What causes fish wounds and scars?

Skin is missing from top of walleye's head. Photos by Larry Pons

Every once in awhile you catch a fish with a gash in its side, a fin missing or a sore of some kind. What causes these things?
It's a dangerous world if you are a fish. There are predators everywhere: other fish, bald eagles, ospreys, otters and people. Often you can make an educated guess about what happened.
It is unusual to see skin missing from the top of a fish's head. If it had a torn lip or gill, you could surmise it was the result of a fisherman. But the wound right atop the head in the photo above makes me think this walleye had escaped from a big northern pike. The entire top of a pike's mouth is a mass of small teeth, all pointed toward the throat. When a pike catches its prey it usually first grabs it in the middle and after a period of time, lets go and grabs it head first.
Most fish swallow their prey headfirst because the fins all collapse as it goes down the throat.
Somehow, this fish may have wriggled loose from the vise-grip-like hold but lost its scales in the process.
A wound that is more frequent is the gash type on the side, shown in the bottom photo. This is most common on northern pike. In this case, the wound has healed entirely, just leaving a scar, so it could have occurred years ago. Many of these types of wounds either come from the pike thrashing in the shallows while spawning or from fighting with another fish. Northern pike become territorial once they reach a certain length, about the mid-30 inches. They will defend their bay or cove against other similar-sized fish.
Sometimes you catch very large pike -- 40 inches or bigger -- with a fresh scar on its side or back. This will most certainly be a territorial fight scar. Nothing is trying to eat a 40-inch pike!
You also see fish with a rash that encircles its body, often right behind the gills. This is the telltale wound caused by fishermen who handled the fish either with dry hands or with a very rough glove. You should always either wet your hands or better, use a wet, soft, plain cotton glove when handling fish that you intend to release. This prevents your stripping away the slime layer on the fish's skin that protects it from viral and fungal infections.
Small red holes in the skin are almost certainly bloodsucker wounds. Incidentally, bloodsuckers that get on live fish are about the size of a pea. The long lake leeches you see swimming around the shoreline only feed on dead fish.
A puncture wound on the side, especially if it is matched by a puncture on the opposite side, was likely made by bald eagles or ospreys.
Chunks of flesh missing from the face of a fish could have come from an otter. This would especially be likely if the fish was small.

Jason Pons with a nice-size pike with an old wound on its side

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