Their dung consumes the oxygen around it, creating lethal pulses of suffocating water.
At first, Chris Dutton and Amanda Subalusky had no idea why the fish were dying.
At a bridge on the border between Kenya and Tanzania, they noticed that whenever the Mara River rose by a few feet, dead fish would wash up on its banks, sometimes in the thousands. Storks, vultures, crocodiles, and hyenas made short work of the carcasses, so “if you weren’t there to see it, you’d never know it was happening,” says Dutton. Local rangers knew about the die-offs, but they blamed the events on farmers who sprayed pesticides in upstream fields.
It wasn’t the farmers. Through an increasingly bold set of experiments, involving remote-controlled boats, computer simulations, a makeshift dam, and vast tankers of excrement-filled water, Dutton and Subalusky identified the real culprits: hippos.