Quirks & Quarks

Flushing hippo toilets smother fish in African rivers

Hippos load their pools with poop, which then washes downstream creating temporary 'dead zones'
Hippo pools in the Mara River are saturated with hippo feces. (Chris Dutton)

Smelly and dangerous

For the sake of their science, Chris Dutton and his team members spent time knee deep in a lagoon full of thick, smelly hippopotamus poop.

Dutton is a graduate student in the Department of Ecology and Evolutionary Biology at Yale University. He was studying an ecological phenomenon that's been going on for ages, but has only just been recognized in African rivers: how hippo pools can cause fish kills.

You can have anywhere between six to a hundred hippos in some of these pools, which are slightly larger than a swimming pool.- Chris Dutton

Hippos deposit so much excrement in the river waters where they congregate that it fertilizes bacteria that suck up all the oxygen in the water which then suffocates the aquatic life downstream.

Dutton was there when thousands of dead fish washed up on the banks of the Mara River in the Maasai Mara National Reserve on the border between Kenya and Tanzania.

"What was really interesting was that within a day or two after the fish kill, all the fishes were gone," says Dutton. "They had been consumed by all the different scavengers and animals in the system."

At the time, he wasn't sure what caused it. But it piqued his interest and so he tried to find out more.

Tracking down the perpetrators

Dutton began by putting equipment in the river to measure the amount of dissolved oxygen and water level. "As we started looking at that data, we saw these huge crashes in dissolved oxygen and they occurred over a period of eight to 12 hours where it would crash and recover," says Dutton. "This is just unheard of in normal rivers."

Pumping 16,000 liters of soiled hippo water into an artificial hippo pool. (Chris Dutton)

This explained the fish kills. The next step was to find the cause, which led them to investigate the effects that the hippos in the region might be having on the water.

What they found was that the 4,000 or so hippos in the Mara River excreted about 8,500 kg of feces daily. "Hippos can generate quite a lot of feces because they're very inefficient digesters," explains Dutton. "You can have anywhere between six to a hundred hippos in some of these pools, which are slightly larger than a swimming pool."

All that feces then settles at the bottom of the pool, and the bacteria start growing on it, and consuming the oxygen in the water. When it rains, the feces from the bottom of the river get dragged into the water column and pushed downstream, which causes the oxygen level in the downstream river to drop dramatically, making it deadly to many aquatic animals.

Dutton confirmed his theory by pumping 16,000 litres of soiled hippo water into an artificial hippo pool.Then they released this water into the river and found that the oxygen levels indeed dropped in the water downstream.

A brown stain on nature's reputation?

Despite the destruction to aquatic life, Dutton says this is not all bad. In fact, he thinks this had been a natural part of the ecosystem before humans came.

"The Mara is a really unique river where we have large amounts of wildlife influence on it," he says. "A lot of these flushing events dump a lot of nutrients into the system and those nutrients fuel other life."