No field studies have been performed on the selectivity of herring and sprat in the southern Baltic Sea in relation to their entire range of prey. Accordingly, we tested in the field the following hypotheses: (i) sprat and herring are selective feeders and (ii) sprat and herring selectivity is size-and season-dependent. The results show that (i) smaller herring and all size classes of sprat are strictly zooplanktivorous, selecting principally Temora longicornis and Bosmina maritima during the autumn and Pseudocalanus elongatus in winter; (ii) larger herring are essentially nektobenthos feeders, predating on Mysis mixta during the autumn and amphipods and polychaetes during the winter; and (iii) herring and sprat seem to avoid Acartia spp. in both autumn and winter. During the autumn, herring are zooplanktivorous up to 18e20 cm, whereas in winter herring feed on nektobenthos starting from 14e15 cm. Selectivity was not an absolute process but it was related to prey relative abundance in the sea and, possibly, to prey profitability (e.g. size, conspicuousness, and reaction time).

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 uslng a bioenergetics model. The annual food consumption was 5.0 X 107 tonnes for herring and 2.4 X 107 tonnes for sprat. For herring ca 90 % of the food consumed was zooplankton and the rest n~ysids 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 thls shift would decrease growth by about 25 % for older age groups. This decrease is similar to that observed in the Baltlc Sea in the late 1980s.

No abstract

Observations during the 1990s indicate that individual growth rate of Atlantic herring (Clupea harengus) decreased by 30% to 50% in the Baltic proper. There are several possible explanations, however, no statistical analysis has been performed to define and describe the changes in herring growth rates. In this study we compared mean weight-at-age and length-specific weight (i.e. condition) data collected in SeptemberOctober 1986SeptemberOctober -1996. Results indicated an increase in weight-at-age between 1986-1989. Thereafter, weight-at-age and fish condition declined in all parts of the Baltic proper. The decreases were evident in 0 through to 8 year old fish. Moreover, the declines were larger in the northern (51%) than in the south-western (42%) part of the Baltic proper and for younger (1-4 years) fish compared to the older fish (5-8 years). Possible causes of the decrease in weight-at-age and condition are discussed. However, the decreases seem to be well correlated with a drastic increase in the number of pelagic fish in the Baltic proper and a consequent reduction in food availability. International Council for the Exploration of the Sea

No abstract