Abstract Ectotherms are predicted to “shrink” with global warming, in line with general growth models and the temperature-size rule (TSR), both predicting smaller adult sizes with warming. However, they also predict faster juvenile growth rates and thus larger size-at-age of young organisms. Hence, the result of warming on the size-structure of a population depends on the interplay between how mortality rate, juvenile- and adult growth rates are affected by warming. Here, we use two-decade long time series of biological samples from a unique enclosed bay heated by cooling water from a nearby nuclear power plant to become 5–10 °C warmer than its reference area. We used growth-increment biochronologies (12658 reconstructed length-at-age estimates from 2426 individuals) to quantify how &gt;20 years of warming has affected body growth and size-at-age and catch data to quantify mortality rates and population size- and age structure of Eurasian perch ( Perca fluviatilis ). In the heated area, growth rates were faster for all sizes, and hence size-at-age was larger for all ages, compared to the reference area. While mortality rates were also higher (lowering mean age by 0.4 years), the faster growth rates lead to a 2 cm larger mean size in the heated area. Differences in the size-spectrum exponent (describing the proportion of fish by size) were less clear statistically. Our analyses reveal that mortality, in addition to plastic growth and size-responses, is a key factor determining the size structure of populations exposed to warming. Understanding the mechanisms by which warming affects the size- and age structure of populations is critical for predicting the impacts of climate change on ecological functions, interactions, and dynamics.

Larger but younger fish when growth outpaces mortality in heated ecosystem