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We investigated the vulnerability of 2 copepod species (Eurytemora affinis and Temora longicornis) to predation by predators with different foraging modes, three-spined stickleback Gasterosteus aculeatus juveniles and mysid shrimps Neomysis integer. Copepods were videofilmed escaping from predators and from an artificial flow field, and the results were used in a model of hydrodynamic disturbance generated by a predator The copepods detected mysids from a significantly larger distance than they detected sticklebacks (0 45 and 0.24 cm, respectively). Consequently, the capture success of the sticklebacks was higher than that of mysids. In the case of sticklebacks foraging on E. affim-S, copepod reaction distance was significantly correlated with stickleback approaching speed; sticklebacks captured a copepod only if they were able to slowly approach to within a strike distance of <0.1 cm from the prey. Also, there was a major difference between the vulnerabilities of the 2 prey species: the capture success of sticklebacks was 92 % with T. longicornis and 53 % with E. affinis. This corresponded with experiments with artificial flow, where the threshold fluid velocity gradient eliciting an escape response in copepods was 4 times higher in T, lonqicornis than in E, affjnis (8.2 and 2.1 S-', respectively). The hydrodynamic model accurately predicted the positive relationship between stickleback approaching speed and copepod reaction distance, as well as the difference between the 2 copepod species. This suggests that, by using simple artificial flow experiments, we can rank various zooplankton species according to their escape capabilities, and thus predict their vulnerability to predation by small fish with different motility patterns. In contrast, the model did not conform with observations on mysids. Apparently, the hydrodynamic disturbance created by a mysid is not related to its swimming speed, but to some other factor, such as the beat rate of swimming appendages.

During the stagnation period of the Baltic Sea the mean weight-at-age of Baltic herring decreased by 50% (between 1977 and 1992). This has usually been attributed to a top-down process, i.e. to the simultaneous collapse of cod stocks and their predation. We present long-term data for 1980 to 1993 showing that bottom-up effects may also have played a role: along with the decline of salinity. the biomass proportion of zooplankton taxa preferred by herring (larger than 20 pg ind:' in wet welght) significantly declined. To support our hypothesis we present a study in which Baltic herring feeding and selective predation were investigated during 1985, a time of good growth and high weightat-age, and 1991, when herring growth and weight-at-age were poor. In this study, herring ston~achs and simultaneously taken plankton samples were analysed from trawl surveys conducted in the northern Baltic proper d'uring the peak of the herring feeding season in late summer. During both 1985 and 1991, herring selectively preyed on the larger zooplankton categones, especially neritic copepods. However, in 1991, a smaller proportion of the prey in herring stomachs consisted of neritic copepods, apparently because their share in plankton had decreased. Consequently, and despite an increase in total zooplankton biomass, the estimated carbon content of the food eaten by herring was lower, and the average stomach fullness index (on a scale of 0 to 5) decreased from 3.9 in 1985 to 1.9 in 1991. Also, the amount of mesenteric fat on herring stomachs declined from 4.2 to 3.2 (scale 0 to 5), indicating a longer-term failure in feeding success. We suggest that, in addition to possible top-down effects (a release of cod predation), bottom-up processes mediated via changes in mesozooplankton species composition have also influenced hernng growth and that both of these processes are affected by the same environmental factor-the Baltic salinity level.

For the first time an international acoustic survey dataset covering three decades was used to investigate the factors shaping the spatial and temporal patterns in the condition of sprat and herring in the Baltic Proper. Generalized additive models showed that the spatial and temporal fluctuations in sprat density have been the main drivers of the spatio-temporal changes of both sprat and herring condition, evidencing intra- and inter-specific density dependence mediated by the size and distribution of the sprat population. Salinity was also an important predictor of herring condition, whereas temperature explained only a minor part of sprat model deviance. Herring density was an additional albeit weak significant predictor for herring condition, evidencing also intra-specific density dependence within the herring population. For both species, condition was high and similar in all areas of the Baltic Proper until the early 1990s, coincident with low sprat densities. Afterwards, a drop in condition occurred and a clear south-north pattern emerged. The drop in condition after the early 1990s was stronger in the northern areas, where sprat population increased the most. We suggest that the increase in sprat density in the northern areas, and the consequent spatial differentiation in clupeid condition, have been triggered by the almost total disappearance of the predator cod from the northern Baltic Proper. This study provides a step forward in understanding clupeid condition in the Baltic Sea, presenting evidence that density-dependent mechanisms also operate at the spatial scale within stock units. This stresses the importance of spatio-temporal considerations in the management of exploited fish

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