Salmonid species (salmon, trout, char, et al) carry nematode parasites that can infect humans. Before salmonid sushi can be eaten, at least in the US, it is required to be frozen, killing the nematodes. Large trout species like rainbow trout are sometimes served as sushi -- I imagine species size and susceptibility to freezing damage factor in too.

Other freshwater fish like sturgeon are also served as sushi.