Why move the fish?
The amount of fish or shellfish that can be produced on a farm depends on the fish density in the production units. Land-based fish farms equipped for intensive farming require high investment per unit farming volume. Continuous high fish density is therefore necessary to attain good production economy; this will require frequent transport or reallocation of fish as they grow (Fig. 17.1).
How often the fish must be moved is mainly decided by their growth rate, but input density and maximum allowed density in the production units are also important. An example can be used to illustrate this.
On a farm for ongrowing fish, the fish will not be moved during production (from input to delivery). Average stocking density is set at 45 kg/m3, and the maximum density to avoid growth reduction is set at 100 kg/m3. Calculations show that the input density cannot therefore exceed 2 kg/m3 (determined by exponential growth in kg/m3). This shows that poor utilization of the production units will result if the fish are not moved. The length of production depends on input size, harvesting size and specific growth rate (SGR) in relation to fish size (daily growth rate expressed as percentage of body weight).
If higher average density is to be achieved, frequent moving of the fish is necessary, as illustrated by the following example. Maximum density must not exceed 100 kg/m3, and required average fish density in the production units on the farm is 70 kg/m3. The SGR is set at 0.9. The intervals between the movements for dividing/splitting of the fish group are calculated to be 3–4 months. The table illustrates the interval between handling (months) in relation to SGR, input density, average density and maximum density where the input weight of the fish is 100 g and the harvesting weight 4 kg. This clearly shows that increased growth rate increases the need for handling.