Rapid life history shifts in a declining fish stock: the case of Western Baltic cod in the Sound

Life history changes in exploited fish stocks, such as shifts in growth and sexual maturation, are often linked to fishing pressure and environmental change. Intense targeted fishing lowers stock reproductive potential and resilience, hampering recovery despite management interventions. The Western Baltic cod (WBC) stock size has declined over the past three decades. Using scientific survey data from 1992 to 2026, along with commercial and recreational fisheries data, we analysed temporal trends in fish length, length-at-age, body condition (Fulton’s K), and maturity parameters of the cod (Gadus morhua) in the Sound, a historically healthy and productive WBC sub-population. We found truncation in size structure, with fewer large individuals, declining length-at-age, reduced body condition and substantial decrease in estimated L50 and A50 (length and age at 50% maturity). Commercial (gillnets) and recreational (rod & line) catches consistently contained larger cod than the survey catches, and recreational fisheries selectively removed cod with higher body condition, indicating strong selective harvesting. These patterns indicate demographic restructuring and a degrading biological state of cod in this important spawning area, potentially contributing to the poor current status of the WBC. Our results highlight the need to incorporate biological indicators beyond Spawning Stock Biomass (SSB) and fishing mortality, such as body condition, size structure and maturity dynamics, into stock assessment. While reducing fishing mortality is necessary for stock recovery, declines occurring after prolonged depletion or under adverse environmental condition may limit rebuilding potential. Revealing and understanding changes in life history traits is therefore critical for evaluating recovery potential and informing precautionary sustainable management.