Investigation of the Differential Susceptibility of Rainbow Trout (Oncorhynchus mykiss), Coho Salmon (Oncorhynchus kisutch) and Atlantic Salmon (Salmo salar) to Infection with Sea Lice, Lepeophtheirus salmonis (Copepoda: Caligidae)       Mark D. Fast1, 2, John F. Burka1, Stewart C.Johnson2 and Neil W. Ross2   1University of Prince Edward island - Atlantic Veterinary College; 2National Research Council of Canada - Institute for Marine Biosciences       Biochemical and immunological parameters of the mucus and head kidney, were compared in rainbow trout (Oncorhynchus mykiss), coho (Oncorhynchus kisutch) and Atlantic salmon (Salmo salar) following infection with sea lice, Lepeophtheirus salmonis. Despite the same initial intensity of infection, a significant decrease in lice density over time was only observed in coho salmon.  Lice development was slowest in coho salmon followed by rainbow trout and then Atlantic salmon.   Both infected Atlantic salmon and rainbow trout exhibited increases in mucous lysozyme at 1 day post infection (dpi).  Mucous lysozyme also increased in infected coho salmon at 21 days post infection (dpi). Sea lice derived low molecular weight (LMW) proteases increased in infected rainbow trout (at 21 dpi) and Atlantic salmon (at 14, 21 dpi).  Increases in LMW proteases were accompanied with decreases in head kidney macrophage phagocytic capacity and respiratory burst in both infected rainbow trout and Atlantic salmon.  Lice development and infection densities on the host, as well as host immunosuppression suggest that Atlantic salmon are the most susceptible species to lice infection followed by rainbow trout and coho salmon, respectively. To determine the response of lice to the mucus of different species, live lice were incubated in the presence of fish skin mucus or seawater. Lice secreted LMW proteases more in the presence of rainbow trout and Atlantic salmon mucus more consistently than in the presence of coho salmon mucus, flounder mucus and seawater.  The resistance of coho salmon to lice infection may be related to factors in the mucus that inhibit LMW protease secretions from the louse. Further work must be done to determine a factor in coho salmon mucus that blocks these secretions, or possibly, a factor in susceptible species that stimulates lice secretions.