Many different animals are blamed for perceived declines in fisheries, this is usually not the case. Here is what a few states DNR's have to say about river otters (these are the people who promote fisheries)

"Otters are opportunistic and will take foods that are most available. The most available fish tend to be the slower-swimming species and those that are most abundant. Although anglers are sometimes concerned that otters will have a detrimental effect on game fish, the fishing activity of otters has not been found to be a problem with sport fisheries. Food-habit studies have generally found game fish to be taken less often than the slower-swimming rough fish. Several authors suggest that otters may even improve a fishery by removing rough fish.
Otters have occasionally caused depredation problems at fish hatcheries and rearing areas. There, only game fish at artificially high densities are present, and the fish cannot escape capture Nebraska Game and Parks

They are a boon to fishermen and prey primarily on non-game fish especially slow moving fish, bottom dwelling rough fish, and secondarily on fish in abundance or in large schools. Crayfish are also important in the diet. Virginia Fish and Wildlife

A quick google search will show a number of studies that show otters feeding primarily on carp and suckers. In my experience, the lakes here in Minnesota that haqve otters usually have better fishing than those without. Come to think of it the lakes with otters are happen to be the ones without very many people.

Kevin