Fisheries and environmental changes in the Black Sea are explored using multi-annual data series. Abundance and fisheries mortality estimates of commercially important fish species are obtained by mean of VPA tuned using independent abundance indices. Performances of three different stock assessment methods (VPA with ad hoc tuning; eXtended Survivor Analysis, XSA; Integrated Catch-at-age Analysis, ICA) on real data are compared. The estimated fisheries parameters are used to analyze effects of fishing and environment on fish stock dynamics. Potential negative effects of fishing on the fish recruitment i.e. recruitment overfishing, are studied.
More than 40 different hydroclimatic (SST, SLP, wind, run-off...), biological (plankton, benthos and fish abundance), and anthropogenic (nutrient input, hypoxia, hydrogen sulfide concentration...) indices covering 20-30 years of data are analyzed in order to describe and compare the long-term patterns in the ecosystem. Low frequency changes (trends and quasi-periodic oscillations) in the environmental indices are studied. Physical environment fluctuations are recognized being an important factor driving fish stocks long-term variations and influencing considerably all ecosystem levels. The responses of the stocks to the environmental changes are modeled using generalized additive models (GAM). Besides the similarities and differences between the species studied (relation to the large-scale circulation and pronounced density dependence of the species reproducing in the open sea: sprat, horse mackerel; dependence on river run-off and strong stock-recruitment relationship of the shelf spawners: anchovy, whiting) the strong sensitivity of the exploited fish populations to environmental changes is shown.
The period after the 70s is marked by a continuously rising anthropogenic impact that tends to influence the whole ecosystem including exploited fishes. Fishing is the most important man-made factor directly impacting the stocks. Effects of fishing can propagate down the food-web and influence lower trophic levels through a trophic cascade. The mechanisms of influence of the anthropogenic eutrophication are not entirely clear but there are some indications of their negative effect on demersal fishes and on some pelagic fishes. The environmental deterioration caused by human activities (including introduction of exotic species) tends to influence fish stocks in a synergy with physical and biological factors and fishing. The negative anthropogenic effects become particularly rigorous in periods of low stock abundance probably caused by climate fluctuations.