A method for the quantification and optimization of hydrodynamics in culture tanks

A method was developed to quantify hydrodynamic mixing parameters, and to optimize the physical environmental conditions, in culture tanks. Improved mixing will result in better tank water quality, more efficient use of available volume by the culture animals (leading to optimal stocking densities and better feed management) and possibly reduced water pumping requirements. Experiments were conducted to determine the influence of a range of flow rates, residence times, water depths and stocking densities on hydrodynamics in juvenile turbot (Scophthalmus maximus (L.)) tanks. Decreases in water depth resulted in significant improvements in mixing and the efficiency with which the water was used, as indicated by reductions in dead volumes. A depth of less than 9.4 cm at a flow rate of 2 l min−1 was expected to minimize dead volumes in the tank. This indicated that mixing was better in shallower tanks. Within the range 0–13 l min−1, increased flow rate improved mixing at a constant depth of 9 cm (and water volume of 18.54 l) though increased flow rates greater than about 2.5 l min−1 produced only small improvements in mixing. Within the range 0–50 fish per tank (equivalent to a mean stocking density of 0–1.84 kg m−2), stocking density did not significantly influence mixing in tanks with a depth of 9 cm and flow rate of 2 l min−1. Such depth reductions, for demersal species, may be a useful means to either decrease water use without reducing residence time, or alternatively to increase the flushing rate without increasing water use, at a given stocking density. The large changes in the efficiency with which the tanks were used, which were achieved with ease, indicates that attention to water mixing can give positive benefits to a wide range of land-based farm operators. Care must be taken when adjusting tank hydrodynamics, that water quality is maintained and that biological parameters such as stress levels, sunlight effects and feed management are optimal.