© 2005 International Council for the Exploration of the Sea
Baltic cod recruitment – the impact of climate variability on key processes
a Danish Institute for Fisheries Research Charlottenlund Castle, DK-2920 Charlottenlund, Denmark
b Leibniz Institute of Marine Sciences at Kiel University Düsternbrooker Weg 20, 24105 Kiel, Germany
c Greenland Institute of Natural Resources PO Box 570, DK-3900 Nuuk, Greenland
d Latvian Fisheries Research Agency Daugavgrivas Street 8, LV-1007 Riga, Latvia
e Institute of Hydrobiology and Fisheries Science, Hamburg University Olbersweg 24, D-22767 Hamburg, Germany
f Sea Fisheries Institute ul Kollataja 1, PL-81-332 Gdynia, Poland
*Correspondence to F. W. Köster: tel: +45 3396 3550; fax: +45 3396 3333. e-mail: fwk{at}difres.dk.
Large-scale climatic conditions prevailing over the central Baltic Sea resulted in declining salinity and oxygen concentrations in spawning areas of the eastern Baltic cod stock. These changes in hydrography reduced the reproductive success and, combined with high fishing pressure, caused a decline of the stock to the lowest level on record in the early 1990s. The present study aims at disentangling the interactions between reproductive effort and hydrographic forcing leading to variable recruitment. Based on identified key processes, stock dynamics is explained using updated environmental and life stage-specific abundance and production time-series. Declining salinities and oxygen concentrations caused high egg mortalities and indirectly increased egg predation by clupeid fish. Low recruitment, despite enhanced hydrographic conditions for egg survival in the mid-1990s, was due to food limitation for larvae, caused by the decline in the abundance of the copepod Pseudocalanus sp. The case of the eastern Baltic cod stock exemplifies the multitude effects climatic variability may have on a fish stock and underscores the importance of knowledge of these processes for understanding stock dynamics.
Keywords: eastern Baltic cod, egg survival, hydrography, larval prey availability, predation, recruitment
Received 10 August 2004; accepted 3 May 2005.
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