The oceans are warming, and marine heatwaves are increasing in frequency, extremity and duration due to climate change. As fish are ectotherms, temperatures above their optimum have a host of physiological and demographic impacts, which result in a net negative effect on population biomass and productivity. However, temperatures decline with depth, which means mesophotic ecosystems, found in the ‘twilight zone’ between approximately 30 and 150 m depth, have the potential to act as thermal refuges.
By analysing 27 years of temperature-depth data, we found that marine heatwaves were buffered at mesophotic depths relative to euphotic depths at Tawhiti Rahi and Aorangi (the Poor Knights Islands) in Aotearoa, New Zealand. We then parameterised a temperature-dependent multispecies size-spectrum model for the Poor Knights Islands coastal reef fish community and used it to run marine heatwave simulations in the presence and absence of a mesophotic thermal refuge. Almost all heatwave strengths resulted in biomass and productivity reductions for almost all modelled species. This thermal refuge effect has significant conservation and management implications, warranting the targeted protection of mesophotic reefs where they occur.