I feed my Yorkie fish all the time. He loves it! He has had shellfish also with no problems (crab meat, shrimp).
I WOULD NOT feed the fish to them RAW. When I was last at the vey, I read in a pamphlet about raw fish leading to some disease. They say you should package up fish trash very well so that the dog cannot get into it.
Canned tuna and salmon are cooked to a certain degree. Here is a link that I found. But I will admit I do feed him some of this stuff, like potatos *he loves them.
http://www.peteducation.com/article....articleid=1030