MacKenzie, B. R., St. John, M. A., Plikshs, M., Hinrichsen, Hans-Harald and Wieland, Kai (1996) Oceanographic processes influencing seasonal and interannual variability in cod spawning habitat in the eastern Baltic Sea. [Paper] In: ICES Council Meeting 1996. , 27.9.-04.10.1996, Reykjavik, Iceland . ICES Council Meeting Papers, C+J:4 .

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Abstract

The volume of water with suitable oxygen and salinity conditions for survival and development of cod/eggs in the eastern Baltie Sea varies significantly within and among years. It has recently been shown that this volume of water (nreproduetive volumen), in addition to spawning stock biomass, is a major factor determining recruitment success in this eod population. However, it is unclear which oceanographie mechanisms are responsible for variability in reproductive volume, and how these processes interaet on a seasonal and interannual basis. In this study, we use time series observations to identify inter-relationships between hydrographie and biological processes (e. g., inflows of North Sea water, winter mixing processes, production and decomposition of organic matter) influencing reproductive volumes from 1952-1992. A frequent pattern of seasonal
variability in 4 eastern Baltic spawning areas includes a spring maximum which decreases
during the summer, followed by an inerease during the fall-winter months. However, this
general pattern is violated in many years, and the magnitude of the seasonality is more
pronounced, in some spawning areas than others. \Ve have begun to analyse these patterns
and interpret them in the context of other fluctuating components of the eastern Baltic
pelagic ecosystem. Preliminary results show that reproductive volumes deerease by an average of 9 - 39% between May and August, and that the decrease in the spawning area most important to long-term recruitment (Bornholm Basin) is temperature-dependen1. The seasonal decline in the Bomholm Basin also tends to be larger and more variable after 1964 than in the previous 12-year periode These patterns of reproductive volume variability may be important to eod reproduetive success beeause of a long and variable spawning period.

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