Optimal harvest size in aquaculture, with RAS cultured sea bream (Sparus aurata) as an example

Abstract

The objective of this paper is to outline a method for determining the optimal harvest size of fish in continuous-production aquaculture, using as an example a particular recirculating aquaculture system (RAS) technology. As in other management decisions, a reliable model of the system is required, including the biological response of organisms to their environment, and as the relevant prices and economic constraints.A model of an idealized steady-state growth system is developed, the required functions and prices being obtained from an actual sea bream RAS. Growth, mortality and feeding rates, as well as prices, are obtained as functions of fish size, and used to evaluate the expected profit for a range of stocking and harvest sizes. Both quota limitation and rearing-volume limitation are considered.For the studied RAS it turns out that: (1) Fish size is an expo-linear function of time, with a slower rate of growth for the larger sizes than previously indicated by a laboratory-based model. This is possibly due to sorting and graded harvest as applied in that RAS. (2) Mortality rate for the very small fish is higher than for the larger ones. (3) Loads as high as 120kg[BM]/m3[tank] (where BM is live fish biomass and m3[tank] is the volume of tank water) do not seem to have a damaging effect on the growth and mortality of the fish.The results of the analysis indicate that the current price system provides an incentive to increasing the harvest size, more so for the quota-limited than for the volume-limited operation. There is also an incentive to increase fish load and perhaps also to reduce fingerling size.