COMPARATIVE SUSCEPTIBILITY AND HISTOPATHOLOGY OF THE RESPONSE OF NAIVE ATLANTIC, CHINOOK AND COHO SALMON TO EXPERIMENTAL-INFECTION WITH LEPEOPHTHEIRUS-SALMONIS (COPEPODA, CALIGIDAE)

The comparative susceptibility of naive Atlantic Salmo salar, chinook Oncorhynchus tshawytscha and coho Oncorhynchus kisutch salmon to infection with the economically important marine ectoparasitic copepod Lepeophtheirus salmonis was investigated under laboratory conditions. Coho salmon were the most resistant to infection followed by chinook then Atlantic salmon. Copepods were lost from the gills of coho salmon by 10 d post-infection and only a few remained on the fins at 20 d post-infection. Although their abundance declined significantly, copepods were retained on both the gills and fins of chinook and Atlantic salmon over the 20 d studied. Rejection of L. salmonis on all 3 host species appears to be due to non-specific host responses. Histological sections of coho fins and gills revealed well-developed epithelial hyperplasias and inflammatory responses to the presence of L. salmonis. Gill and fin tissue responses of chinook salmon to L. salmonis appeared to be intermediate in intensity between those of coho and Atlantic salmon. Only minor gill and fin tissue response to the presence of L. salmonis was observed in Atlantic salmon.