Freshly caught scallops sizzling on the barbeque are a Kiwi summer treat – but their annual abundance can be a changing feast.
Found in suitable habitat throughout the country, especially in Northland, Coromandel, Tasman Bay, Golden Bay, the Marlborough Sounds and Stewart Island, scallops are available for harvest for a set period each year, but it is widely known by fishers that scallop numbers can vary greatly on an annual basis.
In New Zealand, wild stocks of scallops support important commercial and non-commercial fisheries. Recreational and customary fishers harvest scallops by diving and dredging for them. Scallops are found around the coast, mostly in sheltered and semi-sheltered bays, from the low-tide mark out to about 50m depth, on soft sediments such as shell gravel, sand and silt.
Several factors can affect scallop numbers – including fishing rates, scallop biology, changes to the environment and, occasionally, disease.
NIWA fisheries scientists Keith Michael and James Williams have been involved in research to understand these factors for many years, and regularly provide advice to the scallop industry and the Ministry for Primary Industries on the status of the stocks and options to mitigate or remedy some of the factors affecting abundance.
"Scallop populations are well known for their highs and lows in abundance," says Michael. "This is, in part, due to the numbers and condition of spawning scallops and the early life history of young scallops.
"Both factors drive 'recruitment' – the addition of new individuals to the scallop population. When recruitment and early survival exceed mortality – from harvesting or natural causes – the population grows, creating a fruitful season. But the opposite can also be the case."
Scallops usually spawn during spring and summer, typically with an initial spawning in September/October and a second, often larger, spawning in November/December.
The timing varies from year to year depending on environmental conditions. Good feeding conditions before spawning provide an increase in the energy available to scallops to produce lots of good quality eggs and sperm (called maternal provisioning). Scallops release their eggs and sperm into the water, and their spawning behaviour is synchronised among individuals to ensure the eggs are fertilised.
"The more scallops that spawn at the same time in close proximity, the greater the chance of the sperm and eggs mixing, and the higher the levels of fertilisation and lots of young being produced," says Williams.
"Fertilised eggs develop into microscopic larvae which drift in the water column for about three weeks. Another factor affecting recruitment is whether the larvae remain in the same area where they were produced: if the ocean currents take them out of the local fishery area and away from suitable scallop habitat, the entire production from spawning can be lost."
After this three weeks of drifting, scallop larvae settle out of the water column to start life on the seabed as young scallops (known as spat). This settlement involves critical changes in the scallop body, a process called metamorphosis. Many factors determine the success of settlement, metamorphosis and survival of spat – including seabed conditions and predators such as snapper, blue cod, starfish and crabs, as well as the surface they settle on.
Clean surfaces free of silt – such as fine filamentous algae, hydroids and tiny tubeworms on the seabed – provide an anchoring point for the young scallops to attach a fine thread to, until they are large enough to live freely on the seabed from about 5mm in length. Young scallops cannot survive in muddy areas, so sediments can also have an impact on the survival of young scallops.
Scallops become sexually mature at about 70mm in length (by the end of their second winter), and usually take about three years to reach a legal harvesting size of 100mm in length.
But there's one fisherman's myth that can't account for annual variation, says Michael.
"Some fishers believe that whole scallop beds can move away from their fishing area – but it's just not true. Scallops can move a short distance to avoid predators, but they soon tire and cannot effectively move large distances."
"They are not running away from you. The annual variation in the number of scallops is simply related to factors, or a combination of factors, including levels of fishing, currents, sediments and even medium-term trends in the weather (climate).
Occasionally, disease or toxic algal blooms may also affect abundance.
"Climate can determine the delivery of nutrients required for phytoplankton to grow, which scallops feed on, and also the delivery of the food itself into the bays and to the seabed where the scallops are. The weather and climate can also play roles in suspending and transporting fine sediment from scallop fishery areas.
"By learning more about these factors and how they interact, we aim to better understand changes in scallop abundance and provide good advice to inform decisions that may provide for more scallops on the barbeque for summers to come," says Michael.
Keith Michael and James Williams are NIWA fisheries scientists whose research interests include shellfish biology and fisheries (e.g., scallops, oysters and surf clams), and the sustainable development of new shellfisheries. They also undertake research on the linkages between fisheries, shellfish populations and marine ecosystems.
Fast facts about New Zealand scallops:
- Scientific name: Pecten novaezelandiae
- Grow up to 150mm, legal non-commercial harvesting size is 100mm (90mm in the Challenger fishery (Golden Bay, Tasman Bay and the Marlborough Sounds).
- Can live 10 years or longer, but few reach this age.
- Usually found around the coast, mostly in sheltered and semi-sheltered bays, from the low-tide mark out to about 50m depth, on shell gravels to muddy sand substrates.
- Mainly found in Stewart Island, Tasman Bay and Golden Bay, the Marlborough Sounds, Coromandel coasts and Northland coasts.
- Have a fan-shaped bivalve shell with deep ridges that radiate from the hinge, and various patterns and colours ranging from white to brown/orange.
- Muscle flesh is white and the roe is usually two-tone.
- The large white [adductor] muscle holds the shell closed. Scallops are hermaphrodites (both sexes in the same individual): the white part of the roe is male and the bright orange part is female.
For information about harvesting scallops visit:
Scallops & dredge oysters | Fisheries New Zealand
