Comparison of gillnet and trap in relation to retention of salmon lice (Lepeophtheirus salmonis), size selection of sea trout (Salmo trutta) and catch efficiency

Abstract

Salmon lice (Lepeophtheirus salmonis) are marine ectoparasites commonly found on salmonid species, where the infection levels on wild salmonids have dramatically increased the latest decade. Sea trout (Salmo trutta) is being used in salmon lice monitoring studies (e.g. NALO) where different sampling gears are used. The interaction between sea trout and lice is complicated, and therefore methodology studies are highly important to see how much fishing gear can affect estimates of prevalence, abundance and intensity of salmon lice on wild sea trout. It is also important to know the size selection of sea trout in different gears, because different size groups can experience different probability of being infected with lice. This is a methodology study that has three main objectives; (1) Elucidate the impact of fishing tools in relation to estimates of prevalence, abundance and intensity of salmon lice. (2) To see if the trap is size selective in catch, and (3) look into the efficiency of gillnets and trap. The study was conducted in Bjørknesvika on the southwest coast of Norway, and fish were sampled by using gillnets and a trap every other day. The prevalence of salmon lice on wild caught sea trout was not statistical significant between fishing gears and was on average 71% in gillnets vs. 75% in the trap. The mean abundance was significantly higher on sea trout caught in the trap than in gillnets (mean = 18 vs. mean = 7.9, respectively). Sea trout caught in gillnets were significantly smaller than sea trout caught in the trap (191.4 ± 75.6 mm vs. 205.2 ±71.6 mm, respectively). Gillnets were most efficient, representing 81,2% of the total catch where 15 and 17,5 mm mesh sizes caught 89 % of the gillnet catch. To conclude, the study showed that gillnets have significant impact on estimates of abundance and intensity of salmon lice, especially on pre-adult and adult lice by scraping away individuals. The study also showed that there is a probably that the trap are slightly size selective of sea trout, but it needs more investigation. The sampling gear in NALO are not yet ideal for the purpose and future modifications and investigations should be done to find a gear that is efficient, easy to deploy and handle, not size selective in catch and does not induce loss of salmon lice.