Optimal response to lousy circumstances: The impact of salmon lice (Lepeophtheirus salmonis) on depth preference of sea trout (Salmo trutta)
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Sea trout are known for seeking out sources of freshwater to rid themselves of salmon lice. Still, the effect of natural haloclines in fjords on parasite dynamics is not well understood. We tagged 48 wild caught sea trout, naturally infested by varying number of lice, with individual depth sensors. The fish were kept inside a small net-pen (4x4x5m) in Western Norway during four periods in spring 2017. The aim was to investigate how trout respond to salmon lice by changing their depth according to a natural halocline, and further elaborate on how this behaviour ultimately impacts their parasite abundance. The results show that temperature and light were the two most important factors explaining the vertical behaviour of trout. Mobile lice also had a significant effect on depth preference, where fish with higher abundances choose to swim shallower. However, individual variation in depth preference was larger than the impact of infestation levels, with some individuals choosing to stay deeper (and more saline) even though they had a high number of lice. There was a substantial reduction in salmon lice abundance during the seven days in the pen (68 ± 58 to 35 ± 18). The number of attached lice declined more rapidly when the temperature was high, most likely because of higher recruitment to mobile stages. Furthermore, the number of mobile lice showed a more substantial reduction when surface salinity was low. Surface salinity explained this reduction better than the experienced salinity of the individual. In summary, the results indicate that short-time exposure to very low salinities, rather than long-term exposure to moderate salinities, is the driving force behind the use of haloclines for delousing purposes.