Choosing the Right Site Environment for Raising Scallops

Scallops prefer substrates of clean, firm sand, fine gravel or sandy gravel, sometimes with an admixture of mud. This is their natural habitat. They will only survive on fine sediments provided current speeds are reasonably slow to prevent disturbance of the sediment, which could suffocate the scallops, if they became covered. Hard sediments are unsuitable, as the scallops are unable to bury themselves sufficiently to avoid predation.

Depth of water

Sites with depth of water between 15-30 metres are ideal. Allowing for tidal fluctuations, it is essential that nets should not be in contact with the seabed. This avoids predators such as starfish and crabs, which are the two main enemies of scallops, gaining access to the nets. Also, deeper water sites allow on-growing equipment to be sunk to a depth below the influence of wave action and excessive temperature fluctuations.

Salinity

Usually, cultivation sites are chosen at which salinity varies within 30-35 psu (practical salinity units) and where these salinity conditions prevail, it is temperature that is generally considered to be the most important factor affecting performance of the spat. However, cultivation sites may occasionally be subjected to lower salinity conditions, due to increased fresh water input from rivers and land run-off, following heavy rainfall.

An ambient salinity of 28 psu or above is required for successful scallop cultivation. As can be seen from the figure, there is a dramatic decrease in growth rate below this salinity. At lower temperatures growth at 28 psu will be slightly lower than at 30 psu.

Lower salinity is stressful to king scallops. Growth rate is significantly reduced and mortality can result. Scallops are able to tolerate short exposure (for example, up to 6 hours per day for 3 days) to lower salinity (20 psu) although this will be followed by a short-term reduction in growth rate and may also
result in mortality, particularly at temperatures less than 10°C.

It is generally recognised that the stressful effect of low salinity is greater at lower temperatures. For example, one study showed that scallops can survive temperatures as low as 3°C at salinities above 30 psu, but they may die at temperatures below 5°C if salinity falls to less than 26 psu. Other stress factors, such as crowding in the nets during the suspended cultivation phase, will reduce tolerance to the combined effect of these extreme conditions. This shows the importance of considering local salinity profiles at potential sites where the winter seawater temperature tends to be low. If the site is also subject to episodes of low salinity then that site is probably best avoided.

Fresh water is less dense than seawater and in some circumstances, for example in calm conditions in sheltered areas after heavy rainfall, will form a layer, as much as 2 to 3 metres in depth, on the surface.

Scallops will not tolerate this and it emphasises the need to keep suspended cultivation equipment well submerged at sites where this problem may arise.

Temperature

It is well known that seawater temperature has a major effect on the seasonal growth of cultivated bivalve molluscs. Growth rates of scallops measured at existing cultivation and some trial sites in England and Scotland were strongly correlated with temperature. This is seen on research, where the information has come from monthly measurements of performance, including growth, at four sites at which continuous temperature recorders were deployed.