Abstract
Sea trout are known for seeking out sources of freshwater to rid themselves of salmon lice. However, the effect of natural haloclines in fjords on parasite dynamics is not well understood. We tagged 48 naturally infested wild sea trout with acoustic depth sensors. The fish were kept inside a small net-pen (4 × 4 × 5 m), 12 at a time, in western Norway during 4 separate time periods in spring 2017. The sea trout were relatively highly infested with sea lice (prevalence: 100%, mean ± SD: 68 ± 58 lice fish-1), and a relatively large proportion of the individuals did not survive the trials (25% mortality). The results show that temperature and light were the 2 most important factors explaining the vertical behaviour of the surviving trout. Mobile lice also had a significant effect on depth distribution, where fish with higher abundances of lice were observed at shallower depths. During the 7 d periods in the net-pen, total sea louse abundance decreased from a mean of 68 to 35 fish-1. Surface salinity explained this reduction better than the experienced salinity of the individual fish, suggesting that short-time exposure to very low salinities, rather than long-term exposure to moderate salinities, is the driving force behind the effect of haloclines on reduction in sea lice numbers.
Highlights
Brown trout Salmo trutta is an adaptable species with a remarkable level of plasticity in relation to its life history strategy, both within and across populations (Thorstad et al 2016)
The observed data showed that the sea trout, in general, had a very narrow depth range, spending over 80% of their time above
The present study aimed to investigate the potential change in lice abundance on sea trout when only allowed to displace vertically according to the conditions in a natural halocline
Summary
Brown trout Salmo trutta is an adaptable species with a remarkable level of plasticity in relation to its life history strategy, both within and across populations (Thorstad et al 2016). Sea trout is a term for the individuals of brown trout that display an anadromous life history strategy (Thorstad et al 2016) They spawn in freshwater, and the juveniles remain there for 1−8 yr during the life stage called parr. Parr are somewhat tolerant to saltwater but cannot tolerate full-strength seawater Before undertaking their marine feeding migration, they undergo a series of physiological, morphological, and behavioural changes, called smoltification (Thorstad et al.2016). In line with their anadromous life cycle, usually, both smolts and adult individuals of sea trout emigrate from their rivers in early spring and return to their rivers to spawn or overwinter either the first, second, or third autumn following emigration (Thorstad et al 2016)
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