The semi-aquatic mammals died in the Gibe Sheleko National Park, part of the Gibe River, local broadcaster FANA said.
Behirwa Mega, head of the park customs FANA that the animals died between April 14 and 21 and The cause of their deaths is unknown.
The Gibe Sheleko National Park, only established in 2011, is reportedly home to about 200 hippos and covers approximately 36,000 square kilometers in land area.
The IUCN estimates the global population of hippos is between 115,000 and 130,000 and that their conservation should be a "priority" in countries where they exist.
Hippo populations are threatened by poaching, disease, loss of habitat, deforestation, and pollution, according to experts.
They are hunted by poachers who export their long canine teeth from African countries to places such as Hong Kong and the United States where they serve as substitutes for elephant tusks, says the wildlife trade monitoring network TRAFFIC
More than 200 hippos were killed in a massive anthrax outbreak at Namibia The Bwabwata National Park in 2017.
And the hippo population in Africa will face a significant reduction when scheduled culling of the animals in Zambia begins in May despite objections from animal rights groups.
The cull will happen in the Luangwa River Valley in Zambia's Eastern province, the Department of National Parks & Wildlife said in February.