Impacts of fisheries on plankton community structure

There has been much debate on the extent to which resource availability (bo ttom-up) versus predation pressure from fish (top-down) modulates the dynam ics of plankton in marine systems. Physico/chemical bottom-up forcing has b een considered to be the main mechanism structuring marine ecosystems, alth ough some field observations and empirical correlations support top-down mo dulation. Models have indicated possible feedback loops to the plankton and other studies have interpreted a grazing impact from long-term changes in fish stocks. In freshwater systems, evidence for top-down forcing by fish a nd trophic cascading is well documented. First, evidence for equivalent top -down effects in the marine environment is presented, with an overview of r elevant publications. In the second part, time series, averaged for the Nor th Sea (when possible from 1948 to 1997), of fish catch, recruitment, and s pawning stock biomass are related to the abundance of species or larger gro upings of zooplankton and phytoplankton from the Continuous Plankton Record er survey and selected environmental parameters. Preliminary analysis sugge sts that there is strong interaction between different fish species and the plankton and that the fishery, through top-down control, may at times be a n important contributor to changes in the North Sea ecosystem. (C) 2000 Int ernational Council for the Exploration of the Sea.