Fish and zooplankton interaction in the Central Baltic Sea

The pelagic ecosystem of the Baltic Sea has considerably changed since the beginning of the 1980s. The stock size of cod as the top predator in the Central Baltic Sea has significantly decreased and is at a very low level in the 1990s, with the result that a cod-dominated system has changed to a clupeid(sprat)-dominated system. It has been stated that the zooplankton community has also changed in the Baltic Sea and that this phenomenon is mainly explained by changes in the hydrological environment. The long-term zooplankton data collected since 1960 by the Latvian Fisheries Research Institute allowed analyses of the influence of pelagic fishes, the main predators of the zooplankton, on interannual variability of biomass, stock structure and vertical migration of zooplankton. Using a multivariate regression model, two statistically significant and biologically plausible relationships between total stock biomass of sprat and biomass of Pseudocalanus elongatus in spring and biomass of Cladocera in summer were found. Linear correlation analyses indicate that increasing sprat stock size and strong year classes significantly influence the stock structure and vertical migration of Acartia spp. and Temora longicornis , the main food items of sprat. The fast disappearance of Cladocera in autumn could be caused by predation.