Consistent declines in maturation size but variable length-at-age responses to temperature across four fish species in a rapidly warming Baltic Sea lagoon

The temperature-size rule (TSR) states that in warmer conditions, ectotherms grow faster during their juvenile stages but sexually mature at smaller sizes. However, studies assessing this key TSR prediction across multiple species in natural ecosystems remain rare. In this study, we used long-term data (1949-2023) from the shallow and rapidly warming Curonian Lagoon (Baltic Sea) to examine changes in the length at maturity and length-at-age for 4 fish species: common bream Abramis brama , European perch Perca fluviatilis , pikeperch Sander lucioperca and roach Rutilus rutilus . We found that bream and roach exhibited significant and rapid declines in maturation length, with an average decrease of 0.5-1% yr -1 . The maturation length of perch and pikeperch also declined by approximately 0.7% yr -1 , though the statistical significance of these trends varied depending on the model used. In agreement with TSR predictions, temperature generally had a positive effect on juvenile length-at-age. However, its impact on the length-at-age of adult fish was more variable. The lengths of 5-6 yr old perch and 3-4 yr old pikeperch were negatively affected by warmer temperatures, whereas warming had a predominantly positive effect on this length-at-age trait of bream and roach across most age classes. We conclude that rapid changes in maturation size and length-at-age are occurring in fish populations of the Curonian Lagoon and are likely affecting other species as well. Routine monitoring of maturation and body size changes should be a priority for climate-adaptive fisheries management.