LIFECO: Predatory interaction between fish species and top-down control of zooplankton relative to frontal processes

This work package is part of the LIFECO programme: LInking hydrographic Frontal Activity to ECOsystem dynamics in the North Sea and Skagerrak - importance to fish stock recruitment.

The unique physical and biological conditions associated with frontal regions result in enhanced nutrient fluxes to the euphotic zone thereby increasing primary and secondary production, as well as biomass. Furthermore, the unique circulation patterns in these regions cause the aggregation of planktonic organisms. These increased densities and production rates of phyto- and zooplankton provide favourable nursery habitats for larval and juvenile fish, which subsequently exhibit higher growth rates and condition than in adjacent stratified areas leading to lower cumulative mortality probability due to lower vulnerability to predation. In the present work package, we address two hypotheses questioning the importance of frontal areas for recruitment success.

The secondary production and standing stocks of zooplankton in frontal systems may be controlled by planktivorous predators, i.e. juvenile but also adult planktivorous fish as well as gelatinous zooplankton known to concentrate in frontal areas. This predation pressure may become too high to sustain favourable feeding and growth conditions for competing juvenile fish over longer periods.

High abundances of zooplankton and juvenile fish in frontal regions lead to an aggregation of planktivorous as well as piscivorous predators. These results in increased predation pressure on fish larvae and O-group fish, counteracting potentially enhanced feeding conditions and higher growth rates in frontal regions. General knowledge about the food composition exists for most important demersal fish species in the North Sea and Skagerrak systems and to some extend also for their pelagic O-group stages. However, with respect to the coupling of hydrographic features, prey availability and feeding behaviour, data are almost completely missing. Information on the diet composition of important pelagic and planktivorous predators/life stages (e.g. mackerel, horsemackerel, saithe, gelatinous predators), which are hypothesised to have a strong impact on the recruitment process of many North Sea fish species, are only fragmentary.