By David Graham: March 16, 2023
Scroll through the comments section of almost any news headline, photo, or story related to sharks and you'll likely find that one guy/gal proclaiming 'that's why I don't get in the ocean!' A convenient masking of the truth behind a lack of such ambition... but to spoil the party further, you may not be as free from an opportune encounter with a shark in freshwater.
Sharks, namely bull sharks, are quite adept at life in freshwater... and wouldn't you know it, they account for more attacks on humans than any other species! All pandering and fear mongering aside... what article on sharks is complete without obligatory depictions and statistics of the outlier nightmare scenarios.
Still, bull sharks DO go into freshwater... far into freshwater and they do it much more often than the hapless tuber, kayaker, or other recreational water enthusiast may realize. The bull shark is a diadromous species of fish/shark. Meaning it can comfortably move between salt and freshwater. Its a regulatory adaptation that has allowed the species to become one of the most efficient predators on the planet
How Do They Do It?
It's speculated that this evolutionary advantage was established some time during the last ice age. Gridlocked populations of ancestral 'bull shark's of the time may have run into a do or die situations or environmental bottlenecks that separated them from the rest of their subclass of species and favored those equipped to survive in conditions of decreased salinity. Nature demanded such adaptation and it favored the bull shark.
The bull sharks ability to enter freshwater is due to several organs that serve the purpose of maintaining balance between their internal salt and water. The rectal gland, kidneys, liver, and gills all worked together to maintain such balance. The rectal gland is obviously used to excrete excess salts - in freshwater they simply 'excrete' less of it.
When Do They Enter Freshwater?
I suppose the bonus, if there is any.. is that typically larger bull sharks favor estuary mouths or saltwater outright.... it is the smaller specimens occupying more of their time up river in the especially fresh areas. The ability for smaller sharks to retreat far upriver is a survival adaptation. Sharks will leave brackish estuaries to travel out into the sea to breed - but the young will be birthed near shore low-salinity areas and ultimately find their way to the refuge of freshwater estuaries, creeks, and rivers. The idea is obvious... the smaller sharks that could be potential prey for larger predators will move to zones the larger oceanic predators cannot tolerate.
Bull sharks gives birth to live 'pups'... and they may be born as long as 2 feet in length.... so straight out of the womb they are already a formidable sizes predator species by freshwater standards... and will not have to contend with the same sort of predators. Young bull sharks will gradually develop their tolerance for higher salinity as they grow older. So, physiologically they have a better chance of survival in freshwater estuaries because of a low tolerance for salt - AND less threat from larger predators.
Where?
You may be surprised at just how far bull sharks can travel into freshwater. In the US, they've been known to travel as far as nearly 700 miles up the Mississippi river... famously found as far inland as Alton Illinois! In other major river systems like the Amazon, bull sharks have been documented as traveling over three times that distance.
In Florida, bull sharks can also be find in most larger estuaries and rivers that have some access to deeper water. Florida rivers like the Loxahatchee, Caloosahatchee, the rivers feeding into Charlotte Harbor and more - ALL have young bull sharks far upriver. Like their adult counterparts. Young bull sharks are likely in most all major or mid sized river systems along the gulf and southeastern Atlantic coastline of the US.
Eating Habits
Like their adult counterparts - young bull sharks are not particularly picky eaters. In freshwater and brackish ecosystems they will readily consume other species of smaller fish, amphibians, rays, etc. As a result, they are not overly difficult to pursue or capture. In general - cut or live baits fished in deep bends of river systems within 10-15 miles of the bay or harbor they feed into will likely have opportunity to encounter young bull sharks. I have had success catching sharks as far away from the coastline as 20 miles - right alongside longnose gar, channel catfish, bluegill and bass.
The coastal estuaries and rivers of the Gulf of Mexico and the Southeastern Atlantic coastline offer the unique opportunity to encounter a shark in freshwater. While they can at times be something of a nuisance or bait thief - opportunities to catch a shark right alongside typical freshwater species is an interesting angle for any angler!
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