Prevalence and abundance of Lepeophtheirus salmonis (Copepoda: Caligidae) on high-seas salmon and trout in the North Pacific Ocean.

The occurrence of the salmon louse Lepeophtheirus salmonis on Pacific salmon (Oncorhynchus nerka, O. keta, O. gorbuscha, O. kisutch, O. tshawytscha) and steelhead trout Salmo gairdneri caught in the central North Pacific Ocean was investigated. The parasite occurred on all six species of salmonids, but there were marked differences in prevalence and abundance among the host species. Oncorhynchus tshawytscha was the most heavily infested with the parasite followed in descending order by S. gairdneri, O. gorbuscha, O. keta, O. kisutch, and O. nerka. Based on the population size of L. salmonis on each host species population, O. gorbuscha and O. keta were regarded as being the most important hosts in the ocean. Approximately 90% of the total parasite number occurred on these species. In heavy infestations, parasites were often found in clusters on the skin near the anal fin and caused damage to the skin. The prevalence and abundance increased with host size and ocean age.