The effect of sea lice infestation on the salinity preference and energetic expenditure of juvenile pink salmon (Oncorhynchus gorbuscha)

Ocean-going juvenile salmonids heavily infected with salmon louse, Lepeophtheirus salmonis, have been observed prematurely returning to freshwater. This change in salinity preference may be an attempt either to regain osmotic balance or to remove the lice. For either hypothesis to be true, freshwater habitats must provide infected fish with a higher net fitness than saltwater habitats. The objectives of this study were to use behavioural titration to quantify the energetic cost of different salinities to infected and uninfected pink salmon (Oncorhynchus gorbuscha) and to determine if infection alters salinity preference. Results demonstrate that infection changes the salinity preference of fish from saltwater to freshwater. The cost paid by these freshwater-preferring infected fish foraging in saltwater increased with lice density during trials conducted between 13–33 days after infection. Other infection-induced behavioural changes include a 14-fold increase in the jumping frequency of infected versus control fish and a decrease in foraging between 13 and 33 days after infection.