FOOD-CONSUMPTION OF LARVAL, YOUNG AND ADULT HERRING AND SPRAT IN THE BALTIC SEA

The total food consumption of herring Clupea harengus L. and sprat Sprattus sprattus (L.), the dominant zooplanktivorous fish in the Baltic Sea, was estimated from published data on abundances, growth, mortality rates and diets using a bioenergetics model. The annual food consumption was 5.0 x 10(7) tonnes for herring and 2.4 x 10(7) tonnes for sprat. For herring ca 90 % of the food consumed was zooplankton and the rest mysids and benthos. Larvae and young-of-the-year (YOY) of sprat and herring accounted for 50 and 45 % of the total consumption respectively. Peak consumption rates occurred in August-September. Our estimate for annual consumption of zooplankton by clupeids in the Baltic sea is 4 times higher than previous estimates. One major explanation for this is that we included the consumption by YOY and 1 yr old fish. Our simulations also show that a diet shift, from a mix of zooplankton, mysids and amphipods to only zooplankton, could have a significant effect on fish growth rate. At a fixed biomass consumption, the lower energy density of zooplankton would mean that this shift would decrease growth by about 25 % for older age groups. This decrease is similar to that observed in the Baltic Sea in the late 1980s.