Live Freshwater Parasite,(Copepoda: Lernaeopodidae), on the Gills of an Ocean-Migrating Steelhead Trout () and Discussion on the Origin and Survival of the Parasite at Sea.

Abstract

is a parasitic copepod of freshwater salmonids in the North Pacific rim countries. Sixteen adult females of the species were found alive on the gills of an ocean-age 4, maturing steelhead trout,, caught in offshore waters (50°30'N, 179°30'W) of the North Pacific Ocean in July 1997. This is the first evidence of live individuals offrom ocean-migrating salmonids. When found, copepods were attached to the distal ends of gill filaments, and their bodies were observed to be slowly moving in Petri dishes with seawater. Ocean-migrating steelhead trout comprise individuals originating from western Kamchatka (Russia) and western North America. Based on the date and catch location of the infected fish, it is inferred that it originated from western North America, where it acquiredinfection in fresh water. As this fish spent about 4 years in the ocean, the copepods likely survived the same period at sea. However, if the fish was a kelt, the survival period of the copepods in the ocean may be shorter than four years. To confirm identification of the copepods, adult females ofare briefly described using the specimens collected from the fish.