The Fourth International Stock Enhancement & Searanching

Theme D: Developing optimal release strategies

39. FINDING THE RIGHT STARTING POINTS IN STOCKED FISHERIES BY MODELING THE RIGHT END POINTS: EXPRESSING THE CARRYING CAPACITY AS A FUNCTION AND A DYNAMIC EQUILIBRIUM


James A. Smith1, Lee J. Baumgartner2, Iain M. Suthers1, Matthew D. Taylor1
1 Evolution and Ecology Research Centre University of New South Wales, Sydney, 2052, Australia
2 NSW Department of Industry and Investment, Australia
james.smith@unsw.edu.au

Modelling the growth and recruitment of fish populations often relies upon relationships which include a carrying capacity value. These relationships are used with great success in many fisheries models, but when the carrying capacity is needed to direct stocking densities however, current models become inadequate. When an attempt is made to link a static carrying capacity with an ecosystem-productivity model, it quickly becomes apparent that it is the structure of the population, and not its size, that determines the optimum stocking regimes. This is especially true in those fisheries where stock enhancement makes up a large proportion of recruitment, as the very structure of the population can be determined through stocking. Two models of the carrying capacity are demonstrated, which both express the carrying capacity as a function rather than a static value, and thus define the population structure of a stocked species. One of these models, which is based on the theory of energetic equivalence, is also used to examine the trade-off in a fishery between fish size and population density. The scope of this “big fish or many fish” trade-off is revealed as a simple power curve, bounded by the limits of asymptotic growth of a species, and shaped variously by metabolic scaling and density-dependence. The use of these models in matching release densities to ecosystem productivity, and in evaluating the outcomes of these stocking densities, are demonstrated using simulations, and with data from stocked Australian fisheries and manipulative tank experiments.