Bull Shark

The bull shark broke one of the ocean’s basic rules. Almost every shark is locked into salt water by its own body chemistry — but the bull shark can swim out of the sea, up a river, and into a lake, and stay there. That ability puts it in closer, more frequent contact with people than any other large shark.

Overview

Bull sharks live in warm coastal seas all around the world, rarely far from land, in water often too shallow and murky for other big sharks. From there they move into estuaries, up rivers, and into freshwater lakes, and populations exist deep inland on every inhabited continent. Their pups are born and spend their early years in low-salinity river mouths, safe from the larger sharks that would eat them in the open sea.

The freshwater trick

A shark’s blood and tissues are normally kept slightly saltier than seawater, which stops water flooding in through the gills and skin. In fresh water, that same chemistry would cause a shark’s cells to swell and burst. The bull shark manages the transition with osmoregulation: its kidneys ramp up urine production to shed the excess water, and its liver, gills, and a rectal gland near the tail adjust how much salt and urea the body retains. A bull shark can shuttle between full seawater and pure fresh water within days.

Physical characteristics

Bull sharks are stocky and blunt-nosed — the “bull” name is about the shape and the head-butting behaviour observed before some attacks. They are grey above and pale below, with a short, broad body and a powerful bite: for its size, the bull shark generates one of the strongest bite forces of any shark, useful for its varied, hard-bodied prey.

Diet

The bull shark is an opportunist with an extremely broad diet: bony fish of all kinds, stingrays and other sharks (including smaller bull sharks), sea turtles, dolphins, crustaceans, seabirds, and terrestrial animals that end up in the water. In rivers it takes freshwater fish and anything else it can catch. It typically hunts in the low light of dawn, dusk, and night, in water where visibility is poor and prey cannot see it coming.

Reproduction and life cycle

Bull sharks are viviparous — pups develop connected to the mother by a placenta-like structure and are born live after a 10–11 month pregnancy, in litters of 1 to 13. Birth happens in brackish estuaries and river mouths, and the pups remain in these nurseries for years, only moving out to sea as they grow large enough to be safe. This makes healthy, unpolluted estuaries essential to the species.

Threats and conservation

The bull shark is listed as Vulnerable with a declining trend:

• Coastal and estuarine fishing. Bull sharks are caught for the fin trade, for meat and liver oil, and as bycatch, and their inshore lifestyle keeps them constantly in range of nets and lines.
• Nursery habitat loss. The estuaries and river mouths their pups need are among the most heavily developed, dredged, and polluted habitats on Earth.
• Shark control programmes. Baited drumlines and nets set off swimming beaches kill bull sharks along with many other species.