Food consumption by clupeids in the Central Baltic: evidence for top-down control?

Considerable changes have taken place in the pelagic ecosystem of the Centr al Baltic Sea during the last decade. Owing to a combination of high fishin g pressure and unfavourable hydrographic conditions, the cod (Gadus morhua) stock as the top predator in the system was reduced from a high level in t he early 1980s to its lowest size on record in the early 1990s. The preferr ed prey species sprat (Sprattus sprattus) showed a significant increase in population size since the late 1980s to the highest level on record in rece nt years, while the herring (Clupea harengus), another important planktivor e in the system, did not show such a response. We investigate whether fluct uations in clupeid stock size cascade down to the trophic level of mesozoop lankton, based on stomach content data and daily ration estimates in combin ation with stock sizes estimated from Multispecies Virtual Population Analy sis. Estimates of daily consumption by the populations of the two species f or 1978-1990 were compared with standing stocks of mesozooplankton species. No evidence was Found for food limitation as might be expected if clupeids exert a strong top-down control on mesozooplankton. Also no influence on i nterannual variability of mesozooplankton abundance was detected. However, predation did contribute to development of two copepod species (Pseudocalan us elongatus and Temora longicornis). (C) 1999 International Council for th e Exploration of the Sea.