Sediment variability affects fish community structure in unconsolidated habitats of a subtropical marine park

Unconsolidated sediments form extensive seafloor habitats worldwide, yet few studies have examined fish assemblage patterns in relation to specific sediment type or morphology. Baited remote underwater videos (BRUVs) were deployed at 3 sites (3 replicates per site) in the Solitary Islands Marine Park (SIMP), in each of 4 apparent 'habitat types' identified from swath acoustic mapping backscatter imagery and bathymetry. Habitat types ranged in reflected signal backscatter intensity: very dark, dark, light, very light. Additionally, apparent morphological features were associated with each (e.g. very dark = lens-shaped; light = sand waves). Sediments were characterised at each site by conducting granulometric analyses of samples collected using a van Veen grab. Both the darker 'habitat types' were found to be gravels and both lighter types were fine sands. There was a strong pattern in fish assemblage structure in relation to habitat type, and distinct assemblages were seen between 'gravel' and 'fine sand' habitats. Species richness was higher in gravel than in sand habitats. Additionally, 6 species were only recorded in the very dark lens habitat. However, there were no apparent differences in assemblage patterns between the 2 fine sand habitats. Of the different granulometric measures, fish assemblages were most strongly correlated with a combination of median grain size, % gravel and arithmetic sorting. The current habitat classification system for the SIMP includes habitat type (unconsolidated or hard substrata) as criteria for conservation planning. Further division of unconsolidated habitats into 'gravel' and 'fine sand' categories is needed to better represent biotic assemblages.