Elevated fish growth yet postponed maturation during intense marine heatwaves.

In the last decade, the northeast Pacific Ocean has experienced new climatic extremes with the occurrence of several marine heatwaves (MHWs), prolonged periods of warmer-than-normal ocean temperatures, likely as a result of anthropogenic climate change. The temperature-size rule is used to contextualize the impacts of climate change on fish maturity and growth, and predicts earlier maturation, faster growth of juveniles, and smaller adult body sizes with increasing temperatures. We investigated the temporal dynamics of fish reproductive development, maturity, and growth from 2014 to 2021, during intense and less intense MHW conditions. We estimated length and age at 50% maturity with histological sections of ovarian tissue samples collected from 644 female Black Rockfish (Sebastes melanops) caught off the Pacific Coast of the United States (42° N-49° N) and estimated von Bertalanffy growth function parameters with length-at-age data for a subset of 302 females. During intense MHWs, maturation was postponed, reproductive success was lower by a third, and parasite prevalence in ovaries was nominally higher. Younger females were larger at age during intense MHWs, and throughout the last decade, growth rate coefficients were higher than what is typically expected for slower-growing fishes, like rockfishes. The increase in juvenile growth during intense MHW conditions may be explained by the temperature-size rule, but our observation of postponed maturation contradicts theoretical predictions. Our work reveals that MHWs can induce shifts in fish growth and maturation, but that the temperature-size rule may not provide an adequate framework to predict how increasing temperatures associated with climate change may influence reproductive development and maturity for fishes with complex reproductive strategies. An understanding of how anomalous environmental conditions interact with fish life histories may help predict population vulnerability, which will be critical for future fisheries management under climate change.