Trophodynamics of the plankton community at Dogger Bank: The predatory impact by larval fish

The trophodynamics of a coastal plankton community were studied, focusing on fish larvae and their copepod prey. The major objectives were to describe distributional overlap and evaluate the predatory impact by larval fish. The study was carried out across Dogger Bank in the North Sea, August-September 1991. Sampling transects crossed tidal fronts off the Bank and plankton at all trophic levels showed peak abundance within frontal zones. Also vertically there was a significant overlap in distributional patterns of the plankton. Seven species of fish larvae were abundant, of these sprat (Sprattus sprattus) dominated. The abundance of one group of fish larvae peaked in the shallow water close to the Bank, whereas other species, including sprat, were found in deeper water. Prey preference and predation pressure of fish larvae were assessed using information on prey sizes and growth rates of larvae and the copepod prey. We estimated larval removal of preferred prey sizes to 3-4% day(-1), counterbalanced by a 3-7% day(-1) replenishment from copepod production and growth. Additional predation pressure on copepods by an invertebrate predator was estimated to 1-3% day(-1). In conclusion, the dynamics of fish larvae and other zooplankters were closely linked. At peak abundances of fish larvae (>35 mg dry weight m(-2)), the accumulated predation on specific size ranges of copepods, exerted by larvae and other predators, could exceed the ability of copepod replenishment and intra-/interspecific competition among predators might take place.