How whales influence herring school dynamics in a cold-front area of the Norwegian Sea

We present the first acoustic observations of predator prey interactions between fill whales and herring. The school dynamics and predation events of Norwegian spring-spawning herring (Chipea harengus) in a cold front area (about 125 km(2)) in the Norwegian Sea in April were quantified. Data from high-resolution sonar tracking of herring schools combined with echosounder data were integrated with pelagic-trawl samples. A total of 44 herring schools were each observed for an average of 34.8 unit. Altogether, 184 behavioural events were recorded, with ail event Occurring every 8.3 min oil average. Intra-school events (compression, reorganization, ring, pseudopodium, elongate, diving, surfacing) were observed every 15.3 min and inter-school events (approach, join, leave, split) every 22.9 min. We observed 17 fill whales (Balaenoptera physalus), six Atlantic white-sided dolphins (Lagenorhynchus acutus) and five killer whales (Orcinus orca) within the experimental area. predator events occurred every 91 min oil average. Of these marine mammals only fill whales swimming alone or in small groups (2-5 ind.) were observed to attack herring. Attacks from fill whales were observed every 170 ruin. They strongly influenced the density, shape and structure of the herring schools. Large fish such as cod (Gadus morhua) and saithe (Pollachius virens) preying oil herring in coastal areas were not caught Ill the pelagic trawl or detected by acoustics. Herring schools were oil average large (987 m(2)). dense, swain at depth (148 m) and had a moderate swimming speed (1.1 body lengths per second), reflecting a risk-averse, anti-predator behaviour to marine mammals. We discuss differences in schooling dynamics, anti-predator behaviour, attack frequency and predation risk between herring attacked by fill whales ill an offshore area and herring attacked by gadoid predators closer to the coast in a similar study in May. (C) 2002 Published by Elsevier Science Ltd. on behalf of the International Council for the Exploration of the Sea.