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 Ce
ntral 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 clu
peid(sprat)-dominated system. It has been stated that the zooplankton commu
nity has also changed in the Baltic Sea and that this phenomenon is mainly
explained by changes in the hydrological environment. The long-term zooplan
kton data collected since 1960 by the Latvian Fisheries Research Institute
allowed analyses of the influence or pelagic fishes. the main predators of
the zooplankton. on interannual variability of biomass, stuck structure and
vertical migration of zooplankton. Using a multivariate regression model.
two statistically significant and biologically plausible relationships betw
een total stock biomass of sprat and biomass of Pseudocalanus elongatus in
spring and biomass of Cladocera in summer were found. Linear correlation an
alyses indicate that increasing sprat stock size and strong year classes si
gnificantly influence the stock structure and vertical migration of Acartia
spp. and Temora longicornis. the main food items of sprat. The fast disapp
earance of Cladocera in autumn could be caused by predation. (C) 2001 Inter
national Council for thr Exploration of the Sea.