Spatial and temporal density dependence regulates the condition of central Baltic Sea clupeids: compelling evidence using an extensive international acoustic survey

Abstract

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

Has eutrophication promoted forage fish production in the Baltic Sea?

“…This is in line with increased fat content in sprat in the southeastern Baltic Sea (Elwertowski et al 1974). The weights of both sprat and herring in the Baltic Sea have undergone large variations over time, including a substantial decline in the 1990s (Casini et al 2011 and references therein). The reasons for this are not fully understood, but the processes likely involved include climate variability affecting the abundance of favoured prey items and competition (Casini et al 2011).…”

“…The weights of both sprat and herring in the Baltic Sea have undergone large variations over time, including a substantial decline in the 1990s (Casini et al 2011 and references therein). The reasons for this are not fully understood, but the processes likely involved include climate variability affecting the abundance of favoured prey items and competition (Casini et al 2011). Due to complexity of the processes affecting fish growth, it cannot be excluded that the increase in body weight from the 1950s to 1980s coincidentally occurred in parallel with increased nutrients without being a direct effect of it.…”

Environmental cues and constraints affecting the seasonality of dominant calanoid copepods in brackish, coastal waters: a case study of Acartia, Temora and Eurytemora species in the south-west Baltic

“…Trophic cascades documented in offshore areas of the Baltic Sea highlight the tight inter-dependence of copepods and clupeid fishes (e.g. Möllmann et al 2008;Casini et al 2011). In offshore areas such as the Gotland Basin, T. longicornis was more important than Acartia spp.…”

Temperature effects on vital rates of different life stages and implications for population growth of Baltic sprat

“…That study demonstrated that those larvae collected in August had lower (food-limited) growth rates compared to juveniles collected in late November. Furthermore, tight food web coupling between sprat and zooplankton is suggested by the presence of trophic cascades in the Baltic Sea ecosystem (Möllmann et al 2008) and density-dependent changes in the condition of zooplanktivorous fish competing for limited prey resources (Casini et al 2011). All of these studies suggest that prey dynamics (indirect effects of climate) need to be included in models that hope to explore the effects of projected warming of the Baltic Sea on sprat population dynamics.…”