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Topic: Great scallop

Great scallop are common along the coasts of the North-eastern Atlantic Ocean from the Iberian peninsula in the south to Vestfjorden in the north. They are found on sandy bottom from just below the intertidal zone and down to depths of more than 100 m. In Norwegian waters they are most common at depths of 5–30 m in the counties of Sør-Trøndelag, Nord-Trøndelag and Nordland.

Scallops normally recess in the seabed sediments with their flat valve facing upwards, flush with the sea bottom and covered by sediment.

They are generally found in areas with strong currents, on a variety of seabed types, ranging from fine to coarse gravel, which may or may not be mixed with mud and organic matter. They feed on phytoplankton, bacteria, other micro-organisms and dead organic matter (detritus). Their most important sources of food are phytoplankton and microscopic algae found on the seabed substrate. The water currents supplies the food particles, and in many places factors such as the depth, tides and topography will affect variations in their access to food. Seasonal fluctuations in phytoplankton production also cause great variation in both the quantity and quality of nutrition available to the scallops.

The distribution of great scallops in Norwegian waters is significantly limited by low winter temperatures and salinity. Climate change accompanied by milder winters may therefore explain the extension of their distribution northwards that is seen in the Lofoten islands. Scallops have low tolerance of reduced salinity, and any change in the amount of fresh water entering coastal waters may also affect their distribution along the coast.

Status and recommendations

In Norway great scallops are only harvested by divers, with the core area for the fishery being in Trøndelag county. The Institute of Marine Research carry out regular surveys to determine recruitment and age structure of the stocks.

A sustainable long-term development and management of the scallop stock requires biological data and information about catch efficiency. 

Surveys carried out in Trøndelag over the past decades suggest that the reproductive capacity and recruitment of the population being harvested is good, with low variation in age class structure from year to year. The Institute of Marine Research has also participated in the Norwegian national programme of mapping marine habitats, which includes mapping of high density areas of scallops. 

The survey mapping documented great scallop beds at depths of up to 60 meters. This is important in terms of assessing the part of the population above 30 m depth that cannot be harvested by divers, but which probably supports recruitment. The survey helps us to improve our understanding of distribution and recruitment, which in turn may lay the foundations for long-term, sustainable harvesting of great scallops.

Recent surveys of the populations at the northern limit of distribution in Lofoten islands (68oN) indicate stronger recruitment from about 2012 and a northward change in distribution of the Great scallops. 

The fishery

Since 2000, the reported catch of scallops has been in the range 500–900 tonnes. More than 80 % of the landings have taken place in Trøndelag, on the islands of Hitra, Frøya and Froan. In Norway, great scallops are exclusively harvested by divers, who operate in diving teams from registered fishing vessels. There has been a slight shift in catches with increased catches northwards in Nordland county replacing the catches in Trøndelag. Distance to the resource, market demand and changes in regulation of commercial diving operations seem to be the main factors regulating the fisheries.

Great scallop. Photo: Øivind Strand / IMR
Photo: Øivind Strand / IMR