Scallops Feeding Strategies
Scallops feed by filtering mainly microscopic algae (phytoplankton), but also some organic detritus, from seawater. Algae are simple types of plant and, like all plants, contain chlorophyll, which utilises the energy from light to convert inorganic nutrients and carbon dioxide dissolved in the seawater into organic growth.
Primary productivity is the rate at which new algae cells are produced in the sea, and is dependent on various environmental factors, including nutrient availability, light (which may be attenuated depending on the turbidity of the water) and temperature. It has been estimated that when bivalves are grown under similar conditions at different sites, up to 85% of any difference in growth observed between sites can be attributed to water temperature and primary productivity.
Other studies have shown that the growth of small scallop spat is positively related to the concentration of chlorophyll in the water. This indicates the importance of primary productivity for growth of cultivated scallops, yet it is the most difficult factor to assess for a given site.
For the DEFRA-funded LINK Aquaculture study ‘Environmental requirements for successful king scallop cultivation’, water samples were collected weekly from the participating cultivation sites and analysed for chlorophyll, as well as for salinity and particulate organic content. It can be seen from Figure 5 that generally there appears to be abundant chlorophyll in the water during the summer growing season.
The English south coast site exhibits minor peaks of ‘primary production’ in spring and, to a lesser extent, autumn, whereas chlorophyll levels at the Scottish site are maintained throughout the summer.
The amount of food being consumed by the scallops at these field sites was also estimated. This was done from the measurements taken of particulate content of the seawater from the weekly water samples, using estimates of the filtration rate of scallops in relation to particulate content of the water and the prevailing temperature, as determined by laboratory experiments. The result can be compared with estimates of the amount of food that the scallops require at any given temperature, also determined by laboratory experiments.
At low food densities (low ration) in the seawater, estimated by food particle concentration, scallops are able to maintain an adequate food intake by increasing their filtration rate, where the data are taken from measurements made in experimental outdoor nursery systems over several weeks. At higher rations they obtain the food they need by a lower filtration rate. Very high particle concentrations, beyond the range, can inhibit scallop filtration activity.
Above a critical threshold the filtration rates are depressed to such a severe extent that the scallops can no longer obtain sufficient food. For this reason sites where there are regular and intense blooms of algae, perhaps as a result of very high nutrient loading of the seawater coupled with low exchange, are best avoided. A further reason for avoiding such sites is that death and decay of these blooms can use up the available oxygen in the water.
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