Behavioural and welfare implications of a new slipping methodology for purse seine fisheries in Norwegian waters

Atlantic mackerel (Scomber scombrus) change skin colour in response to crowding stress

Wild capture can be stressful for fish. Stress has the potential to induce mortality in released unwanted catches or negative flesh quality consequences in retained ones. Such effects compromise sustainable natural resource management and industry profitability. Mitigating stress during capture is therefore desirable. Biological indicators of stress can objectively inform fishers as to the functional welfare status of catches during fishing operations. If they are to be of practical use in mitigating stress during wild capture events, such indicators must be quantifiable, respond rapidly, reflect the level of induced stress and be easily observable. Atlantic mackerel (Scomber scombrus) are extensively targeted by purse seine fisheries in European waters but are particularly vulnerable to stress. Excessive crowding in the net is thought to be the principal stress mechanism. There is therefore a need to develop indicators of crowding stress for this species so that catch welfare can be improved. Here, we demonstrate that S. scombrus exhibit a skin colour change from predominately green to predominately blue when exposed to crowding stress. In sea cage trials, we induced various degrees of stress in groups of wild-caught S. scombrus by manipulating crowding density and its duration. Skin colour was quantified in air using digital photography. The colour change occurred rapidly (within the typical duration of crowding events in the fishery), and its magnitude was correlated to the severity and duration of crowding. Bluer fish were also associated with higher levels of plasma lactate. No appreciable colour change was observed in uncrowded (control) groups during the treatment period. Nonetheless, unstressed S. scombrus did turn blue <1 h after death. Together, these results indicate that skin colour change has the potential to be a useful real-time indicator of crowding stress for S. scombrus and could therefore be used to improve welfare during wild capture fishing.

Physiological response and survival of Atlantic mackerel exposed to simulated purse seine crowding and release

Understanding how animals physiologically respond to capture and release from wild capture fishing is fundamental for developing practices that enhance their welfare and survival. As part of purse seine fishing for small pelagic fish in northern European waters, excess and/or unwanted catches are routinely released from the net in a process called slipping. Due to excessive crowding in the net prior to release, post-slipping mortality rates can be unacceptably high. Atlantic mackerel (Scomber scombrus) support large and economically important purse seine fisheries but are known to be particularly vulnerable to such crowding-induced mortality. Developing management advice to promote post-slipping survival for this species is currently challenging, due to a lack of understanding of how crowding influences their physiology. Here we examine the physiological response, recovery and survival of wild caught mackerel exposed to various degrees and durations of simulated crowding stress in a series of sea cage trials. The magnitude of the physiological response and its time to recovery was positively correlated with crowding density and duration and was characterized by cortisol elevation, energy mobilization and anaerobic metabolite accumulation. There were also indications of osmoregulatory disturbance. Skin injury and mortality rates showed a similar positive relationship to crowding density. The physiological disturbance was recoverable for most fish. Instead, the rate at which mortalities developed and the physiological profile of moribund fish indicated that skin injury, likely arising from abrasive contact with netting and other fish during crowding, was the probable cause of mortality. Injured fish also exhibited a loss of allometric condition relative to non-injured survivors. Crowding treatments were potentially confounded by differences in ambient oxygen reduction, water temperature and pre-treatment fish condition between trials, and densities were replicated only once. These results contribute to the development of welfare conscious fishing practices that aim to reduce post-slipping mortality.