Forecasting size-at-maturity shifts under global warming: a case study on European hake, a widely distributed species

Abstract As global temperatures rise, the life-history traits of many marine species are changing, with important implications for fish stocks, ecosystems, and fisheries. In fish populations, both plastic and genetic adaptations in traits like body size and maturity timing are influenced by fishing pressures and environmental changes. As ectothermic animals, fish rely on external temperatures to regulate physiological processes, making them vulnerable to warming oceans. While shifts in fish distribution and abundance have received considerable attention, changes in size-at-maturity remain less studied. This study investigates how warming seas influence size-at-maturity, a key indicator of reproductive value and stock productivity, using European hake ( Merluccius merluccius (Linnaeus, 1758)), as a case study. We compiled a dataset spanning 1925–2021, including size-at-maturity records and corresponding temperature data. This dataset covers temperature ranges from the North Atlantic to the Mediterranean, providing a comprehensive look at the hake’s habitat. Using Generalized Additive Models (GAMs), we identified a consistent negative correlation between temperature and size-at-maturity, aligning with theories like James’ Rule and the Temperature-Size Rule (TSR). Under climate change projections (RCP 4.5 and 8.5), our results suggest that size-at-maturity in European hake may decrease by 4–9 cm by 2100, although the extent of this reduction varies across areas. These findings have significant implications not only for European hake, a key fishery resource, but also for other demersal species that may similarly experience reduced size-at-maturity in warming seas. This study highlights the need for adaptive management strategies to address climate change effects on marine ecosystems and fisheries.