© 2005 International Council for the Exploration of the Sea
Synchronous ecological regime shifts in the central Baltic and the North Sea in the late 1980s
a Baltic Sea Research Institute, Seestrasse 15, D-18119 Warnemünde, Germany
b Danish Institute for Fisheries Research, Charlottenlund Castle, DK-2920 Charlottenlund, Denmark
c Latvian Fisheries Research Agency, Daugavgrivas Street 8, LV-1007 Riga, Latvia
d Bundesamt für Seeschifffahrt und Hydrographie, Bernhard-Nocht-Strasse 78, D-20359 Hamburg, Germany
*Correspondence to J. Alheit: tel: +49 381 5197 208; fax: +49 381 5197 440. e-mail: juergen.alheit{at}io-warnemuende.de.
The index of the North Atlantic Oscillation, the dominant mode of climatic variability in the North Atlantic region, changed in the late 1980s (1987–1989) from a negative to a positive phase. This led to regime shifts in the ecology of the North Sea (NS) and the central Baltic Sea (CBS), which involved all trophic levels in the pelagial of these two neighbouring continental shelf seas. Increasing air and sea surface temperatures, which affected critical physical and biological processes, were the main direct and indirect driving forces. After 1987, phytoplankton biomass in both systems increased and the growing season was extended. The composition of phyto- and zooplankton communities in both seas changed conspicuously, e.g. dinoflagellate abundance increased and diatom abundance decreased in the CBS. Key copepod species that are essential in fish diets experienced pronounced changes in biomass. Abundance of Calanus finmarchicus (NS) and Pseudocalanus sp. (CBS) fell to low levels, whereas C. helgolandicus (NS) and Temora longicornis and Acartia spp. (CBS) were persistently abundant. These changes in biomass of different copepod species had dramatic consequences on biomass, fisheries, and landings of key fish species: North Sea cod declined, cod in the CBS remained at low levels, and CBS sprat reached unprecedented high biomass levels resulting in high yields. The synchronous regime shifts in NS and CBS resulted in profound changes in both marine ecosystems. However, the reaction of fish populations to the bottom-up mechanisms caused by the same climatic shift was very different for the three fish stocks.
Keywords: Baltic Sea, cod, ecological regime shifts, herring, NAO, North Sea, sprat
Received 3 July 2004; accepted 28 April 2005.
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