Some like it salty - examining fish assemblages across an extreme salinity gradient, Shark Bay, Western Australia

Variation in environmental conditions drive changes within aquatic ecosystems and their associated inhabitants, with salinity and temperature influencing the spatial and temporal patterns of biological communities. An extreme salinity gradient exists within the Eastern Gulf of Shark Bay UNESCO World Heritage Area, Western Australia, reaching <91.5 psu within Hamelin Pool Marine Nature Reserve. We deployed 288 baited remote underwater stereo-video systems (stereo-BRUVs) across four habitats (sand, seagrass, low relief stromatolites, and high relief stromatolites) and three locations (Eastern Gulf, Faure Sill, and Hamelin Pool) to examine variation in fish assemblages across this salinity gradient. A total of 20,533 fish from 90 species (44 Families) were identified, 86 % of which were carnivores, with only one herbivorous species recorded. Two species accounted for 50 % of fish observed (Helotes octolineatus (30 %) and Pentapodus vitta (20 %)). Variations in assemblages were driven by changes in salinity and habitat. The number of species decreased with increasing salinity, while seagrass habitats consistently had the highest abundance and number of species. However, the composition of seagrass affiliated fish assemblages differed among locations. Seven species observed are targeted by recreational fishers within Shark Bay. Chrysophrys auratus were significantly larger, and Epinephelus coioides and Lethrinus laticaudis more abundant in Hamelin Pool, a potential marine reserve effect. Despite extreme changes in salinity, fish assemblages here are diverse, with Hamelin Pool potentially facilitating the replenishment of species that are fished outside of the Marine Nature Reserve. Under the current effects of climate change, hypersaline environments around the world are likely to see salinity levels increase further, with a potential decrease in the diversity of their fish assemblages.