The influence of habitat structure on nearshore fish assemblages in a southern Australian embayment: Comparison of shallow seagrass, reef-algal and unvegetated sand habitats, with emphasis on their importance to recruitment

Abstract

Assemblage structure and juvenile recruitment of fishes was compared amongst three habitats: seagrass, Heterozostera tasmanica (Martens ex Aschers.) den Hartog; reef-algal; and unvegetated sand. Sampling was conducted monthly from October 1993 to March 1994 at three locations in Port Phillip Bay, southern Australia. A fine-mesh seine net was used to sample subtidally at a depth of approximately 0.5 m. Fish assemblages differed primarily between structured habitats and unvegetated sand, and a number of species previously reported to occur in seagrass habitat were also found to utilise reef-algal habitat. Species richness was highest in seagrass and lowest in unvegetated sand with significant differences amongst all habitats. Total abundance varied between habitats depending on location and month examined, but the most common pattern was highest abundance in seagrass and lowest in unvegetated sand, with a significant difference only between seagrass and unvegetated sand. At the level of individual taxa, pipefishes of the genus Stigmatopora showed a strong preference for seagrass habitat from an early juvenile stage. The King George whiting, Sillaginodes punctata (Cuvier and Valenciennes), showed a complex relationship with habitat, occurring on both seagrass and reef-algae immediately after settlement, but, with growth, showing an increasing preference for reef-algae before finally shifting to unvegetated sand approximately 4 months after settlement. Other species previously found as juveniles on seagrass beds also recruited to reef-algal habitats. Strong locality effects were also found, particularly for King George whiting. This variation was unlikely to be related to habitat structure, because macrophyte biomass showed much greater variation within locations than amongst locations. We conclude that while the presence of structure per se is sufficient for the recruitment of many species, some taxa will discriminate amongst habitats based on structural characteristics.