Physiological responses in sea trout to repeated salmon louse infections and freshwater

Sea trout (Salmo trutta) migrate to the seawater (SW) for increased food availability. However, heavy infestations with salmon louse (Lepeophtheirus salmonis) can make them return to freshwater (FW). The aim of the present study was to map if and how reinfection with salmon louse and repeated FW exposure affects survival, growth rate, hepatosomatic index (HSI), acid base regulation (plasma pH, strong ion difference), osmoregulation (plasma ions, osmolality) and semen quality (fertilization rate, embryo/fry survival) in sea trout. Individually tagged sea trout (~100 g) were infected with louse copepodids in SW and then switched to FW at the louse pre-adult stage. Twelve days thereafter, FW was replaced with SW, and a second similar louse infection and salinity change were performed. Treatment groups were (i) uninfected control, and infected during the first (ii), second (iii) or both (iv) infection periods. The study ended after a final three-month follow-up in FW involving egg fertilization with sperm of previously infected and uninfected control mature male trout. Lice infection did not affect fish mortality or semen quality, but elevated HSI. In SW, lice-infected fish had lower specific growth rate in weight, higher plasma pH, Na⁺, Cl^- and osmolality, and lower plasma strong ionic difference and Na⁺/Cl^- ratio compared to uninfected fish. After 48 h in FW, lice-infected fish still had higher plasma pH, while plasma Na⁺, Cl^- and osmolality were lower and plasma Na⁺/Cl^- ratio higher in infected than uninfected fish. Louse reinfection did not affect any end points compared to single infection. The results demonstrate that salmon louse disturbs sea trout's Cl^- more than Na⁺ regulation, resulting in reduced hypo-osmotic and hyper-osmotic abilities in SW and FW, respectively. Further, a strong effect of lice on acid-base regulation is evident, shown by elevated plasma pH in both SW and FW.