6.5.1 Variability in spat growth between species

The different bivalve species commonly grown in hatcheries have widely differing growth rates when grown at reasonable densities on an adequate diet and ration and at close to the optimum temperature. Oyster spat grow considerably more quickly to a saleable seed size than do spat of the various commercial clams and scallops. Cold water scallops grow more slowly than warmer water species.

Partly this is related to the larger larval size of oysters at settlement and partly the fact that there is no lag phase while metamorphosis takes place.

Figure 98: Comparison of the growth of Pacific oyster, Manila clam and calico scallop spat in similar conditions. Growth is shown as mean shell length (shell height in the case of calico scallop spat) at the beginning and end of a 7-day period.

A comparison is given of the growth of Pacific oyster, Manila clam and calico scallop spat in Figure 98. This contrasts growth of the three species from settlement size by comparing mean shell length at the beginning of a 7-day period with mean shell length 7 days later. Spat of the three species were grown at pilot-scale in systems of the types described previously at commercial densities and with adequate diets, rations and at 23+1oC. In this graph, the more steeply the growth curves are inclined to the left, the faster is the growth rate. Manila clams are in fact growing faster than Pacific oyster spat but they are starting from a smaller size. At the end of a 3-week period from settlement, Pacific oyster spat will grow to approximately 3.4 mm mean shell length compared with Manila clam spat which will reach 1.14 mm. This is mean shell length and the distribution about the mean is much greater in clam spat than it is in oysters. Calico scallops grow more slowly with an equally as large-size distribution about the mean. After 5 weeks growth they will reach approximately 1.5 mm mean shell height (where height is almost the same as length at this stage). Manila clam spat will exceed this size in 4 weeks (1.6 mm).
Cold water scallops such as the Japanese scallop, Patinopecten yessoensis, will take 4 or 5 months to reach 5 mm shell height even when grown in ideal conditions.