The decline in the top predator cod, mainly caused by overfishing, may have triggered a huge ecosystem shift in the Baltic Sea, a new study implies.
The study by the Swedish Board of Fisheries Institute of Marine Research shows that the sea’s ecosystem, historically controlled by temperature and salinity, now sports a much bigger role for the dramatically growing sprat stock. That stock, growing in the absence of the top predator cod, in turn finishes off much of the zooplankton community, which may hinder the recovery of the cod stock. Cod larvae feed on zooplankton.
According to the study, the components of the ecosystem may flip when the number of sprat individuals exceeds a certain critical level. That level, estimated at 170 billions in the Baltic, was surpassed in 1992-93.
The study provides important support for an ecosystem-based approach to fisheries management in the coming discussions around the European Union’s new Common Fisheries Policy (CFP).
The results have been published in “Proceedings of the National Academy of Sciences of the USA, PNAS”, 6 januari 2009.