Ocean warming is changing marine ecosystems globally, including through shifting species distributions, local population dynamics and abundance, and ultimately altering community structure. Shallow water marine reef fish communities are changing particularly rapidly, in large part because widely dispersing larvae and relatively few latitudinal biogeographic barriers result in highly dynamic geographic distributions that are strongly shaped by temperature (Sunday et al. 2011; Stuart-Smith et al. 2017). As such, reef fishes represent a responsive model group for how marine biodiversity is responding to changing sea temperatures, through short-term extreme events (e.g. marine heatwaves) and long-term warming.

The Reef Fish Thermal Index is the Community Temperature Index (CTI) calculated from comprehensive reef fish community structure data from in situ monitoring. It is designed to assist in understanding climate impacts in vulnerable reef habitats such as coral reefs or kelp forests by quantifying community structure changes in reef fishes that occur through local extinctions, immigration and shifting local abundance trends that can be readily linked to temperature.

The mechanism for linking these changes in species composition and abundance to temperature is by using each species’ thermal affinity (a species temperature index) as a continuous measure of the ocean temperature that appears ‘optimal’ for its wild populations. The midpoint of the realised thermal range is typically used, but it appears likely that predictive capacity can be refined by using the temperature at which local population density is greatest across a species’ distribution – i.e. the temperature of maximum abundance (Waldock et al 2019).

A changing balance in the composition and abundance of warmer- versus cooler-water species in a community indicates the influence of temperature change on that community (e.g. Bates et al 2014, Day et al 2018). Larger regional trends across many communities can provide a powerful indication of the scale of biodiversity change that is occurring from environmental change and is a useful communication tool for governments and the public.