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

• Published in: ICES Journal of Marine Science
• Main Authors: Nøttestad, Leif, Fernö, Anders, Mackinson, Steve, Pitcher, Tony, Misund, Ole Arve
• Format: Text
• Language: English
• Published: Oxford University Press 2002
• Subjects: Regular Articles; Norwegian Sea; Balaenoptera physalus; Gadus morhua; Orca; Orcinus orca
• Online Access: http://icesjms.oxfordjournals.org/cgi/content/short/59/2/393; https://doi.org/10.1006/jmsc.2001.1172

We present the first acoustic observations of predator–prey interactions between fin whales and herring. The school dynamics and predation events of Norwegian spring-spawning herring ( Clupea harengus ) in a cold front area (about 125 km2) 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 min. Altogether, 184 behavioural events were recorded, with an event occurring every 8.3 min on 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 fin 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 on average. Of these marine mammals only fin whales swimming alone or in small groups (2–5 ind.) were observed to attack herring. Attacks from fin whales were observed every 170 min. They strongly influenced the density, shape and structure of the herring schools. Large fish such as cod ( Gadus morhua ) and saithe ( Pollachius virens ) preying on herring in coastal areas were not caught in the pelagic trawl or detected by acoustics. Herring schools were on average large (987 m2), dense, swam 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 fin whales in an offshore area and herring attacked by gadoid predators closer to the coast in a similar study in May.