Special-Publication-Aquaculture-Association-of-Canada 5 46 49 St. Andrews, Canada: Aquaculture Association of Canada

Sea lice (Lepeophtheirus salmonis, Kroyer) are a major pathogen of farmed Atlantic salmon (Salmo salar). To obtain a better understanding of the host-parasite relationship, we compared protease secretions of L. salmonis into mucus collected from susceptible hosts (Atlantic salmon and rainbow trout, Oncorhynchus mykiss), a resistant salmonid (coho salmon, O. kisutch) and a non-salmonid (winter flounder, Pseudopleuronectes americanus). Protease zymography of mucus from these fish following incubation with live lice revealed the presence of L. salmonis-derived low molecular weight proteases more frequently in Atlantic salmon and rainbow trout, compared to coho salmon or flounder (P<0.05). The differences in L. salmonis protease secretion in response to mucus from different host species may partially explain why coho salmon are more resistant to infection.