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A Big Fish in a Small Pond

For the enormous majority of people that you ask, they would assume that sharks only live in saltwater habitats, out in the oceans. And these people are right because this is indeed true for some 95% of species,. However, there are some sharks that can survive in freshwater habitats and there have indeed been some rare cases where sharks have ended up in the unlikeliest of places. In one bizarre incident, sharks were left stranded in a golf club lake for 17 years! So how on earth did these sharks get there? How were they able to survive? And what does this teach us about the physiology of these sharks?


Feeling Fresh

All fishes, including sharks and their relatives, have evolved to live in specific habitats. This means you cannot just pick them up and dump them into any old body of water and expect them to be able to survive. The vast majority of sharks are "stenohaline". This means that they have adapted to only be able to live in water with a specific salt level or "osmolarity", that you find in salt water. Therefore, they cannot tolerate living in freshwater environments, like rivers of lakes. They may be able to wander into these waters for a bit, but will not be able to stay there for any extended period of time (Lucifora et al, 2015; Gausmann, 2024).


Comparatively, there are a few species of sharks and rays that can live in lower salinity environments. Some of these can only live in brackish or freshwater, where "euryhaline" species able to roam between rivers and the sea. To learn more about these fascinating characters, check out Getting Fresh.

Bull sharks are amongst only a handful of species of sharks that can tolerate living in freshwater habitats (Image Credit: Pterantula / WikimediaCommons)

One of the most famous examples of a euryhaline species is bull sharks (Carcharias leucas), which are able to survive in both salt and freshwater. In the Amazon River bull sharks have been found as far as 4200 km upstream! In fact, the freshwaters of rivers and estuaries are critical habitats for this species, as they use them as "nursery habitats" for their young (Lucifora et al, 2015; Gausmann, 2021; Gausmann, 2024).


Bull sharks are able to tolerate a range of salinities because they have a suite of physiological adaptations make them remarkable "osmoregulators"; capable of switching from salt to freshwater and back again, and adapting to even sudden changes very quickly. To maintain their internal levels, they can absorb and retain more salt, when they are living in a freshwater environments, but they can exclude and excrete excess salt when they are in higher salinities. To learn the specific of how this works, check out I Need Some Space (Lucifora et al, 2015; Gausmann, 2021; Gausmann, 2024).


Land Locked

As well as living all along the coasts, bull sharks are common in rivers and freshwater systems around Australia. Tracking studies in Albert and Logan river systems near to Brisbane, Queensland, they have been known to travel up to 52 km upstream; spending extended periods in the low salinity (Gausmann, 2021; Gausmann, 2024).


Bull sharks are "euryhaline" - capable of tolerating a wide salinity range (Image Credit: Albert Kok / WikimediaCommons)

This means that, if these water bodies expand because of flooding, bull sharks can gain access to some truly weird and wonderful places. For example, a shark was spotted swimming around the flooded streets of Brisbane, after the Queensland floods of in 2010 - 2011 and after Cyclone Debbie in 2017, bull sharks were washed into the city streets again (Gausmann, 2024).


In 1996 six bull sharks became trapped in a lake on the Carbrook golf course after the floodwaters from the nearby Logan and Albert rivers receded. What was especially interesting about this case, is that six sharks actually ended up staying in this unusual haunt for a very long time - 17 years in fact! (Gausmann, 2024).


Feeling Salty

They were able to flourish because a cornucopia of other smaller, euryhaline fishes, including flathead grey mullet (Mugil cephalus), yellowfin bream (Acanthopagrus australis), Indo-Pacific tarpon (Megalops cyprinoides) and mangrove red snapper (Lutjanus argentimaculatus) were washed into the lake alongside them. All potential prey for the bull sharks. As they were somewhat of a spectacle, the staff at the golf club also occasionally fed them scraps (Gausmann, 2024).

All these good meals meant that, whilst they were relatively small, youngsters when they were first washed into the Carbrook golf club lake, over their years there, they grew. A lot. Towards the end of their stay, one was estimated to to as large as 3 metres in length. Only one shark was ever found dead in the lake during this period and the sharks escaped their confines in subsequent floods in 2013. At this point, they were all thought to be mature adults. They may have sepnd as much as half of their lifespan in the lake (Gausmann, 2024).


There have only been two other examples of bull shark thriving for such extended periods outside of the ocean. Several bull sharks lived in the freshwater system of Lake Bayano, Panamá and another group survived the hypersaline conditions in St Lucia Lake, South Africa, but in neither case did they come close to matching the 17 years achieved by the sharks of the Carbrook golf club lake. This was the longest uninterrupted stay in a low salinity environment that has ever been recorded for bull sharks and highlights just how incredibly adaptive these sharks are. They are true survivors! (Lucifora et al, 2015; Gausmann, 2021; Gausmann, 2024)



References

Gausmann P (2021). Synopsis of global fresh and brackish water occurrences of the bull shark Carcharhinus leucas Valenciennes, 1839 (Pisces: Carcharhinidae), with comments on distribution and habitat use. Integrative Systematics: Stuttgart Contributions to Natural History, 4:1. Access online.


Gausmann P. (2024). 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. Marine and Fishery Sciences (MAFIS), 37:1. Access online.


Lucifora LO, de Carvalho MR, Kyne PM & White WT (2015). Freshwater sharks and rays: A quick guide.Current Biology, 25, 965–979. Access online.




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