Behavioural differences of Atlantic salmon (Salmo salar) and whitefish (Coregonus lavaretus) as the basis for improving the species selectivity of whitefish trapnets

Studies were undertaken in the Gulf of Bothnia, Finnish coast, to reduce the selectivity o f whitefish trapnets for Atlantic salmon (Salmo salar L.). Trapnets for whitefish (Coregonus lavaretus s. str. L.) cannot be used during the spring spawning migration of Atlantic salmon because of the high by-catch of salmon. Several modifications to the traditional trapnet were tested. The nets were modified to catch whitefish and reject Atlantic salmon on the basis o f differences in behaviour o f the two species. One particular modification proved highly selective: a prohibiting net constructed o f large- diameter twine was hung in front o f the wings of the trapnet. Migrating salmon swim parallel to the surface and avoid the trapnet as they sense the prohibiting net. At this time o f the year the whitefish approach the trapnet as they are eating benthic gastropods. The selectivity of the traps for whitefish is not reduced by the presence of a prohibiting net. U se of the prohibiting net reduced the by-catch of salmon by 62% (by weight) but did not affect the catch of whitefish.ailable.