Bigger juveniles and smaller adults: changes in fish size correlate with warming seas.
Increasing sea temperatures are predicted to decrease body size of marine ectotherms based on the temperature size rule. This will impact fisheries yields, but empirical evidence of the process is still limited. We used fishery-independent bottom trawl survey data from 1970 to 2017 to examine the trends of length-at-age of four commercially important demersal fish species (cod, haddock, whiting and saithe) in two study areas facing increasing sea temperatures: the West of Scotland and the North Sea. We then compared the trends of length-at-age with annual bottom sea temperatures. The mean length-at-age of adults declined over the study period, in all species and in both areas, except for cod in the West of Scotland. A common trend of decline in adult length was inversely correlated with bottom sea temperatures. Correlations with temperature at seven yearly time-lags were significant and negative in the North Sea. Correlations were only significant at lags of 1 and 2 years in the west of Scotland, where sea temperature warming was twice as slow. The mean length-at-age of juveniles concurrently increased, which has not hitherto been reported. This trend, shared by all species and both regions, correlated positively with rising temperature, suggesting that our study species have a faster growth rate due to increased temperatures. Synthesis and applications. We examined the body size of a range of commercially exploited fish species, at different age groups, from two management regions. We found that juvenile fish have been getting bigger and adults smaller in both regions. These changes were correlated with rising sea temperatures, providing empirical evidence that global warming is affecting the size of commercial fish species. The effects of these changes on productivity of fish populations and fisheries yield now require investigation. Temperature changes should, therefore, be included into forecasts used in fisheries science in order to mitigate the impact of global warming and maximise sustainable yields.