AbstractOngoing climate change is leading to browning of many lakes and coastal areas, which can impair fish body growth and biomass production. However, whether and how effects of light limitation caused by browning on fish body growth vary over early ontogeny is unknown. In this study, we set up a mesocosm experiment to test whether roach (Rutilus rutilus) body growth responses to browning depend on body size, and if findings are robust over roach densities. We also studied a potential mechanism for size‐specific responses by conducting an aquaria experiment to test if size‐specific prey selectivity in roach changes with browning. We found that roach body growth responses to browning‐induced light limitation vary over ontogeny (independent of roach density), negatively affecting body growth of young‐of‐the‐year (YOY) but not of 1‐year‐old individuals. We also show that this difference in growth response is likely a consequence of browning‐induced alterations in zooplankton community composition and variation in prey selectivity between YOY and 1‐year‐old fish. This suggests that we should account for the diverse effects of browning over fish ontogeny, mediated via altered prey composition and ontogenetic changes in prey preference, when assessing overall impacts of browning on aquatic ecosystems.