Natalia Krasovskaya
Atlantic Scientific Research Institute of Marine Fisheries and Oceanography (AtlantNIRO),
5, Dm.Donskoy Str., Kaliningrad, 236000 Russia

Key words: Vistula Lagoon, Baltic herring, timing of spawning, duration of spawning, water temperature, water salinity, stage of maturity.

Abstract.
The Vistula Lagoon is one of the main spawning grounds of Baltic herring (Clupea harengus membras L.) in the southeastern Baltic Sea. Coastal spring spawners constitute from 94 to 98% of the total herring catches in the lagoon. Using data from 1950-2000, the high amplitude of inter-annual variability of spawning timing of the Baltic herring was shown within a wide range of environmental conditions and stock characteristics. The principal factors which have a prevalent effect on spawning timing included the time of ice breakup and clearing from the lagoon, which is related to the severity of the preceding winter, further warming processes and water salinity values. In spite of the inter-annual diversity of spawning time, abiotic conditions, stock size etc., a noticeable constancy was observed in the spawning run (changes in CPUE, age composition, mean weight-at-age of herring during the spawning period). The trend towards milder winters and the early occurrence of spring should be mentioned, and the distinct shift of herring spawning in the Vistula Lagoon towards an earlier time and at lower temperatures was observed in spite of the positive trend of mean water temperature in spring.