Increasing mean fish length under warming can be reconciled with the temperature-size rule when mortality-to-growth ratios decline

Rising ocean temperature alters fish growth and mortality, but its effects on the length-frequency distribution of fish populations remains poorly understood. Here, we use an equilibrium age-structured fish growth model to simulate population length-frequency distributions under various combinations of the von Bertalanffy growth parameter ( K ) and mortality ( M ) rates, to investigate how widely expected temperature-driven reductions in asymptotic length ( L inf ) may affect mean and maximum lengths in a population. We show that substantial variation in mean length (up to 40%) can occur under empirically observed M / K ratios, with no change in L inf . We also find that mean length can increase by up to 25% despite a 10% reduction in L inf . This only occurs if M decreases or K increases such that population M / K ratios decline by 25-50% across spatially separate populations experiencing different temperature conditions. However, whether such a reduction in the M / K ratio in a population is realistic under climate-driven temporal warming remains uncertain. Length-frequency distributions are a key driver of trophic and non-trophic interactions in aquatic communities and are core indicators of exploitation status in fisheries management. Anticipating their responses under future climate scenarios is therefore critical to assess human impacts and to manage fisheries sustainably.