6.4 DIETS AND FOOD RATIONS FOR SMALL SPAT

6.4.1 Species composition of diets

Foods suitable for growing small spat in closely controlled conditions within hatcheries are the same as those used in larval culture (section 5.1). When spat are in their first week after settlement they are usually fed the same diet they were fed before settlement occurred.

As they increase in size it may not be possible to produce sufficiently large quantities of some of the more delicate and difficult to grow algae. Diets for larger spat tend to be made up of the hardier species such as Tetraselmis sp. and the diatoms Chaetoceros muelleri, Thalassiosira weissflogii and Skeletonema costatum.
The highly unsaturated fatty acid (HUFA) DHA (22:6n3) does not appear to be as important in the development of spat as it is during larval development so that Isochrysis galbana and species with a similar HUFA profile – while useful as minor components of the diet – are not essential. Typically, diets will be approximately a 50:50 ratio of a Tetraselmis species and one of the above named diatoms. Part of the ration may be in the form of an algal paste rather than freshly grown live algae (Figure 97). Some products support satisfactory growth rates. The suggested reading list at the end of this section includes papers on recent research with a range of non-living foodstuffs.

Figure 97: An example of a proprietary algal paste product suitable as a partial or complete replacement for hatchery grown live algae in the culture of bivalve spat. Packs of Tetraselmis and Thalassiosira contain the equivalent of 3 600 l at 410 cells per ?l and 1 800 l at 2 600 cells per ?l respectively. When refrigerated, shelf life is 12 to 14 weeks. A range of useful species is available.