Impacts of eutrophication and oil spills on the Gulf of Finland herring stock

The Baltic Sea is one of the world's most stressed sea areas. Major threats to the ecosystem include eutrophication and oil spills. The progression of anthropogenic nutrient enrichment is lengthy and gradual, while oil spills cause rapid changes in the system, with varying impact time. We quantify the impact of eutrophication and the key ecological covariates on the population dynamics of the major pelagic fish stock, the Baltic herring (Clupea harengus membras), in the Gulf of Finland. The full life cycle of herring is represented with a probabilistic state-space model. Moreover, we analyse the impact of the oil spill from M/T Antonio Gramsci in 1987 on herring survival. The results confirm impact of the spill on the early life-stage survival; the observed high frequency of malformed herring larvae in surveys signaled elevated mortality of the year class. The optimal July-August chlorophyll a concentration for herring reproduction is approximately 5 mu g.L-1. This level is currently exceeded, suggesting recruitment impairment due to eutrophication. The herring stock was also recruitment-overfished. Analysis suggests deceleration of herring growth as salinity descends below 6 psu.