A drop in the number of hippos has led to a reduction in phytoplankton in African freshwaters, and has caused a decline in fish populations.
According to a report from AllAfrica.com, the hunting of hippopotami for their meat is responsible for a change in the composition of the water at Kampala, East Africa, and this has led to a reduction in the catch of food fish.
Hippos spend much of their time in the water, and while they are there, they excrete and urinate in the water, leaving nitrogenous wastes which provide a food source for phytoplankton at the bottom end of the food chain.
When the hippo wastes aren't present in the same numbers, the primary productivity from phytoplankton goes down, which means that there are less zooplankton, and therefore, fewer fish.
Andrew Alio, a senior fisheries officer with the Aquaculture Fisheries Resources Department in Kampala told AllAfrica.com:
"The continuous killings of hippopotamus explain the reduction of natural fish food known as planktons and the consequent reduced fish catch.
"The presence of planktons, algae and detritus is shown by the water changing its colour to light green. These planktons are rich in carbohydrates, fats, minerals, salt and proteins that the fish need."
Alio says that areas in which hippos are found tend to have lots of fish, with omnivorous and herbivorous species such as tilapia multiplying, and carnivores, such as Nile perch and bagrid catfishes moving in to predate on the other species.