Generalized Predatory Impact Model: A Numerical Approach for Assessing Trophic Limits to Hatchery Releases and Controlling Related Ecological Risks

Rigorous assessment of species and ecosystem biology underpins responsible marine stock enhancement. Estimation of limits to stocking density, based on ecosystem productivity and energetic requirements of stocked species, can be used to gauge the appropriate magnitude of release densities, minimizing waste of resources, and the possibility for adverse stocking effects. A generalized mass-balance model (generalized predatory impact model) for stocking density estimation has been developed. The approach is based around the principles of ECOPATH and accounts for dynamic estimation of stocking-related ecosystem relationships at fine temporal (days) and spatial scales. The main parameter inputs include probability distributions for key biological and life-history traits of stocked species and estimates of primary productivity for the target ecosystem. The energetic requirements of stocked fish are evaluated in terms of growth and mortality as well as ontogenetic transitions in diet, habitat use, morphology, and migration. The theoretical carrying capacity for a stocked species within a given arena is assessed from primary productivity, levels of predation by stocked fish on different trophic groups, and a specified level of acceptable trophic impact. A Monte Carlo analysis of uncertainty is used to provide a probability distribution of stocking densities for a given trophic impact. The model is applied for stocking juveniles of snook (Centropomus undecimalis) in Sarasota, FL, USA, and mulloway (Argyrosomus japonicus) in Georges River, NSW, Australia. The model is useful for estimating an appropriate stocking density when planning pilot-scale fish releases. Such releases should be carefully monitored to validate model assumptions and determine density-dependent and other environmental effects.