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We investigated synoptically with two research vessels how the distribution and migration of herring were influenced by zooplankton concentrations and temperature distribution at the Arctic front in early spring. A total of 32 herring schools were tracked continuously for up to 1.2 h by multi-beam sonar. Larger herring schools swam faster and in warmer water masses than smaller schools. Herring had on average a low dynamic tendency, with relatively few split, join and behavioural events per hour, probably due to low temperatures near the Arctic front. No clear diurnal vertical migration was detected based on the acoustic data, and herring were distributed from the surface area down to approximately 360 m. Zooplankton biomass was the highest at the Arctic front and lower on both sides of the front, indicating that herring were successfully searching for high food concentrations there. We suggest that herring were also feeding in the deep (>300 m) cold ‘fridge’ based on stomach samples and high concentrations of larger zooplankton at these depths. The temperature differences were considerable between the surface and 400 m depth, exceeding 5 °C (0.5–5.5) in some areas. Nevertheless, herring did not seem to have any strong vertical preferences with regard to temperature and tolerated very low temperatures. Herring in the eastern and deeper part of the front swam in waters down to 1.0 °C, indicating that these fish can cross the cold Arctic front. This could have pronounced consequences for the migration pattern of Norwegian spring-spawning herring in the Northeast Atlantic. |
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