could be benecial in identifying areas with high

chances of species establishment, enabling efforts

to be concentrated in those regions.

Given the current low abundance and limited

dispersal of M. gigas in the Province of Río Negro,

this early invasion stage presents an opportunity for

implementing preventive control measures. While

complete eradication may be challenging, targeted

removal in key areas, particularly near protected

reserves, could help manage population growth.

Previous studies have shown that early intervention

in non-native oyster populations can be effective

(Guy and Roberts 2010). Based on our ndings of

concentrated populations near El Cóndor and Pie-

dras Verdes, we recommend focusing monitoring

and removal efforts in these areas. Additionally,

considering the speciesʼ proven ability for rapid

growth in other invaded areas, the establishment of

an early warning system through regular surveys

could help detect and respond to sudden population

increases.

ACKNOWLEDGEMENTS

We thank to Tony Brochado and Marcos Brocha-

do Cabarrou for their assistance in diving surveys,

to Raúl Cardón for his support in the initial surveys

at El Condor, and to Guillermo Frías for his collab-

oration in the sampling in El Pescadero. This study

funded by a post-doctoral fellowship awarded to

LH by CONICET and was supported by grants

Marine and Fishery sciences 38 (2): xxx-xxx (2025)

from the Consejo Nacional de Investigaciones

Científcas y Técnicas, Argentina (PIP CONICET

112-201501-00465CO); the Agencia Nacional de

Promoción Cientíca y Tecnológica (ANPCyT),

Argentina (PICT-2018-03107) to MN; and PADI

Foundation (PADI-2021-68836) to LH.

Author contributions

Leandro A. Hünicken: conceptualization; meth-

odology; investigation; formal analysis; writ-

ing-original draft; visualization funding acquisition.

Raúl González: conceptualization; methodology;

investigation; resources; writing-review and edit-

ing; supervision; project administration; funding

acquisition. Dennis Landete: investigation. Maité

A. Barrena: investigation. Juan F. Saad: formal

analysis; resources; writing-review and editing.

Maite A. Narvarte: conceptualization; methodol-

ogy; investigation; resources; writing-review and

editing; supervision; project administration; fund-

ing acquisition.

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APPENDIX

Table A1. Values of environmental variables measured in the three study sites where the presence of the oyster Magallana gigas

was detected. TSS: total suspended solids; TIS: total inorganic solids; TOS: total organic solids; Chl-a: chllorophyl-a.

Site Date Temp pH Salinity TSS TIS TOS Chl-a

(°C) (mg l-1) (mg l-1) (mg l-1) (µg l-1)

Piedras Verdes 11/18/2020 19.2 7.88 ± 0.02 28.73 ± 0.10 92 ± 22 84 ± 22 8 ± 2 1.69 ± 0.68

12/16/2020 18 8.30 ± 0.00 30.08 ± 0.11 260 ± 17 224 ± 19 35 ± 3 2.54 ± 1.03

01/28/2021 20.3 8.09 ± 0.17 28.58 ± 0.06 255 ± 18 223 ± 17 31 ± 2 9.56 ± 2.54

02/24/2021 21.5 8.41 ± 0.00 28.61 ± 0.62 170 ± 8 150 ± 5 20 ± 5 3.65 ± 0.52

03/18/2021 18.5 8.28 ± 0.02 27.17 ± 0.12 345 ± 3 305 ± 21 40 ± 0 10.0 ± 2.22

04/13/2021 17.8 8.19 ± 0.01 28.03 ± 0.02 237 ± 3 208 ± 3 28 ± 3 3.17 ± 0.41

05/14/2021 13.7 8.15 ± 0.04 28.61 ± 0.61 165 ± 5 143 ± 5 21 ± 2 2.72 ± 1.36

06/16/2021 10.6 8.20 ± 0.02 28.98 ± 0.08 146 ± 6 126 ± 4 20 ± 4 2.64 ± 0.65

07/15/2021 9.3 8.19 ± 0.05 29.14 ± 0.04 136 ± 4 120 ± 4 16 ± 0 2.79 ± 0.57

08/12/2021 8.1 8.2 ± 0.02 27.99 ± 2.21 185 ± 8 161 ± 10 24 ± 4 7.25 ± 1.36

09/15/2021 12 8.35 ± 0.04 28.05 ± 0.26 126 ± 8 114 ± 6 12 ± 4 0.90 ± 0.31

10/27/2021 17.3 8.27 ± 0.02 29.50 ± 0.25 273 ± 5 253 ± 5 20 ± 0 0.72 ± 0.31

11/26/2021 23.3 8.36 ± 0.00 31.1 ± 0.36 220 ± 43 190 ± 43 30 ± 0 0.36 ± 0.31

El Condor 10/21/2020 14.7 8.26 ± 0.00 24.66 ± 0.49 386 ± 58 343 ± 55 43 ± 3 10.7 ± 2.23

11/18/2020 18.5 8.09 ± 0.04 25.46 ± 0.11 1,103 ± 102 1,003 ± 92 100 ± 1 28.6 ± 1.74

12/16/2020 18.1 8.38 ± 0.00 26.58 ± 0.10 356 ± 5 306 ± 5 50 ± 1 17.9 ± 1.09

01/28/2021 20.09 8.12 ± 0.01 27.04 ± 0.01 416 ± 23 360 ± 17 56 ± 5 8.52 ± 1.66

02/24/2021 22.9 8.32 ± 0.00 27.01 ± 0.19 170 ± 5 151 ± 2 18 ± 2 3.55 ± 1.08

03/18/2021 19.2 8.13 ± 0.01 22.4 ± 0.09 600 ± 45 523 ± 40 76 ± 5 15.6 ± 2.17

04/13/2021 19.6 8.07 ± 0.01 23.84 ± 0.08 526 ± 35 473 ± 30 53 ± 5 11.7 ± 1.79

05/14/2021 15.3 8.20 ± 0.02 24.46 ± 0.36 410 ± 51 363 ± 40 46 ± 1 3.47 ± 1.45

06/16/2021 11.6 8.25 ± 0.02 24.47 ± 0.19 376 ± 11 330 ± 10 46 ± 5 3.62 ± 0.51

07/15/2021 9.1 8.20 ± 0.01 27.86 ± 0.01 226 ± 15 205 ± 16 21 ± 2 2.98 ± 0.33

08/12/2021 9 8.26 ± 0.00 26.79 ± 0.07 576 ± 12 504 ± 10 72 ± 2 8.76 ± 0.26

09/15/2021 14 8.26 ± 0.01 26.09 ± 0.04 370 ± 20 326 ± 25 43 ± 5 4.66 ± 1.94

10/27/2021 18.3 8.25 ± 0.03 26.33 ± 0.09 313 ± 5 280 ± 10 33 ± 5 1.27 ± 0.41

11/26/2021 21.3 8.10 ± 0.00 26.7 ± 0.09 410 ± 10 356 ± 15 53 ± 5 2.54 ± 0.62

Los Pocitos 10/21/2020 17.2 8.19 ± 0.01 29.81 ± 0.03 151 ± 5 128 ± 6 22 ± 1 1.63 ± 0.47

11/18/2020 24.2 7.88 ± 0.03 30.46 ± 0.06 144 ± 8 128 ± 8 16 ± 0 2.99 ± 1.69

12/16/2020 23.1 8.33 ± 0.00 30.21 ± 0.05 390 ± 26 340 ± 26 50 ± 0 3.74 ± 2.12

01/28/2021 23.1 8.38 ± 0.01 32.28 ± 0.02 84 ± 1 71 ± 1 12 ± 2 2.79 ± 1.90

02/24/2021 22.9 8.06 ± 0.02 30.73 ± 0.02 726 ± 37 633 ± 41 93 ± 5 3.62 ± 3.27

03/18/2021 19.5 8.19 ± 0.01 30.42 ± 0.65 373 ± 15 330 ± 10 43 ± 5 5.38 ± 0.53

Hünicken et al.: Pacific cuPPed oyster in nortH Patagonia 17

Table A1. Continued

Site Date Temp pH Salinity TSS TIS TOS Chl-a

(°C) (mg l-1) (mg l-1) (mg l-1) (µg l-1)

04/13/2021 20.3 8.15 ± 0.01 29.74 ± 0.12 253 ± 17 222 ± 10 31 ± 7 3.90 ± 0.56

05/14/2021 14.8 8.20 ± 0.03 30.06 ± 0.02 113 ± 2 100 ± 4 13 ± 2 0.48 ± 0.83

06/16/2021 9.9 8.25 ± 0.02 28.2 ± 0.07 124 ± 4 104 ± 8 20 ± 4 0.78 ± 0.45

07/15/2021 9.1 8.24 ± 0.02 28.42 ± 0.14 101 ± 2 89 ± 2 12 ± 0 0.24 ± 0.27

08/12/2021 9.4 8.23 ± 0.00 28.09 ± 0.06 200 ± 10 178 ± 10 21 ± 2 2.72 ± 0.81

09/15/2021 16 8.32 ± 0.02 28.67 ± 0.00 125 ± 2 110 ± 2 14 ± 2 0.18 ± 0.18

10/27/2021 24.9 8.37 ± 0.01 31.06 ± 0.01 340 ± 17 296 ± 5 43 ± 1 0.82

Table A2. Correlation matrix showing relationships between physicochemical parameters and chlorophyll-a in the three study

sites where the presence of the oyster Magallana gigas was detected (Los Pocitos, El Cóndor and Piedras Verdes). Values

represent Pearson’s correlation coefcients (r). NA: indicates no correlation was calculated. TSS: total suspended solids;

TIS: total inorganic solids; TOS: total organic solids; Chl-a: chlorophyll-a.

Temperature pH Salinity TSS TIS TOS % TIS % TOS Chl-a

Temperature 1 NA NA NA NA NA NA NA NA

pH NA 1 0.12 -0.2 -0.2 -0.16 -0.24 0.24 -0.15

Salinity NA 0.12 1 -0.47 -0.47 -0.43 -0.23 0.23 -0.58

TSS NA -0.2 -0.47 1 1 0.96 0.16 -0.16 0.81

ISS NA -0.2 -0.47 1 1 0.95 0.18 -0.18 0.82

OSS NA -0.16 -0.43 0.96 0.95 1 -0.05 0.05 0.74

% ISS NA -0.24 -0.23 0.16 0.18 -0.05 1 -1 0.15

% OSS NA 0.24 0.23 -0.16 -0.18 0.05 -1 1 -0.15

Chl-a NA -0.15 -0.58 0.81 0.82 0.74 0.15 -0.15 1