Yes, sometimes you can see them in the fishes fins. It sucks to catch a good fish that is infested with them. The Fish and Game claim the fish safe to consume if prepared properly, no sushi unless you like the dancing rice inside.
A lot of fish (not just gills) caught from smaller ponds have "worms". That is due to them eating the snails that infest ponds. The best solution to a snail infestation is to add shellcrackers to the pond. They will clean up the problem in a single season. And, I wouldn't eat one with worms JMHO :-).
I have run across this problem before also. They seem to be worse some years than others.
Our area fisheries bioligest stocked Red Ear in a strip mine lake with worm problems, and they were basicly gone in a couple years. It could have been part of a cycle ,but I am convinced that the Red Ear helped considerly.
If a fillet has a couple I use the knife point to remove them. If I see them in the fins when I catch one, I release it.
When I fillet one with the little black speck parasites, I consider them pepper. lol ole Mike
We see more grubby or wormy gills and sunnies in lakes and ponds full of weeds most of the time. That being said, if a fillet has only a couple I'll pluck them out with the tip of the fillet knife. If the fillet is loaded with them, and we have a whole mess of fish, that one sees the gut bucket unfortunately.
They are a encapsulated red colored worm, or in the process of becoming one. They are more common in perch. I have seen them appear outside the body of a living fish (walleye) and retract back into the flesh when disturbed. They are pretty repulsive, but not harmful to eat(cooked). I cut them out, or poke'em with a filet knife point, knowing they are there somehow reduces my appetite for that filet. Lance