Kraus, Gerd and Köster, Friedrich, eds. (2001) Duration, frequency and timing of sprat spawnig in the Central Baltic. An analysis based on gonadal maturity. . ICES Council Meeting Papers, J:25 . ICES, Kopenhagen, Denmark, 16 pp.

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Abstract

The duration, frequency and peak of sprat spawning in the 1999 spawning season are described. Spatial and year to year variations in the timing of spawning were analysed utilising maturity data obtained in June 1999 in ICES Sub-divisions 25, 26 and 27 / 28, respectively in April 1995, 1996, 1998 and 1999 in Sub-division 25. All analysis were based on gonadal maturity inspections of catches obtained from hydro acoustic-and trawl surveys as well as commercial fisheries. The sprat spawning season in 1999 lasted from February to July peaking in June with an average spawning frequency of 27% during main spawning time. The course of the 1999 season was validated against egg abundance data obtained from ichthyoplankton surveys conducted in parallel. Marked differences in the timing of spawning were found between the Bornholm Basin / Gdansk Deep and the Gotland Basin with delayed spawning activities of sprat in the latter. Both areas encounter pronounced differences in the temperature regime, whereas the Gotland Basin is characterised by the colder water temperatures. Furthermore, a consistent trend towards later spawning in most recent years has been observed. This trend was negatively related to the recently declining total biomass concurrent with increasing landings, but not to winter water temperatures.

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