What are the black spots on fish?

Minnesota Lakes Association

What About Those Fish With Black Spots?
Are fish with worms and black spots safe to eat, and if so, do they affect the taste?
July-August 2000

All fish have internal parasites. Some you can see; some you can’t. The black spots commonly found on sunfish, bass and perch are larval flatworms or flukes. The complex life cycles of these parasites involve living in the skin of the fish. The yellow spots, often seen under the skin of perch, are parasitic grubs. All these parasites die when their host is cooked. They don’t affect the fillet’s taste, and they are harmless to humans. Parasites are usually harmless even to the fish themselves, which have evolved with these creatures for thousands of years. Only when it is heavily infested with parasites will a fish weaken or die. This usually occurs when fish are stressed from high water temperatures, low oxygen levels or spawning. Reprinted for the DNR’s Fish & Wildlife Today