AbstractLife‐history traits, such as size‐at‐maturity, are key parameters to model population dynamics used to inform fisheries management. Fishery‐induced evolution, density‐dependent effects, and global warming have been shown to affect size‐ and age‐at‐maturity, and resulting spawning stock biomass (SSB) in a wide range of commercial fish stocks. Marked changes in redfish biomass and environmental conditions in the Gulf of St. Lawrence and Laurentian Channel over the past decade called for a review and update of size‐at‐maturity for commercially important deepwater redfish Sebastes mentella and Acadian redfish Sebastes fasciatus stocks. Following a 25‐year moratorium, local redfish biomass has recently reached unprecedented levels, co‐occurring with an overall warming of bottom water temperatures. Our objectives were (1) to perform a histological assessment of redfish reproduction stages, including the validation and fine‐tuning of a robust visual chart to facilitate monitoring of size‐at‐maturity and SSB in a transforming environment, and (2) to evaluate changes in size‐at‐maturity in unprecedentedly strong cohorts of redfish, and consequences for stock status assessment and fisheries management. Each specimen was genetically identified to species, and gonad reproduction stages were determined by histology and macroscopic appearances. The present study enabled a robust visual chart for continued and cost‐effective monitoring of redfish reproduction stages to be refined and validated, and has shown a large decrease in redfish length when 50% of the individuals are considered mature that led to an increase in estimates of SSB during the 2011–2021 period for S. mentella and S. fasciatus. These changes modified the perception of stock status, thus having significant implications for fisheries management. Given that fishery‐induced evolution and community structure changes along with global warming are affecting numerous stocks worldwide, the present study outlines a major and global challenge for scientists and resources managers. As shown by our results, the monitoring and frequent updates of life‐history traits in transforming environments are needed to provide reliable science advice for sustainable fisheries.