Parasite load, morphology, fecundity, and genetic structure of the salmon louse, Lepeophtheirus salmonis, parasitizing three species of pacific salmon

Background/Question/Methods

The purpose of this study was to compare the morphology, fecundity, and genetic structure of (Lepeophtheirus salmonis) as they parasitized three different species of Pacific Ocean salmonid.  Individual chinook or king salmon (Oncorhynchus tshawytscha),  silver or coho salmon (O. kisutch ), and  pink or humpy salmon (O. gorbuscha ) were collected within the western end of the Strait of Juan de Fuca (Washington, USA)  using sportfishing techniques during August and July of 2009 and 2010.   At the time hosts were brought to the side of the boat, host length and mass were measured and all of the parasites were collected from each host and stored in 70% ethanol.   The body length, cephalothorax length, cephalothorax width, and the number of eggs per adult female of individual parasites were later measured.  Lastly, 14 individual hosts were selected at random from each of the host species, and from the adult parasites removed from each of the selected hosts, one individual adult Lepeophtheirus was randomly selected for genetic analysis.  The genotype of each louse was determined at each of five polymorphic  microsatellite loci.  Individual lice were then pooled by host to create host-specific “populations.”   Finally we examined among-host  genetic  variation using Slatkin’s R_ST. 

Results/Conclusions

We observed mean values of parasite load on coho salmon were significantly lower (3.4 parasites/host) than chinook salmon (5.9 parasites/host) or pink salmon (5.4 parasites/host).  Although lice seemed to utilize coho salmon less frequently, we found that Parasite loads were equal among our three salmonid hosts when adjusted for fish body size (p = 0.023).  Interestingly, lice that infected chinook salmon were significantly smaller in total body length, cephalothorax length, and cephalothorax width when compared to lice that infected coho salmon or pink salmon (p < 0.001 for all measures).  Further, L. salmonis produced fewer less eggs when infecting chinook salmon than when infecting coho (p < 0.001) or pink salmon (p < 0.001).   Microsattelite estimates of among population variation suggest little genetic difference in the Lepeophtheirus parasitizing chinook and coho salmon (R_ST = 0.0288).  In contrast, Lepeophtheirus parasitizing pink salmon were far more genetically dissimilar when compared to Lepeophtheirus parasitizing chinook and coho salmon (R_ST = 0.358 and 0.271, respectively).