Experimental transmission, development, and effects of a parasitic copepod, Lernaeocera branchialis, on Atlantic cod, Gadus morhua.

Development and effects of adult Lernaeocera branchialis were studied following experimental transmission to Atlantic cod, Gadus morhua. Growth from the detection of pennella stages to mature adults was approximately 9-10 mo (September-June of the following year) at which time eggs were released and the adult parasites degenerated. Cod from all size groups were susceptible but the prevalence of infection was greater in small fish. Peak mortality, about 30%, occurred within 4 mo after infection and was greatest in young fish with multiple infections. Death was associated with emaciation, blood loss, open lesions, and probable occlusion of branchial blood vessels and/or ventral aorta. Infected fish, especially subadults that survived, consumed less food, gained less weight, had a lower conversion factor, and were substantially smaller than uninfected controls through a 32-wk period. Many of these fish displayed hyperactivity. Large cod that survived the infection and harboured 1 parasite were apparently unaffected but died when stressed. A previous infection conferred no protection against reinfection. Multiple infections delayed gonadal maturation and resulted in significantly lower gonadal somatic indices than in controls. Exposure of infected fish to crude oil fractions or to infection with a hemoprotozoan, Trypanosoma murmanensis, culminated in mortality, weight loss, or low organ somatic indices. It is estimated that considerable losses through mortality and weight gain in young fish occur each year in coastal areas from infections by L. branchialis, particularly in one area where it was estimated that 20% of the population was infected.