"Black-spot disease
These parasitic flatworms appear as tiny black spots on the skin, fins and flesh of fish. No method of control is available for the elimination of this problem. This organism does little harm to the fish. The main problem associated with black-spot is the unsightly appearance it may cause. Skinning infected fish will remove most black spots.
The life cycle of the parasite is quite complex. A fish-eating bird (typically a great blue heron or kingfisher) eats an infected fish. The black spot or worms are released and grow to sexual maturity in the bird's intestine. The adult worms pass eggs with the bird's droppings. When the eggs reach water, they hatch into free-swimming organisms which then penetrate snails for further development. Finally, after leaving the snails they burrow into the skin of fish and form a cyst. The fish surrounds the cyst with black pigment that gives the disease its name. If an infected fish is consumed by a bird, the cycle repeats itself."
https://www.in.gov/dnr/fishwild/3395.htm
