Evidence is accumulating that many fish (and marine ectotherms in general) are undergoing rapid changes in their life-history characteristics, such as growth, maturation and size-at-age. These changes are likely to be caused by fisheries and climate change, but the exact mechanism and relative contributions remain debated. Changes in fish body sizes due to increasing temperatures have been particularly controversial. On one hand, across multiple experiments, ectotherms reach smaller adult sizes at higher temperatures. This “temperature-size rule” is one of the few universal rules in ecology. Yet, the mechanisms behind these rules are far from clear. Most importantly, we do not know whether the experimentally observed “shrinking” with temperature also applies to wild populations, where growth, recruitment, mortality, food limitation, predator avoidance interact. Some models forecast a 15-30% decrease in body sizes due to warming by 2050, but the assumptions of these models have been questioned. So why don’t we look at the data? 

Until now most size data was available for commercially fished species only, where effects of temperature changes and fishing are very hard to separate. Luckily in the recent decades researchers have accumulated a lot of data from underwater visual surveys by divers or from cameras. The data is not necessarily straightforward, but with new statistical methods we can tap into its potential. Together with colleagues, we have analysed ten million visual survey records around Australia, spanning multiple decades and comprising all common coastal reef fish (335 species) (www.reeflifesurvey.com).

We found that temperature indeed drives spatial and temporal changes in fish body size, but not consistently in the negative fashion expected. Around 55% of species were smaller in warmer waters, but 45% were bigger. Interestingly, and in contrast to the “oxygen limitation hypotheses” (check out Pauly et al. and Lefevre et al. debate over the last few years), it was the smallest species that were most likely to shrink with warmer temperatures. Even more interesting and unexpected however, is that by using intra-specific body size patterns through space we make general predictions on how a species will respond to warming through time. This prediction is crucial, because average body lengths of many species in warming locations are changing by nearly 1% per year, but most fisheries and ecosystem models do not account for this trend.