It may not be the same as the dog eating the fishes flesh but I would not take the chance.
The safest action is first get the dog to throw-up whats in its stomach. This is done by forcing it to swallow about 5 or 6 tablespoons of hydrogen peroxide. Hold its mouth open with one hand and pour the h-p down its throat. It can take 2 to 15 min for it to work. Next get it to the vet. I saved mine this way. I am very sorry this is posted so late but I just checked the board before hitting the sack. Good Luck