Magnitude and causes of mortality of Atlantic herring (Clupea harengus) induced by crowding in purse seines

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Releasing, or “slipping”, all or part of a catch has traditionally been used as a method for catch regulation in purse seine fisheries. During the last decades it has been documented that the mortality of fish crowded to densities experienced in the later phases of purse seine fishing may be substantial (Lockwood et al., 1983; Huse and Vold, 2010; Marçalo et al., 2006; Marçalo et al., 2007; Marçalo et al., 2010; Mitchell et al., 2002). Norwegian purse seine fisheries for Atlantic herring (Clupea harengus) are the largest and among the most profitable fisheries in Norway. Reports from the fishing grounds and anecdotal evidence suggest that slipping in these fisheries is a relatively common practice, particularly when herring occur in dense schools or large amounts of small herring are mixed with larger ones on the fishing grounds. Previous experiments have further shown that herring is sensitive to gear contact (Misund and Beltestad, 1995; Suuronen et al., 1996a) and it is therefore likely that slipping mortality is a problem also in these fisheries.