Pink salmon (Oncorhynchus gorbuscha) osmoregulatory development plays a key role in sea louse (Lepeophtheirus salmonis) tolerance

Sea lice (Lepeophtheirus salmonis) of fish-farm origin have been implicated in reducing populations of pink salmon (Oncorhynchus gorbuscha) in British Columbia's Broughton Archipelago. Owing to the physically disruptive nature of louse attachment to fish skin in a hyperosmotic environment, we hypothesize that the impacts on fish performance are ionoregulatory in origin. Therefore, ionoregulatory status was measured in juvenile pink salmon artificially infected in the laboratory and naturally infected in the wild. Body [Na(+)] of laboratory-infected fish (similar to 1 week seawater (SW); 0.2-0.4 g) increased significantly by 12% with a single chalimus-4 louse, and by 23% with 2-3 chalimus-3 lice. Mortality over this 24-day trial was 2.4% for fish initially infected with 1-3 lice. Body [Na(+)] for fish caught with natural infections (similar to 4-12 weeks SW; 0.5-1.5 g) did not differ from uninfected controls. Combining data sets revealed a no effect threshold of 0.5 g for body [Na(+)] of fish infected with one chalimus-4 louse. We propose that this size-related louse tolerance is associated with hypo-osmoregulatory development, adding to a previously suggested multifactorial mechanism based on epidermal and immune system development. We suggest management bodies consider this fish-mass threshold when planning to minimize risk to wild fish populations.