The release of unwanted fish from purse seines whilst still in the water is termed slipping and may lead to significant mortality following release. The objective of this study was to determine the fish welfare implications of a new slipping methodology in which fish are released via a discharge opening formed in the bunt end of the purse seine net. Video analyses of collective and individual level fish behaviour were undertaken in the Norwegian mackerel and herring purse seine fisheries, to quantitively describe slipping behaviour and to determine its driving factors. The majority of fish escaped the purse seine with the schooling structure intact as part of large groups towards the end of slipping process, increasing their speed following escape. However, there was also a tendency (24% of all escapes) to escape in a manner likely to impact negatively upon their welfare, with a breakdown in schooling structure and physical contact with the fishing gear and conspecifics. The tendency to express such welfare compromising behaviour was higher for mackerel than for herring, but was also influenced by the vessel releasing the fish, the amount of fish being slipped, how long the discharge opening had been open and the particular slipping event. These results provide important information for future science-based development of welfare friendly slipping practises.

Blue whiting (Micromesistius poutassou) is harvested in the Northeast Atlantic by a multinational fleet of pelagic trawlers. Occasionally, vessels take catches which exceed their remaining holding capacity and in extreme cases large catches cause codends to burst, resulting in spill of catch. To control catch quantity, a catch limitation system was developed and tested. The system consists of three components: 1) escape opening(s) in front of the codend to release excess fish, 2) a fish lock to prevent loss of fish through the escape opening(s) during haulback and at the surface, and 3) a choking unit to match codend capacity to the desired size of catch. Blue whiting escaped through both longitudinal slots and large (≥ 2 m) meshes in front of the codend. However, video observations showed that with large meshes in the upper panel, large amounts of blue whiting escaped long before the codend was full. Therefore, a design with large openings in the bottom was combined with longitudinal slots in the side and top panels. Two fish locks were tested: an oblique netting panel designed to seal off the codend when vessel speed reduce during haulback, and a cylinder of netting with a choking rope that closed it after the codend was filled. Both fish lock designs inhibited release of fish during haulback and at the surface, but the attachment of the constricting rope in the cylinder frequently broke. The choking unit consisted of a depth triggered releaser connected to a strap of rope wrapped around the codend. Mechanical releasers with factory-set depth trigger and electronic versions were tested. Both freed the choke point as intended, but occasionally at unpredictable depth. A reliable relationship (r2 = 0.94) was attained between codend choking position and catch amount, demonstrating that codend capacity could be adjusted to achieve target catch quantities. Our work shows that controlling catch quantity in the blue whiting pelagic trawl fishery can be achieved effectively through relatively simple modifications to the codend section Future work is needed to optimize the fish lock design and ensure the codend choking rope releasers trigger reliably and at the proper depth.

The impacts of wild capture fishing on animal welfare are poorly understood. During purse seine fishing for Atlantic mackerel (Scomber scrombrus), catches are crowded to high densities to facilitate pumping onboard. This study aimed to monitor fish welfare during crowding events in the Norwegian purse seine fishery, and to identify relevant drivers. We first correlated a suite of neuro-endocrine, physiological and physical stress responses (integrated into a single measure of welfare using multivariate analysis) to the behavioural vitality of individual mackerel in controlled crowding trials in aquaculture cages. Vitality was found to be a useful measure of welfare. We then assessed individual fish vitality onboard a commercial purse seiner. Catch welfare, measured using vitality, was observed to be negatively impacted during pumping related crowding. Larger catches and longer crowding exposure times resulted in greater negative impacts. Vitality was not significantly impacted by crowding density or dissolved oxygen concentrations inside the net, although methodological limitations limited accurate measurement of these parameters. Blood lactate levels correlated negatively with vitality, suggesting that high-intensity anaerobic locomotory activity was associated with the reduction in welfare. Based on these findings, catch welfare could be improved by targeting smaller schools to minimise crowding exposure times.