Changes in size-at-maturity of Baltic cod (Gadus morhua) during a period of large variations in stock size and environmental conditions

Landings of the eastern Baltic cod stock peaked at 390 000 t in 1984 and decreased dramatically to below 50 000 t in 1993. The change in corresponding stock size is estimated at a six-fold decrease by weight until 1992. Suitable levels of oxygen and salinity for successful spawning of Baltic cod were very low in the late XOs with an increase during more recent years. In this study the coinciding changes in size-at-maturity (length and age) and the relation of these observations to population density (spawning biomass and recruitment), variations in environmental stress (volume of suitable oxygen and salinity levels) and fishing mortality are investigated. The data indicate a decrease in mean length-at-maturity from 49.6 cm (total length) in the late 1980s to 33.2 cm in 1996 for females with an increase (36.8 cm) in 1997. Age at which 50% of females were mature also decreased from 3.7 years in 1988 to 2.7 in 1996. The same trend is shown for males. A significant positive relationship was found between size-at-maturity (length and age) acid spawning biomass. No correlation was found between fishing mortality and size-at-maturity for either males or females of Baltic cod, so changes in size-at-maturity may be density-dependent. However, the age-at-maturity was negatively correlated with environmental stress conditions in the spawning area for females and males. Different possible causes of early maturity are discussed. (C) 1999 Elsevier Science B.V. All rights reserved.