Adaptive behaviour of herring schools in the Norwegian Sea as revealed by high-resolution sonar

Fourteen herring schools swimming and actively feeding in 15–40 m depth in the Norwegian Sea were followed for about 1 h each in May 1994 using SIMRAD SA950 sidescan sonar: most of the schools were not recorded by a conventional echo-sounder because of vessel avoidance. Acoustic target identities were validated using trawl samples and stomach analysis. The dynamics of changes in school structure were analysed in relation to stimuli impinging on the schools. School structure changed in response to approaches by other herring schools, likely predators, and the research vessel. Over 230 behavioural events, at approximately 5 min intervals, reveal a surprisingly dynamic regime of approaching, joining, and splitting that could facilitate rapid adaptive adjustment of herring school size to prevailing conditions. Moreover, our interpretation of the visualization of the schools by sonar suggests adaptive responses to different kinds of predatory attacks. Individual cod and haddock predators intimately accompanied the herring schools, their attacks causing frequent modifications to, but not dispersal of, school structure, so that most herring were able to continue feeding while retaining the risk-dilution advantages of schooling. On the other hand, rapid approach by fast-moving schooling predators such as saithe, or by the research vessel, caused the herring to dive steeply. Consequent loss of feeding and reassembly was evidently a cost of evading these more dangerous predators.