Zooplankton diversity and the predatory impact by larval and small juvenile fish at the Fisher Banks in the North Sea

Abstract

The biomass and diversity of the mesozooplankton and fish larvae community were inves- tigated across a frontal zone in the central North Sea in the early summer, to investigate whether larval fish predation is a regulator of mesozooplankton production. Pronounced changes in the meso- zooplankton community were observed across the front off the Jutlandic coast. The biomass and the diversity of copepods changed across the front as the population of Calanus finmarchicus became abundant in the deeper water. The crustaceans (Acartia spp. and Evadne spp.) and polychaete larvae dominated the mesozooplankton in the coastal water. The biomass of fish larvae was dominated by gadoid larvae. As in the copepods, a shift in fish diversity was observed in the frontal zone. On the coastal side of the front, whiting (Merlangius merlangus) dominated the biomass, while offshore from the front whiting were absent and cod (Cadus morhua) was the dominant larval fish species on the deeper stations. The present investigation demonstrates two different trophic pathways related to hydrography in the central North Sea. First, in the shallow coastal water, the abundant small neritic copepods are predominately predated by whiting, while in the offshore region the larger oceanic copepods are predated by cod larvae. However, the predation pressure by the fish larvae was in general low (<10%) relative to copepod production per day. Consequently, in the early summer, the copepod production potentially results in a build-up of copepod biomass on both sides of the front.