MARINE AND FISHERY SCIENCES 37 (1): 5-25 (2024)

https://doi.org/10.47193/mafis.3712024010105

ABSTRACT. This article addresses the history of a resident population of bull sharks (Carcharhi-

nus leucas) in an isolated stagnant body of water in subtropical Australia. From 1996 to 2013, six

bull sharks were landlocked in a golf course lake near Brisbane. The adjacent Logan and Albert

rivers trapped sharks due to major floodings. When floodwaters receded, these sharks remained in

the lake, which is normally isolated from the riverʼs main channel. While this event was extensively

reported in the media and recieved much public attention, it has not been investigated in depth, yet

it provides an opportunity for insights into the tolerance of bull sharks to low salinity habitats and

euryhalinity in this species. Currently, information on the extent of the bull sharkʼs capability to

endure low salinity conditions and its longevity in these environments is scarce. The case reported

here provides information on the occurrence of bull sharks for 17 years, which represents the longest

uninterrupted duration in a low salinity environment that ever has been recorded in this species. Bull

sharks arrived first in the lake as juveniles but through time, they have reached maturity. This occur-

rence presents not just another ordinary bull shark record from a low salinity environment but

instead a record of physiological and scientific importance. Therefore, details of the residency of C.

leucas in an Australian golf course lake are reported here.

Key words: Australia, Carcharhinidae, elasmobranchs, euryhalinity, floods, low salinity habitats.

¿Quién es el pez más grande del estanque? La historia de los tiburones toro (Carcharhinus leu-

cas) en un lago de un campo de golf australiano, con consideraciones sobre la longevidad de

esta especie en hábitats de baja salinidad

RESUMEN. Este artículo aborda la historia de una población residente de tiburones toro (Car-

charhinus leucas) en un cuerpo de agua aislada en la Australia subtropical. De 1996 a 2013, seis

tiburones toro quedaron encerrados en un lago de un campo de golf cerca de Brisbane. Los ríos

adyacentes, Logan y Albert, atraparon a los tiburones debido a las grandes inundaciones. Cuando

las aguas de la inundación retrocedieron, estos tiburones permanecieron en el lago, el cual normal-

mente está aislado del canal principal del río. Si bien este evento se informó ampliamente en los

medios y recibió mucha atención pública, no se ha investigado en profundidad, pero brinda una

oportunidad para comprender la tolerancia de los tiburones toro a los hábitats de baja salinidad y

eurihalinidad de la especie. Actualmente, la información sobre la capacidad del tiburón toro para

soportar condiciones de baja salinidad y su longevidad en estos ambientes es escasa. El caso repor-

tado aquí proporciona información sobre la ocurrencia de tiburones toro durante 17 años, lo que

representa la duración ininterrumpida más larga en un ambiente de baja salinidad que jamás se

haya registrado en esta especie. Los tiburones toro llegaron primero al lago como juveniles, pero

con el tiempo alcanzaron la madurez. Esta ocurrencia presenta no solo otro registro de tiburón toro

*Correspondence:

peter.gausmann@rub.de

Received: 16 May 2023

Accepted: 8 August 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

ORIGINAL RESEARCH

Whoʼs the biggest fish in the pond? The story of bull sharks (Carcharhinus

leucas) in an Australian golf course lake, with deliberations on this speciesʼ

longevity in low salinity habitats

PETER GAUSMANN*

Working Group Biogeography and Landscape Ecology, Department of Geography, Faculty of Geosciences, Ruhr University Bochum,

Universitätsstraße 150, 44801 - Bochum, Germany. ORCID Peter Gausmann https://orcid.org/0000-0003-3577-7349

INTRODUCTION

The bull shark (Carcharhinus leucas Valenci-

ennes, 1839) is known for penetrating far, and for

prolonged periods, into freshwater bodies in trop-

ical, subtropical, and warm-temperate regions

around the globe (Boesemann 1964; Compagno

1984; Gausmann 2021). It is a highly efficient

osmoregulator that can travel between freshwater

and seawater and respond to sudden changes in

salinity with minimal metabolic costs (Pillans et

al. 2005, 2008). This euryhaline species has been

found circumglobally in freshwater environ-

ments, with reported large-scale migrations in

major streams, covering thousands of kilometres,

such as in the Amazon, Mississippi, and Zambezi

rivers (Thorson 1972; Bass et al. 1973; Thomer-

son et al. 1977).

Carcharhinus leucas rely on low salinity habi-

tats during early life stages (Heupel et al. 2010).

These habitats, such as rivers and estuaries, are

crucial nursery areas (Heupel and Simpfendorfer

2011). Young bull sharks spend up to five years in

these low salinity environments (Pillans 2006;

Heupel and Simpfendorfer 2008; Matich and Hei-

thaus 2012), where they are exposed to lower lev-

els of predation by larger sharks (Heupel and

Simpfendorfer 2011). In this context, Curtis et al.

(2011) found out for Floridaʼs Indian River

Lagoon that juvenile bull sharks even remain in

this nursery until they have reached an age of nine

years before they make the full transition to

marine offshore habitats, but this may be excep-

tional and restricted to this particular locality.

Investigations have revealed that C. leucas uses

low salinity habitats across its range and that its

distribution is limited by their availability (Gaus-

mann 2021), emphasizing the importance of these

habitats for the life cycle and occurrence of this

shark. As a result of the worldwide altering of

coastal inshore habitats, as well as estuarine and

riverine systems, in combination with fishing

pressures throughout its range (OʼConnell et al.

2007; Kyne et al. 2012; Seidu et al. 2022), the bull

shark is assessed as Vulnerable (VU) on a global

scale in the IUCN Red List (Rigby et al. 2021).

Out of all elasmobranchs (sharks, skates, rays,

and sawfish), only ~5% of the species can live in

brackish and freshwater environments (Luciflora

et al. 2015; Grant et al. 2019; Kyne and Luciflora

2022), with a minority considered to live perma-

nently in the latter. Within the elasmobranchs,

only freshwater stingrays of the Family Pota-

motrygonidae Garman, 1877 and whiptail

stingrays of the Family Dasyatidae D. S. Jordan,

1888 contain obligate freshwater species that are

confined to freshwater environments of South

America, Africa, and southeast Asia. Because the

seawater/freshwater ecocline provides one of the

sharpest ecological boundaries in nature (Odum

1971), the residency of euryhaline elasmobranchs

in low salinity habitats needs specific anatomical

and physiological adaptions to overcome the

changing salinities. Only a few elasmobranch

species are euryhaline, capable of moving freely

from one habitat to the other due to evolved

osmoregulation, which is achieved through the

control and integration of various organs (rectal

gland, kidney, liver, and gills) in response to

changes in environmental salinity (Reilly et al.

2011). Rectal glands of bull shark individuals that

spend long periods in either freshwater or marine

environments show significant differences

because these glands are no longer functional in

freshwater environments. Oguri (1964) reported

that rectal glands from bull sharks found in fresh-

6MARINE AND FISHERY SCIENCES 37 (1): 5-25 (2024)

de un ambiente de baja salinidad, sino un registro de importancia fisiológica y científica. Por lo tanto, aquí se informan los detalles de

la residencia de C. leucas en un lago de un campo de golf australiano.

Palabras clave: Australia, Carcharhinidae, elasmobranqios, eurihalinidad, inundaciones, hábitats de baja salinidad.

water environments are smaller and show regres-

sive changes in comparison with individuals from

a marine habitat. This is because the rectal gland

of C. leucas undergoes atrophy during prolonged

exposure to freshwater (Oguri 1964).

According to Ballantyne and Fraser (2013),

truly euryhaline species, i.e. species living both in

seawater and freshwater for prolonged periods, are

the rarest within elasmobranchs. Only a few of the

extant elasmobranch species are truly euryhaline,

such as bull shark, Atlantic stingray (Hypanus

sabinus Lesueur, 1824), and common sawfish,

(Pristis pristis Linnaeus, 1758). More recently,

Grant et al. (2019) reviewed the use of non-marine

habitats by elasmobranchs and developed a classi-

fication based on the importance of freshwater

habitats for the life history of each species. From

the sharks, only 4 species of the Family Car-

charhinidae can be considered truly euryhaline: 3

species of river sharks (Glyphis Agassiz, 1843)

and C. leucas, based on their timing and duration

of freshwater (£5% salinity) and/or estuarine (>5

to £30% salinity) habitat use. Due to its ability to

occupy habitats in both freshwater and marine

environments and to rapidly switch between them,

the bull shark has been the subject of numerous

physiological studies (e.g. Thorson and Gerst

1972; Thorson et al. 1973; Pillans and Franklin

2004; Anderson et al. 2005; Pillans 2006; Pillans

et al. 2005, 2006, 2008, 2020; Reilly et al. 2011).

Although physiology related to freshwater toler-

ance and euryhalinity of C. leucas have been stud-

ied extensively, the osmoregulation in this species

and its capability to reside in freshwater is not

completely understood yet.

In Australia, the bull shark is distributed contin-

uously along Australiaʼs coastline from the Collie

River (-33.30° S) ~100 km south of Perth in the

southwest farther to Wollongong (-34.32° S),

~50 km south of Sydney in the southeast (Potter

et al. 2000; West 2011). Further, bull sharks have

been reported from numerous rivers and freshwa-

ter systems in Australia (Thorburn and Rowland

2008; Tillett et al. 2012; Morgan et al. 2014;

Gausmann 2021). Natural disasters have led to

findings of C. leucas in inland waters of Aus-

tralia. After the tropical Cyclone ‘Debbie’ in

northeastern Australia in March 2017, C. leucas

individuals were washed out of the Burdekin

River onto a nearby street (Clamann 2017;

Sandeman 2017). One individual was seen swim-

ming in the flooded streets of Brisbane (Queens-

land, Australia) during the Queensland floods in

2010-2011 (BBC 2011). Several bull sharks were

sighted in one of the main streets of Goodna

(Queensland, Australia) shortly after the peak of

the Brisbane River flood in January 2011 (Garry

2011). During floodwater events in subtropical

Queensland, immature bull sharks were occasion-

ally sighted in overflooded stretches of land, as it

has been reported for recent floods in 2022 in

Maryborough adjacent to the Mary River, some

300 km north of Brisbane (Pizzirani 2022), and

also for the Logan River during the 2017 floods

(OʼBrien 2017).

Carcharhinus leucas was first reported in the

Logan/Albert river system by Thomson (1957)

and examined more extensively later by Pillans et

al. (2020), revealing that neonate and juvenile C.

leucas (74-102 cm total length, TL) remained

within a narrow band of salinity (6-10%)

throughout their tracking period (30 months).

These authors found out that sharks have trav-

elled at least 52 km (Logan) and 30 km (Albert)

upstream, respectively. Werry et al. (2011) docu-

mented the marine-freshwater transition of C.

leucas in the Moreton Bay Estuary system by

acoustically tagging bull sharks and showed that

it was also used by large juveniles (125-

160 cm TL) but also subadults and adults (175-

192 cm TL). Additional information about C. leu-

cas records in the Logan River derived from the

‘Global Shark Attack File’ reported on a fatal

attack in 1903 by a shark of estimated 2.7 m TL

on a swimmer at Loganholme, ~25 km upstream

(GSAF 2022). Even though the involved species

could not be identified, the habitat suggests that a

bull shark was the culprit. This may indicate that,

GAUSMANN: BULL SHARKS IN AN AUSTRALIAN GOLF COURSE LAKE

at least occasionally, not only immature but also

adult sharks make use of the river.

Evidence of reproductive philopatry in bull

sharks has been provided for rivers in northern

Australia (Tillett et al. 2012). Currently, it is not

known if the Logan and Albert rivers fulfill the

criteria of a nursery area according to Heupel et

al. (2007). However, Pillans et al. (2020) tagged

36 neonate and juvenile bull sharks during their

study in the Logan River between March 2013

and January 2014, indicating that a well-devel-

oped immature bull shark population inhabits

these rivers (compare Pillans 2006).

In the mid-1990s, several bull sharks were

trapped in the lake at Carbrook golf club, Logan

City, near Brisbane, because of major floods from

the Logan and Albert rivers, resulting in the

sharks being trapped in the lake when the flood-

waters receded. This situation has been widely

reported by the media and received much public

attention. The event of the involuntary captivity

of several bull sharks provided science with the

rare opportunity to measure the time that this

species can survive in a low-salinity environ-

ment, since until now, information on the

longevity of C. leucas in these environments is

limited and has only been investigated in a few

locations (e.g. Lake Nicaragua by Thorson 1971).

This unique opportunity now allowed scientists to

gain insight into the extent of euryhalinity in C.

leucas, and this case includes several aspects and

findings on the ecology of this species that could

not be studied in depth previously due to a lack of

occasions.

The case reported herein of an isolated bull

shark population that got trapped in a freshwater

lake over a timespan of more than one and a half

decades leads to the following deliberations and

key questions: (i) is the residential time of C. leu-

cas in a nearly freshwater environment with

approximately <5% salinity unlimited, and how

long are bull sharks able to survive in? (ii) are

food resources in this isolated freshwater body

sufficient for energetic requirements of resident

bull sharks? (iii) do trapped bull sharks have

reach maturity in captivity and is there evidence

of reproduction?

MATERIALS AND METHODS

Study site

The Carbrook golf club (-27.68° S-153.24° E)

is located at Carbrook, a suburb of the City of

Logan, southeast of Brisbane, at the junction of

the Logan (main estuary) and Albert (tributary)

rivers in subtropical southeast Queensland, Aus-

tralia (Figure 1 A-C). Southeast Queensland

experiences a subtropical climate with warm, wet

summers and relatively cold, dry winters (Kemp

et al. 2016). In summer, intense storms and occa-

sionally prolonged rainfall are generated by trop-

ical depressions or southeast trade winds. Flood-

ing events are frequent in the area and do occur

regularly at intervals of several years to decades

(Queensland Government 2015).

The concerned Logan/Albert river system

(-27.69° S-153.23° E) has a total catchment area

of 3,875 km2and lies in the southeast corner of

Queensland. Major flooding is experienced in

both rural and urban areas of the catchment (Mid-

delmann et al. 2000). Therefore, Carbrookʼs and

the city of Logan’s subtropical climate combined

with their situation in the floodplain lowlands of

Albert and Logan rivers cause their vulnerability

to natural disasters, such as flooding (Figure 1 D

and 1 E). Both Logan and Albert rivers are tidal

influenced, which extend about 60 km upstream

in both rivers from the mouth of the latter

(Matveev and Steven 2014; Pillans et al. 2020).

Thus their confluence, which is located about

14 km upstream from the Logan River inlet at

Eagleby, is well within the tidally influenced

zone, although following events of heavy rainfall

in the Logan/Albert river basin, salinity can

decrease significantly and the estuary becomes

8MARINE AND FISHERY SCIENCES 37 (1): 5-25 (2024)

fresh at the entrance (Table 1). The Logan River

is 191 km long and largely unregulated (Lonera-

gan and Bunn 1999), with the Albert River as its

largest tributary of 102 km in length.

Water temperature in the Logan River during

the austral summer is most of the time 26-28 °C,

decreasing to 16 °C in winter (Pillans et al. 2020).

No data are available regarding water parameters

for the golf course lake mentioned in this study

and no investigations on these were conducted,

but it can be assumed that the water has a slight

salinity (<5%) and provide oligohaline condi-

tions due to groundwater flow and connectivity

that corresponds to the adjacent Logan River. Pre-

sumably, the salinity in the lake is much lower

than in the adjacent river due to the cut-off from

the tidal influence and dilution by precipitation.

Both Logan River and golf course lake show

remarkable differences in turbidity (Figure 1 F).

The vast lake at Carbrook golf club (Figure 1

A) that was inhabited by several bull sharks is

undisturbed and, despite of being artificial, shows

characteristics of a natural water body. It is locat-

ed northeast across from the confluence of the

GAUSMANN: BULL SHARKS IN AN AUSTRALIAN GOLF COURSE LAKE

DE F

Figure 1. A) Aerial view of the Carbrook golf course lake located nearby the junction of the Logan and Albert rivers. B) Location

of Carbrook on the Australian continent. C) Situation of the Carbrook golf club inside the Brisbane Metropolitan Area

(Queensland, Southeast Australia). Data source 1A-1C: © OpenStreetMap and Bing Maps 2022. D) Historical photo of

the inundated Carbrook golf club course during the flood event in May 1996. E) Aerial view on the inundated course of

Carbrook golf club during the current flood event in February 2022 when the Logan River burst its banks. The view

direction is from north to south. (F) Aerial view of the Carbrook golf course lake and the adjacent Logan River (similar

location as the left figure) under normal river conditions (average discharge). Data source 1D-1F: © Carbrook golf club.

Albert and Logan rivers at a distance of 0.5 km to

the junction (Figure 1 A; -27.69° S-153.24° E).

This lake is situated 14 km upstream from the sea

(Moreton Bay, southwestern Pacific Ocean). It is

~700 m long and ~380 m wide at its widest

extension and is separated from the Logan River

by a ~75-100 m land barrier that is impassable

for fish during periods without floodwaters. The

lake covers an area of ~20 ha with a maximum

water depth of 15 m. Its origin derives from sand

mining activities of a sand mining plant during

the late 1970s, at the time the Carbrook golf club

was established beyond the end of mining activi-

ties (Carbrook Golf Club 2022).

Important flood events

For a complete understanding of the circum-

stances of how bull sharks found their way into

the golf course lake, a short chronological sum-

mary of several flood events that have affected

the Brisbane metropolitan area and surroundings

and which have provided sharks with the oppor-

tunity to invade the lake is given (Table 2).

Severe floods in Queensland on December

2010/January 2011 and March 2017 only slightly

affected the Logan/Albert river system. They

were not leading to the flooding of the land-

bridge, which divides the river and the golf

course lake, although the latter inundated large

parts of Carbrooks golf course (21 October 2022

pers. comm. S Wagstaff).

Data acquisition

As the occurrence of bull sharks in the lake of

Carbrook golf club was reported extensively in

the media but not in scientific studies, these

media reports present an essential part of the ref-

erences that have been investigated. Media refer-

ences investigated included video material pro-

vided by Carbrook golf club staff (Wagstaff

2011a, 2011b), online newspaper articles, and

video footage. An evaluation and assessment

regarding the plausibility and reliability of these

media contents was conducted following the

methodological approach of Schemer et al.

(2008) and Kim et al. (2011). Additionally, an

interview was conducted with the general manag-

er of Carbrook golf club to obtain firsthand infor-

mation on the bull shark population at this site

firsthand (see Appendix for the catalog of ques-

tions). Some additional questions, which arose

later in the course of the investigation were clari-

fied via email correspondence between the golf

club manager and the author.

Calculation of energetic requirements

For extrapolation of energetic requirements of

landlocked bull sharks, a calculation for an esti-

mated bull shark population size of six shark indi-

viduals was conducted, whereby the calculation

is largely based on similar studies that have

already been carried out on this topic by Schmid

10 MARINE AND FISHERY SCIENCES 37 (1): 5-25 (2024)

Table 1. Physiochemical water parameters of the Logan and Albert rivers averaged over the year according to Matveev and

Steven (2014). NTU =Nephelometric Turbidity Unit. In Brackets: minimum and maximum values for each parameter

in the Logan River according to Pillans et al. (2020).

Parameter Logan River Albert River

Salinity (%) 13.63 (1-20) 7.27

Dissolved oxygen (mg l-1) 5.66 (3-10) 5.78

Temperature (°C) 23.20 (14-29.5) 23.4

Turbidity (NTU) 46.40 (0-500) 61.4

and Murru (1994) and Brunnschweiler et al.

(2018). Information on the energetic content and

size calculations of possible prey fish of C. leucas

was largely obtained from literature and FishBase

(Froese and Pauly 2023).

RESULTS AND DISCUSSION

Carcharhinus leucas were observed in a low

salinity environment in the vicinity of Brisbane

for 17 years (Figure 2 A). Sharks got into the lake

after the golf course was inundated three times

over a five-year period between 1991 and 1996,

due to serious floods of the nearby Logan and

Albert rivers. Although the first presence of bull

sharks in the lake was reported in 1996, the date

of their arrival, the number present, and the exact

time of their residency in the lake is unknown.

However, it has been reported by the media that

they were regularly seen in the lake since the late

1990s and the early 2000s.

Reported sightings are consistent with bull

sharks due to their description by Garrick (1982)

and Compagno (1984) such as very small eyes, a

short blunt snout, coloration, and a first dorsal fin

approximately three times as high as the second

dorsal fin. Together with an occurrence in a near-

ly freshwater habitat and occurrences of bull

sharks in the nearby Logan and Albert rivers sup-

port these observations. Given the date of their

first sighting in the golf course lake, it is most

likely that these bull sharks were trapped follow-

ing the flood of the Logan and Albert rivers in

1996 after the major Brisbane floods when these

rivers burst their banks (Boswell 2013; Ward

2022) (Table 2). When the floodwaters receded,

sharks were trapped in the lake.

Sharks have become, and still are, the golf

club’s mascot and flagship. This small population

of bull sharks in the golf club lake was a good dis-

play object for the interested public and most

likely contributed to the understanding of sharks

in general and of supposedly dangerous species in

particular. As there was no immediate risk from

sharks to golf course users, the golf club manage-

ment not only tolerated but welcomed their pres-

ence in the lake. However, for preventing shark-

human incidents, the managers installed warning

GAUSMANN: BULL SHARKS IN AN AUSTRALIAN GOLF COURSE LAKE

Table 2. Chronological overview of flood events during the last three decades affecting the Brisbane metropolitan area including

Logan City and Carbrook.

No Date Affected Rainfall Severity Land-bridge References

area (l m-2) flooding

1 7-10th Logan/Albert 200 Severe Yes AGBM (1991, 2022a,

February 1991 River catchment 2022b)

2 April 1995 Upper reaches of ? Moderate Yes 9 November 2022 pers.

Logan River to comm. S Wagstaff

Carbrook

3 1st-7th Brisbane 600 Very severe Yes AGBM (2022b, 2022c)

May 1996 metropolitan area

4 26th-28th Brisbane River >1,000 Very severe Yes AGBM (2013, 2022b);

January 2013 catchment Pillans et al. (2020)

5 25th-28th Greater 675-1,000 Very severe Yes AGBM (2022d)

February 2022 Brisbane area

signs around the lake to inform the golf players

about the potential risk (Figure 2 B).

History and vitality of the lakeʼs shark popula-

tion

Five flood events (Table 2) have provided bull

sharks the possibility to invade the golf course

lake via the adjacent Logan River. Conversely,

flood events can also be assumed to have borne

the possibility for sharks who reside in the lake to

escape from the lake back into the Logan River.

However, what is certain is the fact that between

1996 and 2013, no significant floods affected the

Carbrook golf club, and in this non-flooding peri-

od of 17 years, the fish-impassable landbridge

was not inundated. Consequently, during this

period, sharks had no opportunity to enter or

escape the lake. Therefore, a complete isolation

of the bull shark population remaining in the lake

can be stated between 1996 and 2013.

It was not possible to investigate precisely how

many sharks and in what year they arrived in the

lake. Similarly, it also cannot be said with certain-

ty whether sharks subsequently entered the lake

during the new millennium flood events in 2013

and 2022, or vice versa, escaping back into the

Logan River. Since individuals were not marked,

it is not possible to know how many individuals

were added during the respective flood events or

migrated out of the lake again. Sharks were seen

for the last time in the lake in 2015 (21 October

2022 pers. comm. S Wagstaff).

Sharks in the lake were juvenile bull sharks at

the time of their arrival, similar in size to those

previously reported by Pillians (2006) and Pillans

et al. (2020) from the adjacent Logan and Albert

rivers. Through time, there was evidence that

sharks within the lake increased in size and were

reported between 1.8 m and 3 m TL in 2013

(Boswell 2013), which is consistent with previ-

ously reported mature Indo-Pacific bull sharks

(Cliff and Dudley 1991; Wintner et al. 2002; Fig-

ure 2 A and 2 C). Based on the information deriv-

12 MARINE AND FISHERY SCIENCES 37 (1): 5-25 (2024)

Figure 2. A) Carbrookʼs golf club staff with a Carcharhinus

leucas in the lake of the golf course in Carbrook,

Queensland, in October 2011. Data source 2A: ©

Jodie Richter/Newspix. B) A warning sign informed

the golf players about the potential risk of bull sharks

in the Carbrook golf club lake. C) A Carcharhinus

leucas close to the shore in 2012. Data source 2B-2C:

ing from the guarantor, six shark individuals

simultaneously inhabited the lake (21 October

2022 pers. comm. S Wagstaff), and this informa-

tion can be valued as trustworthy and reliable.

The author found no reliable information and no

hints that bull sharks in Carbrook golf club lake

have reproduced. Bull sharks rarely breed in

rivers and lakes (Thorson 1982), with estuaries

and river mouths considered to be nurseries for

this species (Heupel and Simpfendorfer 2011;

Gausmann 2021).

Boswell (2013) reported sightings of bull

sharks in the lake in 2013 and confirmed that they

survived major floodings the same year, alleviat-

ing the fears of Carbrook golf club officials that

sharks may have escaped through the floodwa-

ters, suffered or died as a result of the storm

because they were not seen in there after the flood

for some time (Social Golf Australia 2013). How-

ever, following this flood event, the number of

bull sharks in the lake apparently decreased as it

was reported by the guarantor. This observation

of decreasing shark numbers is leading to two

reasonable explanations: either some individuals

must have escaped successfully from the lake into

the nearby river during the flood in 2013, or some

have died, maybe by suffering from the floodwa-

ters. Assuming that affected sharks were the same

individuals that entered the lake in 1996, they

could have reached their maturity age in 2013, 17

years after their arrival in the lake. According to

the guarantor, only one shark of an estimated

2.7 m TL was ever observed floating dead in the

lake over the years, without clearly identifiable

reasons for its demise and no visible external

influences (21 October 2022 pers. comm. S

Wagstaff). Possibly, hunger or diseases are possi-

ble causes here, although suffering from a major

loss of sodium chloride cannot be excluded com-

pletely. The golf club manager recovered the

shark carcass, but an autopsy was not conducted.

It can be not ruled out that more than one of the

landlocked bull shark individuals died in the lake

in 2013 shortly after the flood or that dead shark

corpses have sunken to the ground of the lake

unobserved, or that dead shark bodies were con-

sumed by other sharks that inhabited the lake.

Besides environmental changes and in contrast to

careful efforts of golf club managers and mem-

bers on the treatment of sharks, illegal fishing

operations in the lake provided a further threat to

this small bull shark population. According to the

guarantor, illegal recreational fishing activities in

the lake have verifiably caused the death of at

least one other bull shark over the years (21 Octo-

ber 2022 pers. comm. S Wagstaff).

Food resources and energetic requirements

Bull sharks are opportunistic feeders with a

broad diet that undergoes a shift with increasing

size (Cliff and Dudley 1991; Daly et al. 2013),

including bony fish, elasmobranchs, crustaceans,

birds, and large prey items such as sea mammals

and sea turtles. However, they are assessed as pri-

marily piscivorous (Compagno 1984; Estupiñán-

Montaño et al. 2017; Cottrant et al. 2021). In addi-

tion to sharks, access to the golf club lake was pro-

vided to other fish that normally inhabit the Logan

and Albert rivers, which also entered during

floods. According to information from the guaran-

tor, there is a heap of fish species in the lake, and

plenty of potential prey species for bull sharks,

such as flathead grey mullet (Mugil cephalus Lin-

naeus, 1758), yellowfin bream (Acanthopagrus

australis Günther, 1859), Indo-Pacific tarpon

(Megalops cyprinoides Broussonet, 1782), and

mangrove red snapper (Lutjanus argentimacula-

tus Forsskål, 1775). All these fish are euryhaline

and capable to survive in low salinity habitats

(Froese and Pauly 2023). Some of these fish were

reported to be between 40-50 cm TL (TAASFA

2011). Although the golf course lake is well-

stocked with fish, the golf club staff fed sharks

occasionally with meat (chicken, pork) to encour-

age sharks to come near the surface (Wagstaff

2011b). Sharks within the lake may not meet their

energy requirements with this additional feeding.

GAUSMANN: BULL SHARKS IN AN AUSTRALIAN GOLF COURSE LAKE

To estimate the daily energy requirements of

bull sharks in the lake for an example calculation,

the author adopted routine metabolic rate estimates

from previous studies of captive bull sharks by

Schmid and Murru (1994) that incorporated meas-

ured feeding rates into a bioenergetics model.

Schmid and Murru (1994) informed on the ener-

getic requirements of bull sharks held in captivity

at a water temperature of 24 °C, meaning a food

consumption of 3.4% body mass per week. In this

context, Brunnschweiler et al. (2018) informed

that requirements for wild living bull sharks stud-

ied in Fiji with average water temperatures of

27 °C were lower than in the previous study with

2.7% body mass per week. In detail, Schmid and

Murru (1994) reported a routine metabolic rate of

5.7 kcal kg-1 d-1 for a 78 kg individual C. leucas

held at a water temperature of 24 °C. Overall, this

means a daily energy requirement for a 78 kg indi-

vidual of 444.6 kcal d-1. Brunnschweiler et al.

(2018) calculated the daily energy requirement for

200 kg free-living bull sharks at a water tempera-

ture of 27 °C to be 1,087 kcal d-1 to maintain

growth rates similar to those of captive animals in

the previous study from Schmid and Murru

(1994). Logically, water temperature has an

important influence on energetic demands of fish

but, unfortunately, data on water temperature

from the Carbrook golf course lake are lacking.

Thus, the author calculated with an average water

temperature of 24 °C for bull sharks inhabiting

the Carbrook golf club lake like the study by

Schmid and Murru (1994) because this tempera-

ture is close to the mean water temperature of the

adjacent Logan River (see Table 1). Therefore,

the calculation carried out here is of theoretically

character and the result should only be under-

stood as an approximate value.

To calculate daily energy requirements for six

bull sharks, which are regarded as the minimum

population size that inhabited the golf course

lake, the author have used Mugil cephalus as a

prey item for C. leucas. Whitfield (2021)

assumed that C. leucas feeds heavily on the glob-

14 MARINE AND FISHERY SCIENCES 37 (1): 5-25 (2024)

al distributed flathead grey mullet in South

Africaʼs St. Lucia Estuary. Although general con-

ditions between the present study and the study by

Schmid and Murru (1994) are only comparable to

a limited extent, since these authors calculated

energy requirements for sharks in captivity, it can

be assumed that the range of movement of sharks

in the golf course lake was also restricted.

The weight-length relationship of fish can differ

significantly between sexes and locations due to

sexual dimorphism and differences in environmen-

tal conditions. Luther (1963) gave information on

the weight-length relationship of M. cephalus

caught at Mandapama along the east coast of

India, but this author used the fork length (FL) for

its measurement. For the comparability of data,

information derived from Luther (1963) was trans-

formed by converting the length from FL to TL

using the length-length table for M. cephalus pro-

vided by FishBase (Froese and Pauly 2023).

Therefore, a M. cephalus individual of 210 mm FL

represents an individual of 231 mm TL after con-

verting. Luther (1963) calculated an average

weight of 100 g for individuals of grey mullet of

231 mm TL. This transformation was necessary

because Marais and Erasmus (1977) reported a

caloric content of 187.4 kcal 100 g-1 for undried

flesh of M. cephalus individuals >230 mm TL

deriving from South Africaʼs Swartkops River

Estuary.

Bull sharks inhabiting the golf course lake were

estimated to have the lowest reported length for

this locality of 180 cm TL, which represents the

smallest reported size at maturity of Indo-Pacific

bull sharks (Cliff and Dudley 1991). According to

Branstetter and Stiles (1987), this length corre-

sponds to an estimated weight of 50 kg. To correct

for the smaller body mass of bull sharks at the

Carbrook golf course lake in comparison to the

study by Schmid and Murru (1994), the author

used the allometric exponent of 0.79 identified in

a comparative analysis of shark mass versus

swimming metabolic rate (Payne et al. 2015). This

correction is leading to an estimated energy

requirement of 50 kg bull sharks of 8.22 kcal

kg- 1 d-1 and 411 kcal d-1. To meet its daily caloric

requirements, a single bull shark thus needs 2.2

M. cephalus individuals of 100 g day-1. This cor-

responds to 0.44% of its body weight per day and

3.08% of its body weight per week.

Considering the energy content of M. cephalus

as prey for C. leucas, and the calculated energetic

requirement of a 50 kg and 180 cm TL bull shark

derived from Schmid and Murru (1994), a model

of the amount of food demands over time for six

adults in the lake might looks like: If six bull

sharks require 0.44% of their body weight in prey

per day and resided in the lake for 1 week/1

month/1 year, then 9.240/40.101/482.143 kg of

fish would be needed based on the caloric content

of the potential prey (M. cephalus), respectively.

In concrete terms, this means that six bull sharks

of 1.8 m TL require an amount close to half a ton

of fish per year to meet their energy needs.

Residential time in the golf club lake

These previously reported finding increase the

longest known continuous residence of bull shark

individuals in a low salinity environment to 17

years, and are remarkable in that regard because

they present the world record for longest resi-

dence in an ex situ non-marine environment for

this species that has ever been recorded. The lat-

est reliable bull shark sighting in the Carbrook

golf club lake by golf club officials was in 2015

(21 October 2022 pers. comm. S Wagstaff).

Therefore, last bull sharks sighted in 2015 could

have reached theoretically an age of 19 years,

assuming that they were the same individuals that

had already entered the lake in 1996, and not sub-

sequently in 2013. Theoretically, they could even

have reached an age of 24 years, assuming that

they entered the lake with the first flood event in

1991 (Table 2) and survived until 2015. Today, in

2023, the last verifiable shark sighting in Car-

brook golf course lake can be dated back eight

years, and their vanishing remains undiscovered.

Information is lacking on whether new sharks

migrated to the lake during the recent severe

flooding in 2022, as it may have provided an

opportunity for bull sharks to recolonize the golf

course lake. Only increased observation of the

lake can provide clarity on this matter. According

to the general manager of the golf club, the lake

was rapidly recovering from the impact of the last

flood and returned to good health in late April

2022, just two months after the flood (21 October

2022 pers. comm. S Wagstaff). However, officials

at the Carbrook golf club were disappointed of

the vanishing of sharks because they have lost

their flagship mascot. The general manager of the

golf club visited the sharks in the golf course lake

nearly 100 times over many years and has never

observed a decreased vitality or symptoms of

sickness (e.g. sluggish behavior) in the residential

bull sharks (21 October 2022 pers. comm. S

Wagstaff). Therefore, the disappearance of sharks

from the lake beyond 2015 remains a mystery.

Carcharhinus leucas can inhabit both natural

and artificial bodies of water with access to the

sea (Werry et al. 2012). Along the east coast of

Australia, occurrences of bull sharks in artificial

freshwater habitats are neither unusual nor excep-

tional. Dunn et al. (2014) reported that the highly

populated Gold Coast Broadwater, an estuary of

the Nerang River including its artificial water-

ways, provides an additional habitat opportunity

for C. leucas. However, because of the bull

sharkʼs ability to occupy low salinity environ-

ments, coupled with the expanding settlement

activity in many riverine and coastal areas world-

wide, this shark appears near urbanized areas and

in unusual inland locations where people do not

normally expect it.

From a global perspective, there are further

examples of landlocked bull shark populations.

Bull sharks have also been cut off from the sea

and trapped in South Africas’ Lake St. Lucia

Estuary system after historic periods of near-con-

tinuous mouth closures, in some cases lasting

over 10 years. Lake St. Lucia’s Estuary system is

GAUSMANN: BULL SHARKS IN AN AUSTRALIAN GOLF COURSE LAKE

known to be an important key nursery for bull

sharks in the southwestern Indian Ocean (Daly et

al. 2021), and it is well-known for the develop-

ment of increasing salinities (up to 100%) in the

system during periods of droughts (Bass et al.

1973). In contrast to Carbrook golf course lake,

which is characterized by very low salinity condi-

tions, bull sharks from Lake St. Lucia had to

resist long-lasting hypersaline conditions, as

drought has led to increasing salinities in most

parts of this estuary system during times of mouth

closures. Bass et al. (1973) reported that during

times when the estuary mouth became blocked,

many trapped fish and sharks might die, presum-

ably resulting from high salinity levels in this

estuary system. It is confirmed for Lake St.

Lucia’s Estuary system that an unspecified num-

ber of bull sharks remained landlocked in the lake

for many years after its mouth was closed and the

connection to the sea was interrupted. Herein,

they withstood hypersalinity and extreme temper-

atures and survived in this system for more than

10 years of mouth closure in a period from 2002

to 2013 (29 October 2022 pers. comm. R Daly).

Some individuals died during a drought period

when the water in some parts of the lake was too

shallow, and prey became limited. However,

other individuals have survived as long as envi-

ronmental conditions were tolerable and there

were enough preys (29 October 2022 pers. comm.

R Daly). As a species withstanding a broad salin-

ity range from 0-53% (Compagno 1984), bull

sharks have endured successfully the extreme

habitat conditions in Lake St. Lucia for an

extended period, in fact for more than 10 years.

However, a study that was conducted by Bass et

al. (1973) in Lake St. Lucia based on tagging-

recapture methods suggests that individuals of C.

leucas also lived continuously for considerable

periods (up to 537 days) in the lake when access

to the sea is given.

In addition to the landlocked Australian bull

sharks that once inhabited Carbrook golf club

lake and South African bull sharks trapped in

Lake St. Lucia, until nowadays, to the knowledge

of the author, there exists only one other docu-

mented case of a local bull shark population that

was trapped and survived for years in an isolated

body of water, in fact in Panamaʼs Lake Bayano,

Central America. This artificial impoundment is

characterized by pure freshwater conditions and

was disconnected from the sea by damming off

the Bayano River. This historical case was accu-

rately documented by Montoya and Thorson

(1982) who already hypothesized that due to the

ability of C. leucas to osmoregulate, it theoreti-

cally could survive in landlocked situations for

many years. These authors reported that three

dead mature female bull sharks of 222-226 cm TL

were found in Lake Bayano in 1980, four years

beyond the dam closure, indicating that the life-

time of C. leucas in freshwater could be limited

by physiological constraints, but they presumed

that other factors (e.g. herbicides) could have

caused the death of these sharks. Further, due to

the lack of signs of reduced vitality in the sharks,

these authors argued that it does not seem reason-

able that either terminal aging or osmoregulatory

failure would occur in three sharks simultaneous-

ly. However, isolated bull sharks of Lake Bayano

survived at least four years trapped in this pure

freshwater environment, while Montoya and

Thorson (1982) speculated that they almost cer-

tainly would be capable of survive longer.

Deliberations on trapped bull sharks lead to

two possibilities of being landlocked. Firstly, nat-

urally by natural events such as storms and cou-

pled flooding, which mislocate sharks into stag-

nant water bodies normally cut off from rivers

(e.g. lakes, backwaters, billabongs, dead river

branches); and secondly, by human alteration of

rivers (e.g. reservoirs, dams) through man-made

constructions that could leave sharks behind a

dam wall or a barrage after completion. The first

scenario also includes mouth closures of rivers

and estuaries by sand accumulation or prolonged

drought, the latter initiated when the discharge of

the river is drastically reduced, both leading to a

16 MARINE AND FISHERY SCIENCES 37 (1): 5-25 (2024)

cut off rivers from the sea and preventing bull

sharks from returning to the marine environment.

Because many large streams are characterized by

pronounced dynamics, natural alteration of river

flows occurs in many places, especially in the

tropics, which are characterized by high rainfall

totals. Considering these scenarios, future captiv-

ities of bull sharks can be predicted for several

regions within the speciesʼ global range. Thus,

temporary or permanent landlocked bull sharks

should be expected for the numerous freshwater

billabongs of Australiaʼs Northern Territory, from

which they have been already reported by

Stevens et al. (2005) and Pillans et al. (2009), or

sections of rivers isolated from the main tidal

stream. Due to the recent trend of new hydropow-

er dam construction in the developing world, such

as along Brazilʼs Amazon River system, Iranʼs

Karun River, and southeast Asiaʼs Mekong River,

from which C. leucas either have been verifiably

reported or is suspected to occur (Gausmann

2021), isolated bull shark populations should also

be expected in the upper reaches and cut off sec-

tions of those rivers. Considering this, to mitigate

the risk of shark bites, it is advisable not to swim

in stagnant bodies of water adjacent to rivers with

access to the sea that are isolated from the main

stream of the river in warm temperate, subtropi-

cal, and tropical regions. The same applies for

reservoir lakes and connected upper river sections

after the recent completion of dam constructions

because it has now been shown that bull sharks

can survive for many years in low salinity envi-

ronments and in landlocked situations.

Compagno (2002) once stated that C. leucas

should be expected in any warm-temperate, sub-

tropical, and tropical river and lake with access to

the sea that is inside the worldwide range of this

species. In addition to this statement, this species

should also be expected in unusual shark habitats

in regions with risks of natural disasters. Extreme

natural events such as floods may provide C. leu-

cas individuals the possibility to navigate into

water bodies that are normally inaccessible to

them. Simultaneously, bull sharks are facing the

risk of being trapped in isolated bodies of water

when they enter inland waters that are cut off from

the main river or stream. Stranding of bull sharks

in freshwater bodies containing only sporadic and

temporary water, such as Australiaʼs billabongs,

carries some additional risks to bull shark life, par-

ticularly when these waters dry up, and residence

here may result in the death of sharks.

CONCLUSIONS

The finding of bull sharks that have survived

over a 17-year period in an ex-situ environment

with low salinity, coupled with the knowledge that

they have also survived for many years in land-

locked situations at other locations worldwide,

contribute to the knowledge that C. leucas has the

physiological capability to spend a large propor-

tion of its lifespan in fresh and brackish water

habitats. Along with the case reported here, there

are now three confirmed records of bull sharks

worldwide being entrapped for several years,

either in freshwater habitats (Lake Bayano), in

low salinity habitats (Carbrook golf club lake), or

in extremely high salinity habitats (Lake St.

Lucia). Results of this study may contribute to our

understanding of the extent of euryhalinity of C.

leucas, a species of broad halotolerance and virtu-

ally no osmoregulatory limits. Moreover, these

findings impressively demonstrate the bull

sharksʼ adaptability to environments with salini-

ties lower or higher than seawater.

The period of 17 years that has been proven for

the residency of the golf course lake bull sharks

represents more than half of the given longevity

of 29-32 years that have been reported for free-

ranging Indo-Pacific C. leucas by Wintner et al.

(2002). In similar future cases of landlocked bull

sharks, these individuals should be monitored by

scientists to gain more insights into the complex

biology of this species, as only a small number of

GAUSMANN: BULL SHARKS IN AN AUSTRALIAN GOLF COURSE LAKE

elasmobranchs held so much interesting informa-
tion on adaptation to low salinity environments in
this ordinarily marine fish group.
It would be desirable if newcoming sharks in
the golf course lake were tagged, monitored long
term, and examined in depth to gain more data on
physiology and other life parameters. Finally, the
status of C. leucas in the Carbrook golf course
lake remains unclear. The last time the sharks
were seen in the lake by the golf club officials
was in 2015, eight years ago, and it is currently
unclear if new sharks arrived during the latest
flood event in 2022. Intensive baiting and fishing
activity or the use of further investigation meth-
ods such as environmental DNA (eDNA) could
bring clarification on this matter. For their detec-
tion and future observation, the collaboration
between the officials of the Carbrook golf club
and shark scientists would be desirable.
ACKNOWLEDGEMENTS
The author is deeply grateful to Scott Wagstaff
at Carbrook golf club (Carbrook, Logan City,
Queensland) for providing worthful information
on the history of the bull shark population in the
Golf course lake and, further, for the permission to
use photo material for documentation purposes.
He also sincerely thanks Philip Matich (Texas
A&M Galveston University, Department of
Marine Biology) for his helpful and valuable com-
ments and remarks on the preliminary version of
the manuscript, which have improved the quality
of the content greatly. Further, his thanks go to
Ryan Daly (Oceanographic Research Institute
–ORI, Durban) who provided information on bull
sharks in South Africaʼs Lake St. Lucia Estuary
system and access to important literature. Addi-
tionally, he also gives thanks to Butian Wang
(Australian Government, Bureau of Meteorology,
Flood Forecast and Warning) for providing worth-
ful information on the Queensland floods. More-
over, the author acknowledges the reviewers and
editors for their worthful comments that signifi-
cantly contribute to the quality of this publication.
Declaration of interest
The author has nothing to declare.
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24 MARINE AND FISHERY SCIENCES 37 (1): 5-25 (2024)

APPENDIX

Catalog of questions used for this study:

1) In which year do you have started your profes-

sion at Carbrook Golf Club?

2) When were sharks spotted for the first time in

the lake of Carbrook Golf Club (date, year)?

3) How many sharks were spotted for the first

time at the point of their discovery?

4) How many individuals of sharks were spotted

in the lake overall (absolute numbers)?

5) Was the number of sharks in the lake stable

over time?

6) Currently, are there still living sharks in the

lake, and how much individuals (absolute

numbers)? If not, when they disappeared

and why (please provide possible explaina-

tions)?

7) Were you able to distinguish between different

individuals of bull sharks by distinct markings

(such as fin shape, damages in the dorsal fin,

etc.) or deviating sizes over time?

8) Are there information on food resources for the

sharks in the lake? What kind of fish and food

items the sharks prey upon?

9) How many times the sharks get additionally

feeded by the golf course staff and with what

kind of food items (e.g. meat, slaughterhouse

waste)?

10)Do you observed hints on sickness or limited

vitality of the sharks that inhabit the lake (e.g.

sluggish behaviour)?

11)Do you ever have observed dead sharks in the

lake? If yes, what do you believe to have cau-

sed this deaths (e.g. another flood event,

aggression by another shark, sickness)?

12)How many flood events have affected the lake

since the first appearance of the sharks?

13)Were the bull sharks of the Carbrook Golf

course lake already the subject of a previously

conducted scientific investigation?

GAUSMANN: BULL SHARKS IN AN AUSTRALIAN GOLF COURSE LAKE