Fungus. Probably saproligina to be exact. Its a secondary invader of a fish that has been attacked by bacteria due to the stress of spawning, injury, handling in cold water or other reasons.

It's common on salmon during their spawning runs. Here's an example.

http://www.salmonfishingforum.com/forums/showthread.php?p=61205

If pigs could fly bacon would be harder to come by and there would be a lot of damaged trees.