Potential climate-mediated changes to the distribution and density of pomacentrid reef fishes in south-Western Australia

© The authors 2018. Climate change and associated increased water temperatures pose a substantial threat for the future of marine ecosystems. Temperate Australia is a global biodiversity hotspot which has experienced ocean warming rates 2 to 4 times faster than the global average. To better understand the effects of these temperature changes on marine fish distributions and densities, we surveyed territorial damselfishes across 2000 km of temperate coastline in south-Western Australia. Diver-operated stereo-video was used to determine if the distribution and density of 4 pomacentrids (Parma occidentalis, P. mccullochi, P. victoriae and Pomacentrus milleri) and their biotic habitat changed between 2006 and 2015, a time period characterised by gradual warming trends and an extreme marine heatwave. Surveys showed that the density of the warm-water pomacentrids P. milleri and P. occidentalis increased, while cool-water P. victoriae and intermediate species P. mccullochi showed no changes in density. In northern, warmer waters, the density of habitat-forming algal species such as Ecklonia radiata decreased, while turf algae species increased. In general, 2015 was characterised by a shift toward non-canopy algae habitats when compared to 2006. The observed changes in fish assemblages were likely caused by a combination of increased temperatures and changes in habitat-forming algal species. These changes along the Western Australian coast provide insights into the different nature of cool- and warm-water affiliated species' responses to ocean warming and biogenic habitat changes associated with climate change