Many ecologists in Africa have noticed a phenomenon, that is, a large number of dead fish appear in the Mara River at a fixed time every year. Ecologists visited the residents near the Mara River, and the residents said that it happened once a year, which has become commonplace.

But this unusual phenomenon has aroused great concern of ecologists, because the Mara River is an important river in Africa, with a total length of 395 kilometers and a drainage area of 13504 square kilometers. Although the Mara River is in eastern Africa, it will not stop even in the dry season.

Most importantly, the Mara River has almost become a "boundary river" across Tanzania and Kenya. Every year, animals from East Africa cross this river when they migrate from Serengeti Prairie in Tanzania to Masai Mara Prairie in Kenya during the dry season.

There are a large number of Nile crocodiles in the Mara River, and they will have a good meal when animals migrate across the river. Because the number of animals crossing the river exceeds one million, and many animals will die in the Mara River, the behavior of animals migrating here to cross the river is also called crossing the sky.

If a large number of dead fish in the Mara River are caused by pollution, the survival of Nile crocodiles will be threatened, and animals crossing the river will get sick or even die in large numbers because of pollution, so it is very urgent to find out the reasons.

The researchers found that in the water where a large number of fish died, the oxygen content in the water was extremely low, and the water also contained certain toxic substances. At first, the researchers thought it was caused by someone poisoning the fish. However, no fish poisoners were found through investigation. In three years, there were 13 collective fish deaths in two sections of the Mala River.

So what caused the fish to die? The researchers found two strange phenomena: first, there are almost no fish in the river with dense hippos; Secondly, 13 collective fish deaths all occurred after heavy rainfall, and in the downstream of hippopotamus gathering place.

Both discoveries point to the same animal-hippopotamus. Hippos, as the third largest animal on the African continent, belong to semi-aquatic mammals. They almost stay in the water except going ashore for food at night. Most importantly, hippopotamus is a highly gregarious herbivore, and a hippopotamus group ranges from a dozen to hundreds.

Through further investigation, ecologists found that there are about 4000 hippos living in the Mara River. Although these hippos are not all gathered together, the roads where they gather are still relatively close.

So, what makes there almost no fish living in the densely distributed hippos? The reason is feces. As a large herbivore, hippopotamus needs to eat at least 40 kilograms of grass every day. The more they eat, the more they pull. 4,000 hippos excrete 8.3-9.3 tons of feces in the Mara River every day.

Although hippopotamus will wag its tail constantly when defecating, and the feces will be scattered all over the floor, after a large amount, the feces will sink to the bottom one after another, forming a large area of water and feces deposition. These feces need a lot of microorganisms to decompose, and microorganisms will consume a lot of oxygen in the water when they decompose. At the same time, the fresh manure itself contains some harmful substances, and the harmful substances produced by microbial decomposition of manure make it impossible for fish to survive in the river section with dense hippos.

When this problem is clarified, the mystery of the collective death of fish in the lower reaches of the river is solved. Every time the collective death of fish in the downstream occurs after heavy rainfall, it will make the Mala River flood and speed up, and a large amount of feces deposited on the bottom will be washed downstream. The density of feces downstream will increase, and the demand for oxygen content by microorganisms will increase, resulting in hypoxia in water. At the same time, the harmful substances in the process of manure decomposition will make the fish that are already anoxic worse, and finally a large number of fish will die.

Therefore, although animal manure can be used as a supplement to aquatic ecology, once the amount is large and it is not fermented, it will change from "nourishment" to "poison", reduce the oxygen content in water and produce some harmful substances.

Although more than 4,000 hippopotamus droppings are a disaster for some fish in the Mara River, as we said at the beginning, animal droppings are a natural fertilizer, and hippopotamus droppings are no exception. According to the researchers' analysis of hippopotamus feces in Mala River, they are one of the main nutritional sources of aquatic plants and algae in Mala River. Usually, some undeposited feces will drift with the water and be decomposed by microorganisms, becoming nutrients for aquatic plants and algae.

At the same time, the decomposed feces will also be directly used by fish and other aquatic animals. Because there is more feces in the water and the water is more fat, aquatic animals, especially fish, grow and reproduce faster, and the death of fish caused by hippopotamus feces is less than that of fish after nutrition supplementation.

Moreover, the main cause of fish death is lack of oxygen, so such dead fish can also be eaten. Whenever there are a large number of dead fish, many scavengers will come to the water's edge to eat them at will, so the feces of hippos have become an important part of the ecology of the Mara River, and its contribution is far greater than before.