Climate change could shrink fish sizes

An IMAS-led study has revealed that global climate change will affect fish sizes in unpredictable ways and consequently impact complex food webs in our oceans. The study analysed three decades of data from 30,000 surveys of rocky and coral reefs around Australia, confirming that changes in water temperature were responsible for driving changes in average sizes of fish species across time and spatial scales. The findings from the study were published in the journal Nature Ecology and Evolution.

“Cold-blooded animals, especially fish, have long been noted to grow to a smaller size when raised in warmer temperatures in an aquarium. If fish grow to smaller sizes in warmer aquaria, it is only natural to expect that global warming will also lead to shrinkage of adult fish size. However, average fish body size in wild populations are affected by growth, mortality [and] recruitment as well as interactions with other organisms and their environment simultaneously, and it is unclear how all of these factors are affected by temperature,” said Dr Asta Audzijonyte, a scientist at the Centre for Marine Socioecology.

Researchers found that while temperature has a significant impact, it caused different fish species to react differently. In 55% of the species the average fish body size got smaller as predicted, while it increased in others (around 45%). In general, larger species tended to get even bigger in warmer climates, while smaller species tended to get smaller. Tropical species were more likely to be smaller at the warm end of their distribution ranges. The species that were smaller at the warmer edges of their habitat ranges were also more likely to get smaller with global warming.

“At Tasmanian survey locations, where some of the fastest rates of warming were observed, up to 66% of species showed clear changes in body size. As well as happening quite quickly, some of the size changes can also be surprisingly large. For example, the change in a median-length temperate fish corresponds to around 12% of its body mass for each 1°C of warming. At the current rate of warming, in 40 years this would result in around a 40% change in body length, either increasing or decreasing depending on the species,” said Dr Audzijonyte.

The varying responses of species to warming could have implications for food webs and ecosystems, including their stability and resilience to other external stressors, such as fishing, coastal pollution and other climate change impacts.

The study was supported by a collaboration between University of Tasmania scientists and government managers across Australia, and by the efforts of over 100 volunteer Reef Life Survey divers, who have undertaken regular surveys at over 1000 sites around the continent.

Image credit: Antonia Cooper.