Hippopotamus excrement could be lethal for river fish, a new study says. In fact, it might be the reason why fish are dying in Kenya's Mara River.

A common belief among ecologists is that agricultural and sewage pollution can damage marine life. A study published May 16 in the Nature Communications journal expands upon this, claiming the waste from hippos can have a similar effect in the Mara River, which passes through the Kenya's Maasai Mara National Reserve, home to over 4,000 hippos.

Hippos Might Be Indirectly Killing Fish

Hippos spend their nights in the savanna grazing and return to the river during the day to keep cool. As they wade in the waters, they excrete wastes. The study found that these hippos excrete 9.3 tons of waste into the Mara River every day — according to the researchers, this volume of waste can trigger fish kills since hippo waste depletes oxygen from the water during decomposition. In addition, microbial activity creates chemicals such as ammonium and sulfide, which are known to be harmful to fish.

"During dry periods, oxygen-poor water accumulates in hippo pools. Periodic intense rains eventually flush the water downstream," said Emma Rosi coauthor and a freshwater ecologist at the Cary Institute of Ecosystem Studies. "This sudden pulse of deoxygenated water can cause temporary hypoxia and fish kills."

Two Major Factors Of Dissolved Oxygen Decline

The researchers monitored water chemistry and flow downstream from 171 hippo pools. Out of the 55 documented flushing flows — which happens when rain doubles river flow — dissolved oxygen concentrations declined during 49 of the events, dipping low enough to cause fish kills as much as 13 times. The team linked dissolved oxygen decline to two major factors: low-oxygen water from hippo pools and hippo pool sediments. When low-oxygen water mixes with upstream river water, the river's overall dissolved oxygen concentration is reduced. Simultaneously, hippo pool sediments washed out during flushing flows continue to use up oxygen as decomposition continues.

While the findings definitely sound negative, dead fish offers scavenging opportunities for birds and crocodiles that frequent the Mara River. There's a popular idea that asserts rivers aren't supposed to have dissolved oxygen crashes, but according to Yale University researcher Christopher Dutton, this comes from scientists studying places that no longer have intact large wildlife populations. The Mara River, on the other hand, does.

"This system offers a window into the past and illustrates how ecosystems might have functioned before human impacts."

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