Bull Sharks: The Ocean Predator That Swims Up Rivers and Into Lakes

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From 1996 to 2013, a lake on a golf course in Queensland, Australia held a thriving population of bull sharks. This episode explains how a marine predator ends up there. The bull shark is stocky and built for shallow, murky water, with females reaching 13 feet and over 1,300 pounds when pregnant. Its bite, measured at up to 1,330 pounds of force, is the strongest for its weight of any cartilaginous fish tested, which lets it eat nearly anything, turtle shells and rival sharks included.

Its real trick is fresh water, which kills most sharks by flooding their cells. A bull shark's kidneys flush the excess water, its rectal gland shuts down to save salt, its gills pull sodium and chloride from the river, and its liver adjusts both urea and buoyancy. Mothers use that ability to give birth in estuaries and rivers that larger predators cannot enter. The episode also covers the shark's reach inland, its close contact with people, its intelligence, and its vulnerable conservation status.

• Bull sharks have been documented 2,500 miles up the Amazon in Peru and as far up the Mississippi as Alton, Illinois.
• The sharks of Lake Nicaragua were long thought to be a separate species until tagging in the 1960s showed ocean sharks reaching the lake through the San Juan River in 7 to 11 days.
• Hunting by touch in zero visibility, they use a bump and bite technique, and they are widely believed responsible for the 1916 Jersey Shore attacks.
• Juveniles tracked in Florida's Caloosahatchee River ride the tides upriver and back down to offset the energy cost of regulating their salt balance.
• A barrier of flexible pipes holding permanent magnets overloads the shark's electrical sense and turns it away from swimming areas without nets or harm.