Parasites of Fishes in the Vicinity of Steller Sea Lion Habitat in Alaska
Fish serve as intermediate hosts for a number of larval parasites that have the potential of maturing in marine mammals such as Steller sea lions (Eumetopias jubatus). Members of the Auke Bay Laboratories (ABL) Habitat Assessment and Marine Chemistry Program examined the prevalence of parasites from 229 fish collected between March and July 2002 near two islands used by Steller sea lions in Southeast Alaska and four island habitats in the Aleutian Islands. Even though the fish samples near the Southeast Alaska haulouts were composed of eight species of forage fish and the Aleutian Islands catch was dominated by juveniles of four commercially harvested species, the parasite species identified from the fish were similar at all locations.
Twenty-two species of parasites were found in the fish from the four Aleutian Islands habitats: two protozoa, one monogenetic trematode, seven digenetic trematodes, four each of cestodes and nematodes, and two each of acanthocephalans and copepods. Eleven of the 20 parasite taxa identified were in their larval stage in the fish hosts; several of these larval stages have been described from mammalian final hosts.
Four species of parasites were statistically more prevalent (P < 0.05) in the Southeast Alaska samples: Pleistophora sp., Lecithaster gibbosus, Tubulovesicula lindbergi, and Phyllobothrium sp. In contrast, seven species of parasites were statistically more prevalent in the Aleutian Island samples: Steganoderma formosum, Abothrium gadi, Anisakis sp., Hysterothylacium aduncum, Pseudoterranova sp., Corynosoma sp., and Echinorhynchus gadi. The larval nematodes Anisakis sp. and Pseudoterranova sp. were found in 95% and 54%, respectively, of the Aleutian Islands samples and are potentially pathogenic to marine mammals.
These results suggest that a few of these parasites may serve as sources of indirect information about the habitat of the fish hosts. The greater prevalence of the two species of larval nematodes in the Aleutian Islands probably reflects the greater number of final hosts (sea lions), and changes in host intensity of these two species of nematodes often mirror changes in the density of marine mammals. The larval acanthocephalan Corynosoma strumosum was present in 20% of the Aleutian Islands samples but was detected in less than 3% of the Southeast Alaska samples, suggesting that amphipods are a less prevalent part of the diet among benthic-feeding fishes in Southeast Alaska. The protozoan Pleistophora sp. was present in significantly greater proportions among walleye pollock, Theragra chalcogramma, (21%) and Pacific cod, Gadus macrocephalus, (23%) from Southeast Alaska, in contrast to 5% and 0% among these fish species in the Aleutian Islands, and may serve as a stock separation marker for these fish. This is the first time most of these host-parasite associations have been reported in Alaska waters.
