Size structure represents a key demographic characteristic of fish populations, playing an important role in maintain stability and reproductive output. However, fishing is a size-selective process, which truncates the size structure. In addition, warming can shift the population toward smaller size composition, owing to the metabolic constrains. Here, we examined the relative importance of fishing and warming in affecting size structure of exploited stocks, using data in the Northeast Pacific, Mediterranean Sea, and North Sea. We used variance partitioning approach to quantify the relative contribution of temperature, fishing, and combined effects on the size structure.

Our results showed that the variance (on average of 18%) of size structure was mainly affected by fishing; in comparison, temperature effects contributed less than 10 % of the variation. The combined effect on size structure was very subtle. Moreover, we found life history effects: stocks with a larger asymptotic length experienced more temperature effects, whereas stocks with a lower growth rate and a higher maturation age experienced higher fishing impacts.