Trap design for catching fish unharmed and the implications for estimates of sea lice (Lepeophtheirus salmonis) on anadromous brown trout (Salmo trutta)

► The trap captured all size classes of sea trout without net-induced mortality. ► Gillnets removed significantly more adult sea lice than the trap. ► This trap is an appropriate tool for ectoparasites assessments. The decline in stocks of sea trout, i.e. anadromous brown trout (Salmo trutta L.), on the west coast of Norway has given rise to a series of conservation measures and highlighted the need for a non-destructive capturing method. For this reason, a trap was developed for use in marine environments, with the overall goal of capturing sea trout unharmed and obtaining accurate assessments of sea lice (Lepeophtheirus salmonis) infection levels. The trap design captured all size classes of sea trout, and a variety of other coastal fish species, none of which suffered net-induced mortality. The trap was used to compare the abundance and intensity of sea lice on sea trout captured in that particular trap, as opposed to gillnets. The abundance of adult sea lice on trout caught in the trap was significantly higher than on trout caught in gillnets, but not significant for larval sea lice. The intensity of sea lice was only significant for the total number of lice (adult and larvae). These results indicate that estimated infection levels for trout captured by gillnets, which are frequently used in assessment programmes for sea lice, are likely to be underestimated. Consequently, the use of this particular trap can be recommended for the capture of sea trout for sea lice infection assessments. In general, the trap is an appropriate tool for studies where live, unharmed, coastal fish are required for research or conservation purposes.