Unaccounted mortality in purse seine fisheries. Quantification and mitigation of slipping mortality
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Purse seining is an efficient fishing method that is used to catch most of the world’s pelagic fish species. In some of these fisheries slipping is used to adjust catch size, increase catch value or to release illegal species and sizes. Slipping involves the release of fish from the net before they are brought on board. Little information is available on the frequency of slipping, but anecdotal information indicates that slipping is a common practice in some fisheries, including the Norwegian mackerel (Scomber scombrus) and herring (Clupea harengus) fisheries in some areas and seasons. Previous studies have shown that the mortality of mackerel and sardines slipped from the net may be high, especially if the density before slipping is high and the fish are held for a long time in the net. Paper II studied the mortality of herring crowded and released from a purse seine. The results showed a density-dependent mortality and a considerably higher tolerance to crowding compared to mackerel. Even so, a mortality rate above 50% was registered at high crowding densities, suggesting that herring slipped in a late phase of hauling may suffer unacceptably high mortality.
Identification of the main causes of slipping mortality is necessary for the adoption of appropriate mitigation measures and regulations. Physiological sampling of crowded herring in paper II showed that some fish display a maladaptive physiological stress response, including disturbances in their osmoregulatory balance, and energy exhaustion. A common assumption is that mortality occurs due to impairment of the skin barrier, leading to osmoregulatory deficiencies and substantial leakage of body fluids. Paper III showed that a loss of more than 25% of the scales can cause significant mortality in herring. Large quantities of scales can be observed in the water during the late phases of purse seining when the fish are crowded in the net, but to exactly what extent herring lose scales is not known. There is some evidence for scale loss not being a single cause of mortality, but the mortality is rather a result of the synergistic effects of several stressors (scale loss, hypoxia, exhaustion and stress).
Slipping of dead or dying fish is illegal in Norway. The upcoming landing obligation in EU will have the same approach where fish can be released if high survival is ensured. Crowding experiments indicate that slipping in an early stage and in a careful manner can ensure high survival. There are currently no available estimates of fish densities inside the net during purse seining or any practical methods for monitoring the catches or the net during commercial fishing, which makes the regulation of slipping difficult. Paper IV described a method for monitoring purse seine net volume and estimating catch densities. The results indicated that even in large catches, lethally high densities of mackerel and herring are unlikely to occur up to the point when 80% of the net has been hauled. The results further showed that catch densities between purse seine sets vary significantly due to differences in net volumes and catch sizes. Regulations on how late in a haul slipping can be permitted will have to take such variations into account in addition to any differences in mortality rates caused by different environmental, fishing and biological conditions. In order to ensure that purse seines can continue to provide high-quality catches efficiently even with a precautionary limit to slipping, slipping regulations will have to be combined with technological solutions for improved identification of catch size, species and quality before the net is set or in an early stage of purse seining. In addition slipping techniques and gear modifications that raise survival rates will need to be developed and tested for survival.
Slipping mortality is a source of unaccounted mortality and contributes to biased stock assessment. The Northeast Atlantic mackerel stock is an example of how a high level of unaccounted mortality and a lack of fishery-independent indicators of stock biomass have resulted in unreliable estimates that were eventually rejected in 2013. In this fishery slipping is one source of mortality among many, but it is important to mitigate all the sources. In paper I, a tag recapture data-set collected by the Institute of Marine Research was used to estimate mackerel stock size. The results indicate a stock size that has been as much as 2 to 2.3 times as large as the official estimate. The estimates involve some uncertainty that needs to be taken into account, especially related to variable tagging mortality and detector efficiency, but could potentially improve the current assessment if the data were included in the ICES stock assessment on a regular basis.