TOKYO—Fatty food isn't a problem just for humans. A Japanese aquarium is putting its dolphins on a diet after their portly figures got in the way of their water tricks.

"These days animals at the sealand are a bit ... fat!!!" a zookeeper confided on the website of Kinosaki Marine World in western Hyogo prefecture.

Audiences have apparently noticed, with the dolphins failing to jump into the air as high as before, he said.

"But we ask you, please be lenient with them as they are making efforts to lose weight," he said.

The problem lies in their diet of mackerels, which are popular at this time of year in Japan. Mackerels are known in Japan for their rich taste but also their high fat content.

"We are trying to make them return to their best weight by feeding them other kinds of fish with less fat or by offering fewer mackerels," the zookeeper said.

The marine park offers audiences shows by dolphins, sea lions and other marine animals.

Dolphins are popular attractions in Japan, even though the country has come under fire abroad over the annual tradition of slaughtering thousands of dolphins for meat in the western town of Taiji.