Oysters as fish habitat: a seascape perspective

Intertidal oyster reefs were once a widespread and conspicuous habitat of temperate and subtropical Australian estuaries. These reefs covered vast areas and formed complex three-dimensional structures providing a myriad of regulating and provisioning services. Today, oyster reefs are considered functionally extinct, occupying less than 5% of their distribution prior to industrialisation. Historic overharvest using destructive fishing practices and more recent water pollution and disease have placed this important habitat at the brink of national extinction. With growing recognition of the plight of oyster reefs in Australia, interest and investment in oyster reef conservation and restoration has grown. Yet despite the importance of understanding the ecological role of oyster reefs in setting restoration targets in Australia, their ecology remains largely unexplored. Furthermore, it is unclear how oyster farms, which were established in response to the collapse of wild oyster fisheries, replicate some of the ecological functions of natural reefs, and consequently may serve as de novo reefs in areas where oyster reefs cannot be restored. This thesis investigated the role of remnant intertidal Sydney rock oyster (Saccostrea glomerata) reefs and oyster farms in providing food, habitat and a nursery to fish. Remnant oyster reefs and oyster farms are located within a mosaic of other natural biogenic habitats, such as mangroves, seagrasses and bare sediment, among which fish may migrate tidally, seasonally or ontogenetically. Consequently, this study compared the fish communities of oyster reefs and farms with those of adjacent habitats. Within oyster farms, it also compared fish utilisation of two of the most extensively used cultivation methods: rack-and-rail and longlines-with-baskets. This study found that despite their degraded state, remnant oyster reefs were key habitats to fish within the temperate estuarine seascape, providing food, refuge and shelter, and serving as a nursery to juvenile fish. As expected, based on their structural differences, oyster reefs consistently supported more species and greater observations of both adults and juveniles when compared to bare sediment. However, oyster reefs also supported at least as many species, individuals and juveniles, and in many instances more, than other biogenic habitats, broadly regarded as essential fish habitats. Several recreationally or commercially fished species utilised multiple habitats, but displayed unique behavioural profiles on oyster reefs. A stable isotope study demonstrated that the role of oyster reefs extended beyond habitat attraction, to trophically underpinnig estuarine food webs. Oyster farms in many instances supported higher richness and observations of fish than natural habitats, and similar richnesses and observations to oyster reefs, particularly in the case of rack-and-rail farms. This challenges the view that aquaculture negatively impacts ecosystem services. Overall, this thesis demonstrates the crucial role oyster reefs play as fish habitat within the temperate estuarine seascape of south-eastern Australia. The knowledge generated by this study will serve as a baseline against which to establish realistic conservation and restoration objectives and will assist in identifying those settings in which restoration may reap greatest ecological benefits.