Topic: Great scallop

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.

The fishery

Commercial harvesting of the great scallop in Norway is carried out by divers operating from registered vessels. The recorded sales over the past five years have ranged between 270-450 tonnes per year, a significant reduction from the peak year of 2008 when 900 metric tonnes of scallops were landed. Until 2012, almost all harvesting of the great scallop took place in Trøndelag County, mainly from the areas around Hitra and Frøya (65^oN), but in the last 10 years, harvesting further north in the Helgeland area (Nordland County, 67^oN) has increased and now accounts for 40-45% of the total landing. In 2023, the catches amounted to a total of 359 metric tonnes, of which 161 tonnes were harvested in Nordland.

Photo: Øivind Strand / IMR