A scallop is a bivalve - a mollusc with a hinged shell. Several species occur off the coast of Western Australia but only the southern saucer scallop (Amusium balloti), is abundant enough to support a commercial fishery. In the southern saucer scallop, the lower valve is white while the upper valve is pink/brown. It can grow to 14 cm in length.
Scallop meat is a delicacy and in demand on local and international markets, creating a lucrative industry for WA scallop fishers.
Some scallop species can swim. The southern saucer scallop is an active swimmer, which affects the fishing method used - these scallops are trawled rather than dredged.
Although they settle on the bottom, the fishing gear disturbs them and they swim upwards, which is when they’re caught in the net.
Scallop stocks can vary greatly annually depending on environmental factors. We work closely with the industry to monitor stocks to predict where and when to catch scallops.
Distribution and habitat
The southern saucer scallop is found off the coast of WA between Broome and east of Esperance (as far as Israelite Bay). It occurs in greatest numbers in Shark Bay and the Abrolhos Islands. It lives where the ocean floor is sandy and often in sheltered environments found in bays or the lee of islands and reef systems.
The Southern saucer scallop can live for up to three years.
In Shark Bay (where the State’s biggest scallop fishery is based) scallop spawning occurs between April and December.
Eggs and sperm are released into the water and fertilisation takes place externally.
Tiny veliger larvae hatch from the fertilised eggs. During this period, larvae are passively transported by tides and currents. Generally scallops have a larval phase of 12 to 24 days.
After this time they settle to the bottom. Once settled, the tiny juvenile shells, known as ‘spat’, grow into juvenile scallops. By nine to 12 months of age, they are mature enough to spawn, and the lifecycle begins again.
Most scallops from one year’s spawning reach fishable size (shell length of about 90 millimetres) by April of the following year.
In WA, the Leeuwin Current carries warm water southward and influences coastal fisheries. While the relationship is not completely understood, in years when the Leeuwin Current flow is strong, scallop recruitment (addition of young scallops to the overall population) in Shark Bay is low, and vice versa. The effect may be caused by currents flushing scallop larvae out of the bay.
Scallops feed on minute plants and animals that they strain from the water by a filtering mechanism involving the gills and cilia.
Predators and parasites
Scallops are eaten by pink snapper, loggerhead turtles, rays, octopus, crabs and sea stars. A nematode worm sometimes infests scallops and although it presents no danger to humans, it can lower the value of scallops by ruining the appearance of the meat in older animals. However, if scallops are caught at the optimum age (about a year old) risk of worm infestation is almost eliminated.
Illustration © R. Swainston/www.anima.net.au