6.5.4 Survival

The percentage of spat that will survive to be sold is extremely variable between species, within and between years and between hatcheries. As a general rule spat are not as vulnerable as larvae to pathogenic micro-organisms but, occasionally, abnormal rates of mortality will occur in smaller-size spat coincident with mass larval mortalities.

The survival of oysters is usually in the region of 50 to 70% from set to 2 to 4 mm shell length. For clams and scallops it may be in the 10 to 20% range (Figure 101). Much of the mortality takes place in the first week following settlement in oysters and during the first two weeks for clams and four weeks for scallops. Many larvae that set fail to survive metamorphosis, presumably because they have insufficient food reserves to complete this critical life history stage. Early mortality does not appear to be as much of a problem with the oysters which set and complete metamorphosis within a day or two. However, it has frequently been observed within hatcheries that a higher than average set does not necessarily mean that more viable spat will be obtained. Conditions may favour a good set but they do not necessarily improve the levels of reserves in larvae that may not be fitted for survival through metamorphosis.

When spat are set on cultch, survival is dependent on the density of set. This applies mostly to oysters that cement themselves to the substrate. Clams and scallops are able to change their position relative to their neighbours if overcrowding occurs. Where density of set is intense in oysters the stronger will overgrow the weaker which will inevitably die.
Mortalities will occur if oyster spat are grown at too high a biomass per unit volume in closed systems. The first symptoms are when the shells of the spat gradually or suddenly turn pale in colouration. If they are not reduced in density at this time then the calcium carbonate crystals in the shell will dissolve. This only happens when biomass grossly exceeds the recommended or if a water change has been missed. A check of the water contained in the tank system with a pH meter will show that the pH level has dropped sharply. It normally decreases between water changes from pH 8.2 to pH 7.6 or thereabouts, but if for the reasons mentioned above husbandry has been neglected, it may drop to below pH 7.0. The reason is partly the build-up in CO2 in the system from the respiration of the biomass of spat and the numerous bacteria in the water. The only remedy if the problem is recognized soon enough is to change the water and reduce the biomass of spat.

Figure 101: The survival (blue line) and growth (orange line) of calico scallop, Argopecten gibbus, spat during a 6-week period post-settlement. Estimates were made of survival at 2-weekly intervals.