Early development of resistance to the salmon louse, Lepeophtheirus salmonis (Kroyer), in juvenile pink salmon, Oncorhynchus gorbuscha (Walbaum)

This study examined the effect of fish weight on the susceptibility of post-emergent pink salmon to Lepeophtheirus salmonis (Kroyer). Three trials were conducted, each with two stocks of pink salmon, Oncorhynchus gorbuscha (Walbaum), at starting weights of c. 0.3, 0.7 and 2.4 g, respectively. In each trial, duplicate tanks of fish were exposed to 0, 25 (only in Trial 1), 50 or 100 copepodids per fish. Mortality in Trial 1 was c. 37%, regardless of stock following exposures to 50 or 100 copepodids. Mortalities occurred up to 26 days after exposure, and more than 80% of the lice on the dead fish were chalimus stages. Infections with adult or preadult lice were observed on c. 35% of fish surviving to 37 days after exposure. Mortality was 5% in Trial 2 and there was no mortality in Trial 3. The abundance of L. salmonis was lower in Trial 3 compared with Trials 1 or 2. Histological changes in the skin coincident with fish growth included a thickening of the epidermis, infiltration of the dermis with fibroblasts by the end of Trial 1 and the first evidence of scales by the end of Trial 2; scales were evident throughout Trial 3. These results showed that the previously reported innate resistance to L. salmonis displayed by pink salmon develops in fish heavier than 0.3 g and appears to be functional by 0.7 g. This resistance coincided with changes to the epidermis and dermis, including the formation of scales. The present results indicate that elevated risk associated with L. salmonis infection among migrating post-emergent pink salmon may occur during a relatively brief period before the fish reaches 0.7 g.