Long-term variation in dates of mass spawning of cod Gadus morhua callarias (Gadidae) in the southeastern part of the Baltic Sea

Abstract

Long-term data on the numbers of eggs of cod Gadus morhua callarias in ichthyoplankton and the seasonal variation of their distribution in the Gdansk Deep of the Baltic Sea are considered. The spring peak of spawning was observed for the years 1948–1954 and 1960–1964; the spring-summer peak for 1968–1977; and the summer peak for 1955–1959 and 1995–2001. Maximum egg numbers of cod were recorded from the late 1940s to the early 1950s; minimums were recorded in the late 1980s and the late 1990s. The decrease in the numbers of cod eggs in the ichthyoplankton, in general, coincided with a decrease in the proportion of spawners maturing at the beginning of the season of mass reproduction and a decrease in the oxygen content in the near-bottom layer of the Gdansk Deep. It is suggested that a shift of the spawning peak of cod from spring to summer is related to a decrease in the volume of near-bottom waters providing favorable conditions for the reproduction of this population.