6.4.2 Calculating food ration

Ration is calculated on the basis of the biomass of spat held in a tank unit whether it is a closed downwelling or upwelling system or a system operated with partial water exchange. Spat of most bivalves have similar requirements in terms of the quantity of food required per unit biomass.

Thus, a ration calculated for a given biomass of oyster spat will be equally as suitable for the same biomass of clams and mussels although growth responses may be very different. For example, clams will initially grow more slowly than oysters even in the best possible conditions. Scallops are again the exception and respond best to lower rations per unit biomass.
Ration in terms of the dry weight of algae required is calculated from the equation: F = (SxR)/7
where, F = the dry weight of algae per day (mg); R = ration as dry weight of algae (mg) per mg live weight of spat per week and S = the live weight of spat (mg) at the beginning of each week.
A worked example is given below together with an extension of this equation to calculate the volume of harvested algae required for the daily ration.

 

Example:

Basic Information:

Live weight biomass of spat at the beginning of the week = 600 g = 600 000 mg
Ration = 0.4 mg dry weight of algae per mg live weight of spat per week
Diet: Tetraselmis suecica at a harvest cell density of 1 500 cells per ?l

Calculation:

F=(600 000x0.4)/7 = 34 286 (mg dry wt of algae)

Therefore, the daily ration fed to 600 g of spat will be 34 286/1 000 = 34.286 g dry weight of algae.
Reference to Table 1 (section 3) shows that 1 million cells of Tetraselmis suecica weighs 0.2 mg.

The volume of Tetraselmis required to provide the daily ration is calculated from the equation:

V=(Sx0.4)/(7xWxC)

Where, V= the volume of harvested algae (l) required to provide the daily ration W = the weight of 1 million algal cells of the required species, and

C = the harvest cell density of that species (cells per ?l)

Thus,

V = (600 000x0.4)/(7x0.2x1 500) = 114.3 l

Therefore, 114.3 l of Tetraselmis at a harvest cell density of 1 500 cells per ?l provides the daily ration for 600 g biomass of spat.
Note: A ration of 0.4 is satisfactory for oyster and clam spat of any size within the range likely to be grown on the hatchery premises.
A diet made up of Tetraselmis and Skeletonema in a 50:50 ration by dry weight will be 57.2 l of the former at 1 500 cells per ?l and 76.5 l of Skeletonema at a harvest cell density of 7 000 cells per ?l. The dry weight of one million cells of Skeletonema is 0.032 mg.
A biomass of 600 g of oyster or clam spat will need to be grown in a 3 000 l volume. Adding the above ration will result in an initial algal cell density within the system of 57 cells equivalent in size to Tetraselmis per ?l (57 000 cells per ml). This algal cell density is too high to support optimum growth if it is delivered as a single batch feed. The optimum food cell density in this respect is 10 000 cells per ml. The solution is to add (10/57x114.3) l = 20 l of food as a batch feed and the remainder by drip feed or dosing pump over the 24-h period.
A ration of 0.4 mg algae per mg live weight of spat per week is towards the upper limit for spat of warm water scallops, such as Argopecten species, which are grown at the same temperature as the oysters and warm water clams (i.e. 23+2oC). Ration needs to be reduced for cold water scallop species.
Calculations given in the example above apply equally to systems operated with a partial daily water exchange. Ration is calculated for the biomass of spat held and not the water volume in which they are grown.
When spat systems are operated on flow-through and food supply is from a nutrient enhanced pond or tank it is not possible to accurately assess the species composition of the food supply or the ration that needs to be provided. It will vary from day to day according to the state of the bloom. An experienced technician will be able to judge whether the pond water will need to be diluted with non-bloomed seawater in order to keep the daily ration within reasonable bounds. Excessive pseudofaecal production by spat indicates that the food supply is too high.