Centurial Variation in Size at Maturity of Eastern Baltic Cod (Gadus morhua) Mirrors Conditions for Growth

The status of Eastern Baltic cod (EBC) Gadus morhua has remained poor despite low fishing mortality for over a decade, including a fishing ban since 2019. Although the decline in productivity can be explained by lower individual growth and survival rates, other aspects of life-history changes such as maturation patterns for EBC has so far not been sufficiently explored. According to current stock assessments, the median size at maturity (L ₅₀) has halved from 40 to around 20 cm in total length since the 1990s, while the overall size distribution has become increasingly truncated. It has previously been suggested that changes in L ₅₀ can be attributed to both fishing-induced evolution and phenotypic plasticity induced by growth rates. However, since L ₅₀ is currently occurring around 20 cm, the maturation process must have been initiated at much smaller sizes, that is, long before the fish could be caught in the dominant trawl fishery at around 35 cm. In this study, we aimed to further investigate what drivers may have led to reduced productivity in EBC by determining variations in size at sexual maturity in longer time series than has been done before (1930s to 1980s) and include prey productivity and quality. We found that L ₅₀ declined already in the 1930s and thereafter remained stable at around 40 cm up to the 1990s. On a centurial perspective, L ₅₀ has been positively correlated to growth potential (L ₉₅), length diversity, total stock biomass, total catch and yield per recruit, while Fulton's condition factor was not related to L ₅₀. Our results suggest that the links between life-history parameters and external drivers are complex, but the present unprecedented early onset of maturity and hence decline in L ₅₀ since the 1990s signals a decline in growth potential, which also has hampered the productivity of EBC.