Distribution range, occurrence and bycatch risk of a large pelagic shark, the porbeagle (Lamna nasus), in Norwegian waters

Abstract

Environmental changes are affecting a great number of marine ecosystems. Many top predators are particularly vulnerable to such changes as they are contingent on the various prey species along the food chain and their responses to shifts. In addition, many top predators, among those many shark species, are long lived, mature late, and have low fecundity, making them particularly vulnerable to anthropogenic stressors like overexploitation. The porbeagle (Lamna nasus) is a large pelagic shark that inhabits cold-temperate regions of the oceans on both hemispheres. During the last century, it was heavily targeted by fisheries in the Northeast Atlantic (NEA), which caused a dramatic decrease of the population. Today, the NEA subpopulation is considered critically endangered by the International Union for Conservation of Nature (IUCN), but there is still little known about the status of porbeagle in many areas of its distribution range. Fundamental information on porbeagles in parts of the NEA, such as post-fishery life-history traits, occurrence and population size, is still lacking. This study addressed the distribution range and abundance of porbeagles in Norwegian waters. Analyses of catch data from fisheries from the past decades revealed (i) the presence of porbeagle all year round in Norwegian waters, (ii) a decreasing trend in landings of porbeagle, consistent with the implementation of zero catch regulations, (iii) a catch peak in the summer months, and (iv) two potential hotspots in the Norwegian Sea for being caught. Bycatch through nets was identified as the main threat for fishery mortality for porbeagle in Norwegian waters. These findings provide important information for informed advice to fisheries in order to further reduce porbeagle bycatch, and to develop and implement sustainable strategies for an efficient management of the NEA porbeagle.