Filtering New Life to Port Phillip Bay

Most of us have enjoyed a dish of mussels or oysters at a family Christmas celebration, but many may not know the benefits shellfish provide to their natural habitat or their previous abundance in our very own Port Phillip Bay.

A shellfish reef.  Image: The Nature Conservancy

A shellfish reef. Image: The Nature Conservancy

Like many other places around the world, Port Phillip Bay was once home to a number of temperate habitats including shellfish reefs, soft corals beds, seagrass meadows and salt marshes. Tragically, however, dredging and overfishing have seen a vast depletion of these important habitats. Since the 1800s, native flat oysters were dredged in large numbers for food and for their shells used to make limestone. This practice continued into the 1970s, leaving an estimated 90% reduction of the mussel beds and oyster reefs. Despite the discontinuation of the practice, the populations have never recovered, leading to large portions of the seafloor existing as ghost towns.

Filtering the water column

Shellfish are filter feeders and their ecological importance has not been fully appreciated until recently. Shellfish reefs are essential to the health of marine ecosystems, yet they are seen solely as assets managed by fisheries. They provide a host of ecosystem services including improving water quality, providing a substrate for other species of plant and animal to grow upon, and the creation of habitats that allow other invertebrates to exist. Amazingly, shellfish can filter several bath tubs worth of water a day (up to 150 litres), leading to a reduction in dangerous algal blooms and improving the clarity of water to allow more sunlight to penetrate the depths of the bay. They are also a food source for snapper and other commercially important fish species.

Reefs at risk

The recent report Shellfish Reefs at Risk revealed that shellfish reefs are the most threatened marine habitat on earth. At a global level, 85% of oyster reefs have been completely lost and many of the remaining reefs are functionally extinct. According to the report, shellfish reefs are at less than 10% of prior abundance and wild capture rates of shellfish are down millions of tonnes per year compared with 50 to 100 years ago. 

Baby oysters attached to scallops and mussels for deployment.  Image: ABC / Cameron Best

Baby oysters attached to scallops and mussels for deployment. Image: ABC / Cameron Best

A bright future: Business working with nature

While there appears to be considerable reason for alarm, a new and exciting three-year initiative worth $270,000 is taking place between The Nature Conservancy, Fisheries Victoria and the Albert Park Yachting and Angling Club, which aims to restore Port Phillip Bay’s shellfish reefs. This exciting project is part of a larger Conservancy project called the Great Southern Seascape that hopes to restore a host of marine habitats in South Australian waters. The exciting project was initiated by Fisheries Victoria and the Albert Park Yachting and Angling Club, whose members have identified the loss of productive snapper habitat in and around Hobsons Bay as early as the 1980s.

The first part of this ambitious project seeks to restore shellfish beds to three areas of Port Phillip Bay, including Hobsons Bay (St Kilda), Geelong and Chelsea by attaching up to 40,000 juvenile flat oysters and mussels to empty shells of scallops and limestone beds. I know what you’re thinking: “How many shells are going to be needed to house 40,000 juvenile oysters?” Well, the daunting task of sourcing empty shells has been made considerably easier with the involvement of local brewery Little Creatures that has arranged to donate thousands of their used shells to the project instead of throwing them out. Once collected, the shells get mixed together with lime to create the limestone bedrocks needed as a substrate.

A deployed shellfish reef.  Image: Paul Hamer

A deployed shellfish reef. Image: Paul Hamer

The next step is for divers to place the young oysters and mussels from the hatchery on top of the limestone beds where it is hoped they will settle and successfully establish the reef. If the initial deployment is successful, the reefs will develop into house-sized blocks that will improve the overall health of the bay and increase the populations of octopus, crab, rockling and snapper.

This novel initiative optimistically signifies a fresh relationship between government, environmental groups, and businesses collaborating for a common purpose and paves the way for similar projects to be undertaken. If successful, this will vastly improve the wellbeing of the Bay, as well as create sustainable practices for recreational fishing in the future.