Beady eyes and tiny ears may be all you see of a hippopotamus wading through the water, but there’s a lot more going on under the surface—and a lot of it is poop. The giant African mammals generate 52,800 metric tons of dung each year, enough—according to two new studies—to pollute ecosystems and kill fish by the hundreds.
Until now, the daily migration of Africa’s 70,000 hippos between wading pools during the day and grasslands where they feed at night was thought to be beneficial. With their poop, the animals transfer nutrients to African aquatic ecosystems. But in the dry season of Kenya’s Mara River, when 4000 hippos crowd together in the 100 or so few pools deep enough for them, hippo dung accumulates at the bottom, feeding microbes that rob the water of oxygen needed by other underwater creatures. When this water is flushed downriver by periodic floods, it can kill fish downstream.
Over 5 years, one team documented 13 such incidents on the Mara River, and the resulting loss of oxygen killed large numbers of fish during nine of them, the researchers report today in Nature Communications. They used robotic boats to survey the pools and built small dams to construct and flush an artificial hippo pool to verify this role of hippo dung.
In a second study, a different team measured water quality and biodiversity in isolated hippo pools in the Great Ruaha River in Tanzania, where water management practices have caused the river to stop flowing altogether in the dry season. During the dry season, the diversity of species in those pools decreases, and there are 41% fewer tilapia, a major local food source for local residents, that team reports this week in the Proceedings of the National Academy of Sciences.
Some fish may be adapted to survive these periodic conditions. But the newly documented detrimental effects should be a call to arms for eastern Africans to be very careful in planning dams and other water management projects that could exacerbate them by restricting water flow even more, both groups warn.
*Correction, 16 May, 12:30 p.m.: This story misstated the number of fish killed; actually there were nine incidences wherein lots of fish died, not nine fish killed.