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.