What are those little black spots?
“What are the black spots on the trout I just caught” is the most common question we hear this time of year. As soon as June arrives and anglers all over the State hit their favorite fishing hole, trout with “those black specks” start showing up. Here is some good information from former IF&W biologists to answer common questions such as: What is it? What causes it? Can I eat it?
“Black spot disease” or “black grub” in trout is caused by a trematode worm in its larval, or immature, stage. The tell-tale sign is small, sand-grain sized black spots on trout and other fishes. The level of infestation can range from just a few, barely noticeable spots to heavy with the fish nearly covered as in the large trout in the photo.
Heavy (larger trout) and low black spot infestation in brook trout from a northern Maine pond.
The species of trematode worm that causes black spot uses snails and fish as intermediate hosts in their immature stages. Loons are the most common host for the adult worm living in the bird’s mouth and producing eggs that are eventually passed through the loon’s gut and into the water. The eggs mature and produce ciliated miracidia that penetrate snails and other molluscs (the first intermediate host). The worm develops into cercariae within the snail. As trout ingest and digest infected snails, cercariae become mobile, penetrating tissues of the second intermediate host (the trout). The mechanical damage caused by this mobile stage causes hemorrhaging. Once the cercariae become stationary in the fish’s skin it produces the visible scarring in a slightly raised black spot. The cycle is completed when a loon ingests an infected fish.
Anglers are usually most interested in whether trout with black spot are edible. Proper cooking kills the parasite and renders the fish completely safe to eat.