Impact of grazing from capelin (Mallotus villosus) on zooplankton: a case study in the northern Barents Sea in August 1985

The distribution of capelin was mapped in the area east of Hopen. Zooplankton was sampled with Juday net and 1 m² MOCNESS sampler, and analysed with respect to hydrography and capelin abundance. The capelin "front" coincided more or less with the physical Polar Front, and this complicated the interpretation of the results. Strong indications for a grazing impact by capelin on zooplankton were nevertheless obtained. The zooplankton biomass was significantly lower in the area with high abundance of capelin than in the area with no capelin. This effect was due to a lower biomass of relatively large zooplankton (> 1 mm size fraction) and seen most clearly in data obtained with the MOCNESS. The biomass of zooplankton in the upper 100 m was very low where capelin was present, suggesting rapid depletion of the major prey items. The biomass (m ^?2) of capelin in the capelin front area was about three times higher than the biomass of zooplankton in areas without capelin. The capelin front would therefore have the potential to graze down the available prey in 3-4 days. Light seems to be an important factor for the predation impact by capelin, resulting in strong interactions between capelin predation and zooplankton vertical distribution.