Fine-scale spatial association between baleen whales and forage fish in the Celtic Sea

Abstract

Baleen whales can be regularly observed in the Celtic Sea; however, little is known about their local foraging behaviour. This study's objective was to determine whether or not baleen whales selectively prey upon particular forage fish species or, on the contrary, is predation on the Celtic Sea plateau driven by random encounters between prey and predator? Concurrent sighting surveys for fin (Balaenoptera physalus), minke (Balaenoptera acutorostrata), and humpback (Megaptera novaeangliae) whales were carried out simultaneously from 2007 to 2013 during dedicated fisheries acoustic surveys assessing the abundance and distribution of forage fish. Probabilities of spatial overlap between baleen whales and forage fish were analysed and compared with the probability of a random encounter. For estimations of foraging threshold and prey selectivity, mean fish biomass and fish length were calculated when baleen whales and forage fish co-occurred. Whales were dominantly observed in areas with herring (Clupea harengus) and sprat (Sprattus sprattus), while areas with mackerel (Scomber scombrus) were not targeted. A prey detection range of up to 8 km was found, which enables baleen whales to track their prey to minimize search effort. Fish densities within the defined foraging distance ranged from 0.001 to 3 kg·m−2 and were correlated to total fish abundance. No prey size selectivity according to fish length was found. By linking baleen whale distribution to high-density herring and sprat areas, it was possible to identify the Celtic Sea as a prey hot spot for baleen whales during autumn.