Altering the natural flow regime (an essential component of healthy fluvial systems) through hydropower operations may negatively impact freshwater fish populations. This systematic review investigates how changes in flow magnitude due to hydropower operations influence fish abundance and biomass. Following the guidelines of the Collaboration for Environmental Evidence, I examined all available evidence identified by a recent systematic map and search update. Of 103 articles (133 studies) included for data extraction and critical appraisal, 58 studies were included for quantitative meta-analysis. Fish abundance and biomass had variable responses to flow magnitude changes; overall mean effect sizes ranged from positive to negative, differing by study design and taxa. No consistent patterns in fish abundance and biomass responses to alterations in flow magnitude were identified and fish responses were context dependent. Responses may not be generalizable across systems impacted by hydropower where specific features of the system may be highly influential.