Parasite Identification in Rockfish

Type: Chesapeake
Region: NA
Location: Chesapeake Bay

Everyday we receive questions from Maryland fishermen on a wide variety of subjects here at the Fisheries service. Recently fisherman Paul Puher sent in this question and it proved worth sharing with fellow fishermen.I recently caught a rockfish that looked very healthy. I filleted it and saw these black spots deep in the meat. I cut thought it, and it appeared to be a dead something. Can you please tell me what these things are; should the fish be thrown out, is this normal?

We went right to the leading expert on this subject, fish health biologist Mark Matsche at the Cooperative Oxford Laboratory. The black spots (arrows) are the larval form of a parasitic worm known as digenean trematodes in the picture below of a striped bass fillet. This infection is often called �black spot disease�, and in some fish, particularly small individuals, the worms may be visible through the skin. There are many different species of digenean worms, and most are white or yellow in color. The black appearance of the worms is a result of pigments that may accumulate around the parasites as part of the fish�s immune response. This black pigmentation of the worms doesn�t always occur, and the spots may appear white or yellow (�white spot� or �yellow spot� disease). Most species of fish may be susceptible to digenean infections. Digenean worms have a fairly complex life cycle, which involves aquatic snails or other invertebrates as the initial host; fish as an intermediate host; and mammals, birds or other fish as the final host; the intermediate form of the worm penetrates and burrows into the flesh of fish. While most digenean parasites are not dangerous to humans. When few in number, black spots can be trimmed from the fillets and thorough cooking will kill any remaining worms.