Are restricted periods of over-stocking of recirculating aquaculture systems advisable? A simulation study

Fish stocking is an important element of aquaculture management. A constant stocking rate is a convenient strategy in intensive aquaculture, being also the best under steady physical and economic conditions. If, however, these conditions vary with time, the best stocking rate may also vary. This problem has been addressed in a previous paper, where water temperature, fish price and/or market demand were allowed to vary sinusoidally over the annual cycle, while a constant upper limit on the water-treatment capacity constrained the process. Considering only situations where feeding is always to satiation, sinusoidal stocking rates were shown to perform often significantly better than the best constant stocking-rate strategy.In this paper a further step is taken in that over-stocking (more than hitherto) for certain periods of time is allowed. This leads to restricted feeding at a later time (to avoid overload of the water-treatment system) and to some loss of feeding efficiency. Often, however, the loss in feeding efficiency is more than compensated for by better utilization of the rearing space.Optimal constant and sinusoidal over-stocking strategies were applied to eight sample combinations of sinusoidal temperature and market conditions. Restricted over-stocking often resulted in a considerable improvement of economic performance over the previous results. The potential for increase of yield and profit by over-stocking depends on the amount of under-utilization of the rearing space by the previous solution, namely when only feeding to satiation is allowed. On average (for the system under consideration) out of 10% of under-utilized space when feeding to satiation, 4.4% (almost half) can be recovered by properly applied over-stocking. Expressed in terms of feed processing capability, this 4.4% recovery is equivalent to about 16 kg[feed]/(m3[tank]y), and to an additional production of about 6.6 kg[fish]/(m3[tank]y). The gain in profit is of the order of 25 $/(m3[tank]y) per 10% of previously unutilized capacity.