Population dynamics of calanoid copepods and the implications of their predation by clupeid fish in the Central Baltic Sea

Population dynamics of major Baltic calanoid copepod species in the Gotland Basin during the last two decades were characterized by a decline of Pseudocalanus elongatus associated with declining salinities, and an increase of Temora longicornis and Acartia spp. potentially due to warmer conditions. Additionally this study investigated the effect of predation by the major planktivorous fish species herring (Clupea harengus) and sprat (Sprattus sprattus) for the period 1977-1996 in the Gotland Basin (Central Baltic Sea). Examination of consumption by these fish species in relation to copepod production estimates showed a switch by herring from consuming mainly CV/VI of P. elongatus and T. longicornis, to preying on CII of the latter copepod. This switch was potentially due to increased competition with the drastically increased sprat stock since the late 1980s. Further, an increased predation pressure by sprat on CV/CVI of both copepod species in spring resulted in higher copepod mortality rates. In consequence, based on these results we suggest that the increase in the sprat stock since the late 1980s contributed to a decline of P. elongatus, and additionally prevented an even more pronounced temperature-driven increase in the T. longicornis stock, as was observed for Acartia spp., which was not significantly consumed.