WŁODZIMIERZ Grygiel and Mirosław Wyszyński
Sea Fisheries Institute, Kołłątaja 1, 81-332 Gdynia, Poland

Key words: herring, sprat, first sexual maturity, maturity ogive, southern Baltic basins.

Abstract.
The percentage of sexually mature males and females of herring from the southern coast spring spawning population and sprat at age groups and length classes were investigated according to aggregated years and the Bornholm and Gdańsk basins. The temporal (1980- 2001) changes in the proportion of mature clupeoids at age groups in ICES sub-divisions 24 and 25 were also analyzed. Moreover, the length of herring and sprat at first sexual maturity for particular year classes was calculated, and logistic curves of maturity ogives were elaborated. The possible influence of stock size, mean weight and length, body condition and seawater and air temperatures on the maturing of clupeoids in the southern Baltic is discussed. The results of the investigations indicated that there was a larger numerical share of mature herring and sprat in younger age groups than the one assumed by ICES working groups. In the 1990s the mean percentage of herring from age group 2, which begins spawning for the first time, decreased from 80 to 75% in the Bornholm Basin and from 92 to 82% in the Gdańsk Basin. In these decades the fraction of sprat from age group 1, which begins spawning for the first time, increased in comparison with the 1980s from 26 to 38% in the Bornholm Basin and from 14 to 18% in the Gdańsk Basin.