Remnant Oyster Reefs as Fish Habitats
As awareness of the extent of oyster reef loss has grown, so too has interest in oyster reef restoration. The development of realistic goals for restoration requires knowledge of the services provided by remnant oyster reefs, and the habitat contexts under which these services are maximised.
Francisco Martinez Baena of Macquarie University is addressing the role of remnant oyster reefs in providing food and habitat to wild fish communities. First, using underwater video surveys, comparing the fish communities supported by remnant oyster reefs to those provided by: (1) non reef-forming oysters in mangroves and on rocky shores; (2) other key habitats of the estuarine seascape (seagrass, mangroves, unvegetated sediments); and (3) oyster farms, which provide structure and a source of organic matter. Second, a combination of stable isotope and gut content analyses is being used to determine the trophic role of oyster reefs in estuarine seascapes.
In combination, video sampling and stable isotope analysis will provide critical information on the role of remnant oyster reefs in supporting wild communities of fish, many of which are recreationally and commercially harvested. The results will contribute to a business case, targeted at recreational fishing groups and fisheries managers, outlining the fisheries benefits of oyster reef restoration.
Oysters once created vast reefs along temperate Australia. However, historic overharvest using destructive fishing practices, and more recent disease and pollution, have rendered oyster reefs functionally extinct throughout much of their range, including south-eastern Australia, with > 85% of reefs lost (Beck et al. 2011). The loss of oyster reefs is of significance due to the important ecosystem services that they provide (Beck et al. 2011).
Currently, oyster reef restoration is gaining popularity in Australia and is increasingly being supported by recreational fishing groups, seeking to enhance fisheries productivity. This goal is based on the role of oyster reefs in enhancing fisheries productivity in south-eastern USA (Humphries & La Peyre 2015), but little is known about the role of oyster reefs in south-eastern Australia. Oyster reefs contribute to a mosaic of biogenic and abiogenic estuarine habitats, among which fish may migrate to reproduce, forage, and avoid predators and other stressors (Gillies et al. 2015). Consequently, the role of oyster reefs in supporting fish communities may vary spatially according to the identity, area and configuration of other estuarine habitats with which they co-occur and cannot necessarily be inferred from studies done elsewhere.
This research project is addressing: (1) the extent to which fish utilize remnant oyster reefs in south-east Australian estuaries for food and habitat; and (2) how this role of remnant oyster reefs compares to that of non reef-forming configurations of oysters in mangroves and on rocky shores, to other adjacent habitats (high productive seagrass beds), and to oyster farms, which comprise the largest aquaculture industry in NSW and add organic matter and structure to estuaries; and 3) how habitat context (i.e. the identity of adjacent habitat patches) influences this role of remnant oyster reefs.
Remote underwater videos will be used to couple censuses of fish community structure, in two NSW estuaries with remnant oyster reefs, Botany Bay and Port Stephens, with behavioural observations, and stable isotopic analyses.
At SIMS, the trophic role of oyster reefs will be analysed using stable isotope and stomach content analysis of key primary producers, invertebrate and fish species found on oyster reefs and in adjacent habitats. The diversity of estuarine habitats and producers therein makes an isotopic approach to trace the feeding of fishes particularly useful, by increasing the likelihood of finding habitat-specific isotopic signatures (Herzka, 2005). They are a powerful tool to identify the main trophic pathways and functional links between organisms at different trophic levels and their connectivity between these habitats.
The anticipated outcomes of this project are to, 1) provide the first information about the trophic ecology of our remnant oyster reefs in Australia, 2) define the trophic levels of this habitat and its connectivity to other adjacent estuarine habitats, 3) determine the importance of oyster reefs for recreationally and commercially important fish species in NSW estuaries and 4) determine the likely benefits to fisheries productivity of oyster reef restoration, and assess in which habitat contexts restoration aimed at enhancement of fisheries productivity is likely to be most successful.
In addition to the support of supervisor, A/Prof Melanie Bishop, Francisco Martinez Baena is collaborating with scientists at UNSW, NSW DPI Fisheries – Port Stephens, and James Cook University and receiving financial support from the Holsworth Wildlife Research Endowment Grant of the Ecological Society of Australia.