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

Kirsty Richards¹, Euan Harvey¹, Ben Saunders¹, Shaun Wilson^{2, 1}, Erica Suosaari^{3, 4}

¹Curtin University, Perth, Western Australia - ²Australian Institute of Marine Sciences, Western Australia - ³National Museum of Natural History, USA - ⁴Bush Heritage Australia, 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. Areas of extreme salinity are rare and understudied. An extreme salinity gradient exists within the Eastern Gulf of Shark Bay UNESCO World Heritage Area, Western Australia, reaching <91.5psu 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.