Recruitment Dynamics of the Gulf of Riga Herring Stock: Density-Dependent and Environmental Effects

The Gulf of Riga and open-sea stocks of the Baltic herring have displayed remarkably consistent inverse recruitment and stock development patterns since the 1970s: the open-sea stocks steadily declined, whereas the Gulf stock increased rapidly in the early 1990s, reaching a peak abundance in the early 2000s and exceeding the level of the 1970s by a factor of 2¿3. The increase was accompanied by a decline in the mean weight-at-age and the condition factor. The estimated decline (by 30¿40%) in the average annual consumption rate per individual and changes observed in the zooplankton community suggest that density-dependent effects may have increased since the 1970s. The current period of high stock sizes is also characterized by greater recruitment variability. Historical fecundity investigations have established that the average egg production per individual has decreased in all age groups by 20¿ 50%, along with a decrease in mean weight and condition. Yet, the effect on recruitment has been low so far, because lower fecundity has been compensated by the greater abundance and population fecundity has been maintained at the original level. Recruitment appears to be more influenced by environmental conditions than by spawning-stock biomass.JRC.G.4-Maritime affair

The impact of selected Baltic ports on adjacent fish fauna: revealed effects and uncertainties

The paper provides an insight on possible impacts and problems connected with the estimation of the effect of big harbours on adjacent fish fauna. The data has been collected annually in the impact and reference areas of the three largest deep-water ports of Estonia, the eastern Baltic Sea: Muuga Port (1994-2008), Paldiski Southern Port and Paldiski Northern Port (both in 2002-2008). We assessed the species composition and abundance of different macroalgal, benthic invertebrate and fish species as well the spawning condition and food consumption of several key fish species. The results showed that the impact of harbour activities on local fish communities and on year-class abundance remain largely uncertain due to the masking effects of ecosystem changes. Nevertheless, the vanishing of spawning grounds of herring and changes in trophic interactions due to the introduction of alien species can be highlighted as the major potential effects. Recently, the round goby Neogobius melanostomus, an aggressive bottom dwelling invasive species, were found in Muuga Bay. This introduction is discussed in the context of the discharge of the ballast water.JRC.G.4-Maritime affair