Kinosaki Marine World in western Japan said Tuesday that all its 19 dolphins have been on a low-fat diet since late August, when they started failing to hit jumping targets and keep upright while treading water.
"We were puzzled by their poor performance, then we noticed they looked rounder," said park spokesman Haruo Imazu.
Keepers measured their weight and found all had gotten plumper, some up to 22 pounds heavier just during the summer.
All had the same menu - about 31 pounds of mackerel mixed with some white fish - but keepers found the mackerels had gotten fattier, adding too many calories for the dolphins.
Keepers immediately put them on a weight loss program, feeding them more white fish and less fatty mackerels, while instituting a routine exercise regime, Imazu said.
Less fat and moderate exercise seemed to be working, he said.