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

Considerable changes have taken place in the pelagic ecosystem of the Central Baltic Sea during the last decade. Owing to a combination of high fishing pressure and unfavourable hydrographic conditions, the cod (Gadus morhua) stock as the top predator in the system was reduced from a high level in the early 1980s to its lowest size on record in the early 1990s. The preferred prey species sprat (Sprattus sprattus) showed a significant increase in population size since the late 1980s to the highest level on record in recent years, while the herring (Clupea harengus), another important planktivore in the system, did not show such a response. We investigate whether fluctuations in clupeid stock size cascade down to the trophic level of mesozooplankton, based on stomach content data and daily ration estimates in combination with stock sizes estimated from Multispecies Virtual Population Analysis. Estimates of daily consumption by the populations of the two species for 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 interannual variability of mesozooplankton abundance was detected. However, predation did contribute to the seasonal development of two copepod species (Pseudocalanus elongatus and Temora longicornis)