Bull Shark (Carcharinus leucas)
The bull shark, with its characteristic blunt snout (image from http://www.abyssdivecenter.com/
Bull sharks, sometimes called the Zambezi shark in parts of Africa, is so named for their short, blunt snout, feisty attitude and tendency to ‘head-butt’ or ‘bump’ their prey before attacking. The bull shark is a requiem shark, one of the largest and best known families of sharks in the order Carcharhiniformes. They are migratory, live young bearing sharks that inhabit warm seas. They have the ‘classic’ shark look, with torpedo shaped bodies and round eyes, ideal for such active predators capable of taking down large prey. Depending on their location, they will eat anything from bony fish, squid, marine mammals, turtles and even other sharks and rays. They are often called the ‘trash cans’ of the ocean, as sharks have been autopsied with actual garbage in their stomachs (cue the famous scene from Jaws where they find a car license plate in the belly of a shark.)
From the rivers to the sea: freshwater tolerance
The bull shark is well known for tolerating freshwater and is perhaps the commonest of the 43 elasmobranch species regularly reported in estuaries and rivers, alongside stingrays and sawfish. This ability makes them a euryhaline fish, capable of adapting to a range of salinities. In most elasmobranchs, entering saltwater is dangerous as their blood is as salty as seawater so that they can maintain an osmotic balance in their marine habitat. For those species without euryhaline adaptations, entering freshwater causes disruption to this osmotic balance, and their cells rupture as water moves out of their cells towards the less salty water around them. This means the shark will bloat and die even in just a short time in freshwater. Salts like urea and triethylamine oxide accumulate in shark blood to allow them to maintain their osmotic balance in saltwater, but in sharks that travel into freshwater, like the bull shark, the concentrations of these compounds are significantly reduced in comparison. Bull sharks have a specially adapted rectal gland that enables them to excrete sodium and chloride when they change between saltwater and freshwater. Although theoretically bull sharks could happily live in freshwater their whole lives, studies on shark movements and lifestyles have shown that they will starve in freshwater as their primary source of food is in the ocean. Generally, newborn and younger sharks are found in freshwater, with the adults moving into the open ocean to reproduce. The age of the bull shark affects its ability to tolerate extremes of salinity, and scientists believe this is why adults travel up into rivers; to birth their young in an environment that will improve their chances of survival, as juvenile sharks cannot cope with the high salt concentrations in the ocean. The juveniles stay in the brackish estuaries and rivers until they have matured enough to survive the marine environment.
‘Maneater’ or just misunderstood: human wildlife conflict
Unfortunately, requiem sharks tend to be the species implicated in most attacks on humans, although identifying sharks in during an attack is always particularly difficult. The bull shark is, however, one of the ‘big three’ in the shark attack world, alongside the tiger shark and the infamous species of Jaws fame, the great white shark. Due to their size and power, these species of shark can, in very rare instances, cause significant injuries to unlucky swimmers and surfers. They tend to spend a great deal of time in shallow waters and they are incredibly territorial, making them a significant concern for swimmers and surfers, and the shark biologists trying to protect sharks from fatal retaliation to attacks (which is proven to be highly ineffective in preventing shark incidents, by the way). In fact, some shark biologists even believe that bull sharks were the actual perpetrators behind the spate of infamous 1916 shark attacks in Jersey shore, rather than the juvenile white shark that was pulled from the waters a few weeks later. These famous attacks kick started the fear of sharks that persists today and were the inspiration behind Peter Benchley’s iconic novel Jaws, and the accompanying film that frightened generations of beachgoers out of the water. The ability of bull sharks to comfortably spend time in brackish water coupled with their sometimes-nasty attitude makes them possibly a more likely suspect for the 1916 attacks.
However, the fact remains that even though bull sharks, as well as other species, spend a lot of time cruising around the same waters as every beachgoer’s favourite swimming spot, the likelihood of being bitten by a shark is 1 in 3,748, 067. In fact, death by selfie is a more common way to meet your maker than a shark attack, and it can almost be guaranteed that no one is afraid of their iPhone.
Many shark biologists are now considering changing the language around sharks, to help tackle the misconception that every interaction with a shark will be fatal. Instead of using the term attack, there is a movement towards using the word ‘incident’ and ensuring to distinguish between bites and the vanishingly rare fatal bites. Very often, sharks engage in a ‘bite and spit’ behaviour, when it is suspected that sharks, especially white sharks, mistake humans for seals and take an exploratory bite, realise they’ve got an unpleasant tasting bony human and release them. Of course, we also have to consider human behaviour, and how we might be frightening sharks and causing them to react with aggression. Pressure is also being applied to the media to stop their hysterical reporting of shark incidents and apply some more rational discussions to sharks and maybe even highlight the huge numbers of sharks and rays we kill every year. There is definitely a dangerous, murderous predator out there, and it certainly isn’t the bull shark.
Biting back: shark conservation
As apex predators, sharks are an essential part of the marine food chain, but they have long been overexploited in the fisheries industry, as intentional catch and bycatch. In the last 50 years, we have lost around 70% of the global shark population through our activities. Their slow growth, late sexual maturity and low reproductive yield makes them especially vulnerable to intensive fishing. Although Bull Sharks currently are classed as vulnerable, many species of sharks are endangered, and a vast number, including bull sharks, have declining populations. Sustainable shark fishing is possible and is considered a better option than an outright ban, but there is not enough legislation in place currently to enable this. The shark fin trade, driven by demand from Asian countries where shark fins are considered a delicacy, is one of the most serious threats facing all shark species. Huge numbers of sharks are caught on baited longlines trailed behind huge ships, their fins hacked off, and the butchered sharks thrown overboard, still alive, left to suffocate on the ocean floor, unable to swim. A study from 2020 identified that between 2010 and 2017 between 591 and 859 metric tonnes of shark fin were exported through the United States alone. From studies of records of the Hong Kong fish market, a global hub for the trade in shark fins, it was identified that two thirds of identified species are threatened with extinction, especially concerning for commonly finned species like shortfin mako, blue shark, silky shark and scalloped hammerhead, all of which are in real danger of extinction.
For bull sharks in particular, they are targeted by beach protection programmes in South Africa and Australia that target large sharks through the use of SMART (Shark-Management-Alert-InRealTime), aquatic traps that consist of an anchor, two buoys and a satellite linked GPS tag attached to a baited hook. When the shark takes the bait and puts pressure on the line, an alert is sent to a team of scientists and boat crew that use the GPS tagging to find the shark and release it. This was introduced as an alternative to simply indiscriminately catching sharks on drumlines and allowing them to die slow deaths in an attempt to protect beaches from sharks. Unfortunately, these kinds of interventions can have serious negative consequences on shark populations, even though SMART drumlines are an infinitely better choice than allowing sharks to die on drumlines or in nets strung across bathing areas.
Due to their euryhaline lifestyle, travelling up into rivers and estuaries, and the reliance of juvenile sharks on freshwater habitats, bull sharks are also particularly vulnerable to degradation of freshwater through damming, coastal development and agricultural run off polluting rivers.
But attitudes towards sharks are changing rapidly, with even the most beach-phobic people understanding the issues facing sharks and appreciating them for their role in our oceans.
Bite Back is a UK based charity dedicated to shark and marine conservation with a focus on shark overfishing, working to tackle the fin trade and bringing attention to smuggling of shark meat. They have also worked to pressure major supermarkets and grocery stores to stop selling unsustainable seafood like monkfish, swordfish and species of skates and rays. Changing policy around the transport and trade of shark products is arguably the most important conservation action that can help dwindling populations of sharks around the globe, and Bite Back work to spearhead change from the UK and across the world. You can check out their work at Bite-Back Shark & Marine Conservation – Save the sharks, protect the oceans.
The shark trust, established in 1997 are also based in the UK but now work internationally with fisheries and marine scientists to safeguard the future of sharks and rays. They work across three key areas: species protection through better legislation, improving the sustainability of fisheries, and promoting responsible trade and reducing demand for shark products. You can see their most noteworthy achievements from 2020 in the image below, and you can check out their work at The Shark Trust.
Awesome videos!
References
Cardenosa, D. Shea, S.K. Zhang, H. Fischer, G.A. Simpfendorfer, C.A. and Chapman, D.D. (2022) ‘Two thirds of species in a global shark fin trade hub are threatened with extinction: conservation potential of international trade regulations for coastal sharks.’ Conservation Letters.
Friedrich, L.A. Jefferson, R. and Glegg, G. (2014) ‘Public perceptions of sharks: gathering support for shark conservation.’ Marine Policy
Heupel, M.R. and Simpfendorfer, C.A. (2008) ‘Movement and distribution of young bull sharks Carcharhinus leucas in a variable estuarine environment.’ Aquatic Biology
Midway, S.R. Wagner, T. Burgess, G.H. (2019) ‘Trends in global shark attacks.’ PLOS ONE
Neff, C. and Hueter, R. (2013) ‘Science, policy and the public discourse of shark ‘attack’: a proposal for reclassifying human-shark interactions.’ Journal of Environmental Studies and Science.
Pillans, R.D. and Franklin, C.E. (2004) ‘Plasma osmolyte concentrations and rectal gland mass of bull sharks, captured along a salinity gradient.’ Comparative Biochemistry and Physiology.
Pillans, R.D. Good, J.P. Anderson, W.G. Hazon, N. and Franklin, C.E. (2005) ‘Freshwater to seawater acclimation of juvenile bull sharks: plasma osmolytes and Na+/K+-ATPase activity in gill, rectal gland, kidney and intestine.’ Journal of Comparative Physiology.
Pushaw, C. (2015) ‘The impact of the Queensland Shark Control Program on local populations of threatened shark species, 1962-2014.’ SIT Graduate Institute.
Shiffman, D.S. Bittick, S.J. Cashion, M.S. Colla, S.R. Coristine, L.E. Derrick, D.H. Gow, E.A. Macdonald, C.C. O’Ferrall, M.M. Orobko, M. Pollom, R.A. Provencher, J. Dulvy, N.K. (2020) ‘Inaccurate and biased global media coverage underlies public misunderstanding of shark conservation threats and solutions.’ iScience.
Simpfendorfer, C.A. de Jong, S.K. and Sumpton, W. (2010) ‘Long term trends in large shark populations from inshore areas of the Great Barrier Reef World Heritage Area: results from the Queensland Shark Control Program.’ Marine and Tropical Sciences Research Facility Transition Program Report.’
Werry, J.M. (2010) ‘Habitat ecology of the bull shark on urban coasts in eastern Queensland, Australia.’ Griffith University of the Gold Coast.
