Key words: shellfish reefs, native flat oyster, blue mussel, ecological restoration, marine ecosystem
Background. Globally, shellfish reefs are the most threatened marine habitat on earth. Research published by The Nature Conservancy documented that that over 85% of shellfish reefs have been lost from coastal areas worldwide, with 99% of shellfish reefs ‘functionally extinct’ in Australian coastal waters, including within Port Phillip Bay (Shellfish Reefs at Risk Report).
This dramatic loss of shellfish reef habitat in Port Phillip Bay had occurred by the mid to late 20th century, caused by over-harvesting through destructive dredge fishing, further compounded by pollution, predation and disease in later years.
In an Australian first, The Nature Conservancy Australia (TNC) are part of a research partnership that are trialling different approaches to restoring Port Phillip Bay’s lost shellfish reefs (video link).
Shellfish reefs are intertidal or subtidal three-dimensional habitats formed by oysters and/or mussels at high densities. Shellfish reefs can vary in appearance depending on the dominant reef-forming species. There are many common attributes of shellfish reefs including:
- They provide habitat and refuge for other species including sessile and mobile organisms, supporting high levels of species diversity and unique assemblages;
- They can accrete dead shell material such that the reef grows in size and mass over time;
- They provide food for other organisms, either when consumed directly or through the species assemblages they support.
Figure 1. Clumping native Flat Oysters at 9ft Bank in Port Phillip Bay
Figure 2. Remnant Oyster Reef in Georges Bay, St Helens, Tasmania. (Photo: Chris Gillies)
Restoring the Lost Shellfish Reefs of Port Phillip Bay. A three-year trial was established in late March 2015 to investigate the following research questions:
- Can the oysters simply grow on the bottom or do they need a rubble base?
- Can oysters be deployed at a young age and survive, or is it more beneficial for a grow-out on aquaculture leases to gain a ‘headstart’?
- At what densities do we need to deploy mature mussels? (i.e. Can they create mussel beds naturally on the sediment or require substrate?)
Reference ecosystem. Historical information and relictual evidence shows that the shellfish reefs of Port Phillip Bay were subtidal with the dominant species being native flat oyster (Ostrea angasi) and Blue Mussel (Mytilus (edulis) galloprovincialis). Healthy reference sites for such reefs are very limited in Southern Australia. Within Port Phillip Bay the only site found so far is a dispersed clumping reef called 9ft Bank (Fig 1). A remnant shellfish reef also occurs in Georges Bay, off St Helens in Tasmania (Fig 2). Further research is planned for the Tasmanian site to complete a biological assessment to inform long-term restoration targets and reef design at Port Phillip Bay and other future sites in the region.
Locations of the restoration trials: The intent is to conduct restoration trials in three locations within Port Phillip Bay, although currently works are occurring at only two sites: Wilson Spit (Outer Geelong Harbour) and Margarets Reef (Hobsons Bay) (Fig 3). These are both old shellfish reefs that are largely dead and covered by sediment (Fig 4). The depth range is between 6 to 8 metres depth with Wilson Spit being a silty mud bottom and Margarets Reef sand.
Figure 3. Port Phillip Bay Shellfish Reef Restoration sites.
Figure 4. Relictual evidence of previous oyster reef at Wilson Spit restoration site. (Photo: Paul Hamer).
Works Undertaken. As Port Phillip Bay is both reef substrate- and recruitment-limited a reconstruction approach (involving rebuilding substrates and reintroducing oysters and mussels) is a necessary starting point for the restoration, with the longer term expectation of natural colonisation.
The trial has involved the deployment of a total of 6 tonnes of limestone marl substrate in a patchwork of 1m x 1m plots at both sites. Native flat oysters are being raised at the Victorian Shellfish Hatchery and their larvae settled on recycled scallop shells (called cultch) (Fig 5). The larvae are then left for a 3-6 month period on an aquaculture lease before being deployed onto the substrate base (Fig 6). To date over 20,000 live oysters have been deployed to seed the reefs. In addition, over 6 tonnes of blue mussel have also been deployed at different densities and in 3 x 3m plots (Fig 7).
Figure 5. Cultch spat growing out at the Bates Point Aquaculture Lease. (Photo: Ben Cleveland)
Figure 6. Limestone rubble base with cultch spat. (Photo: Paul Hamer)
Figure 7. Deployed mussel bed at Margarets Reef. (Photo: Paul Hamer)
Monitoring Methodology. The University of Melbourne are contracted to lead the monitoring in Stage 1 of the restoration trial. Baseline sampling was conducted of the trial pre-deployment (trial layout is shown in Fig 8) and subsequent monitoring to be carried out 6 months and 12 months after deployment. Monitoring includes measuring:
- Oyster survival per shell on the various substrate treatments
- Oyster growth on the various substrate treatments
- Mussel survival (inner cores only) and mussel growth as well as shell cover and predator density
- Baseline community sampling (pre-deployment) of mobile fish, cryptic fish, mobile invertebrates, benthic biota and benthic substrate.
Figure 8. An example of the oyster reef experimental design at the Margaret Reef site.
Lessons Learned and Future Directions. Early monitoring results from both sites show that oyster spat survival is greater if deployed on a rubble base than directly to the seabed, with cultch loss high on sand, due to burial. Oysters grew on average five times as fast on rubble than sand over winter. We conclude from this that elevation is important for both the survival and growth of oysters.
For the mussels the highest density treatment had the highest mortality at both sites, suggesting that the low density treatment improves survival and may be the most cost effective approach.
The most abundant predator was the native Eleven-arm Seastar (Coscinasterias calamaria).
We consider that scale is important in helping to minimise early losses and this hypothesis will be tested in the second stage of the trail. Planning is in place to scale-up the trial to 20 x 20m plots in late 2016, with a mixed-species approach, combining mussels and oysters rather than having separate treatments. Elevation through large and small limestone rubble will also be tested, integrated with recycled shells sourced from restaurants and wholesalers.
Stakeholders and Funding. The Restoring the Lost Shellfish Reefs of Port Phillip Bay Project is a key element of The Nature Conservancy Australia’s Great Southern Seascapes Program and delivered in partnership with the Victorian Government (Fisheries Victoria) and Albert Park Yachting and Angling Club. All partners have contributed funding towards the project and continue to fundraise.
Contact. Simon Branigan, Estuaries Conservation Coordinator, The Nature Conservancy Australia, Suite 2.01, The 60L Green Building, 60 Leicester Street, Carlton, VIC 3053, Australia. Tel: 0409087278. Email: firstname.lastname@example.org
WATCH FIRST VIDEO: Shellfish reef restoration in Port Phillip Bay
WATCH SECOND VIDEO: Trialling shellfish reef restoration techiques for potential application across Australia