Size selection of fish in the trap fisheries of the Baltic and Bothnian Seas

Sammanfattning: A sustainable fishery in the Baltic Sea requires fishing gear that fishes selectively and at the same time excludes raiding seals. A successful type of trap recently developed in response to the seal problem is the pontoon fish chamber, which significantly decreases damage to gear and catch losses caused by grey seals. However, a common problem with traps is the bycatch of juvenile and non-marketable fish which constitutes a threat to the sustainability of the fishery and a time-consuming problem for the fishers. This thesis deal with bycatch reduction of young herring, whitefish and perch in pontoon traps. Rigid grids and square mesh panels were installed in traps during commercial fishing operations and continuously monitored with underwater cameras. The selection efficiencies were calculated for different species and selection panel designs. The importance of abiotic and biotic factors for the selection efficiency, the diurnal activity levels of species, and the positions of escape through a selection panel were analysed. This thesis also addresses the survival changes of herring after being released from a trap and the potential size-structuring effects on the herring stock after a size-selective fishery. The results showed that several tonnes of young fish were able to escape through selection panels from the traps during a season. 70-80% of young herring and whitefish escaped through an encircling selection panel while 90-100% of young perch and roach escaped through a rigid grid. Both biotic and abiotic factors were influencing the selection efficiency of herring. The factors which had most effect were the quantity of fish in the trap, the season of the year, the time of day, and the presence of seals. The diurnal activities were significantly different between species. Herring and roach preferred to escape during night while perch escaped mostly during dusk and dawn. The passing through a rigid grid did not affect the short term mortality of young herring and the risk that extensive use of traps will induce selection for phenotypic changes in mature herring, leading to an evolutionary change on the Bothnian Sea herring population is low. The overall conclusion of this thesis is that bycatch can be reduced by equipping traps with selection panels. The appropriate design of the selection panel depends mainly on the behaviour and physiology of the fish. The survival changes of released fish seem high and the risk that extensive use of size-selective traps will induce evolutionary changes on the herring stock is low.