it's part of a nutria's diet. but the shellfish is crucial to the reproduction of these fish known as bitterling, they're in danger of extinction. they lay their eggs in ishigai shells. the shell harbors the eggs and later the fish. >> translator: the empty shells are concentrated in this area. >> reporter: norihiko has been observing the river's ecosystem for many years. he says that nutrias eat more shellfish during the winter when there are fewer plants. more than 90 out of about 100 empty shells found in this area were ishigai shells. >> translator: the young fish live in the shells during the winter. and i'm very worried that they will be devoured. >> reporter: nutria not only destroy the spawning ground for the bitterlings, but they also kill the young fish. >> translator: i think we need to be responsible, and take measures to restore the ecosystem. because these alien species were introduced by us human beings. >> reporter: the damage keeps growing. the government will study the problem further and draw up

it's the work of an invasive alien species known as nutria. they live in ponds and slow moving rivers. they grow to 80 centimeters in length. they were brought to japan from south america in the 1930s for their fur. some were set loose in the wild. they began to set up colonies mainly in western japan. they mostly eat plants that grow on river banks. they also consume rice plants and vegetables in fields as much as a million dollars worth each year. despite the effort of local government to capture the animals, their numbers keep growing. but in recent years they have expanded their diet to shellfish. you are viewing the first footage ever captured of one eating clams at night. this was digging up clams in the sand with its paws. he ate nonstop for two hours. why did they start eating clams? >> it's entirely possible their diet changed because shellfish were easily attainable. so they shifted to a diet richer in calories. >> it's reached a stage where they are upsetting the ecosystem even more. the yodo river is home to many fish and shellfish