Strategies to Minimize Fitness Impact of Cultured Individuals on Wild Populations

As fishery catches have plateaued in recent years, aquaculture and stock enhancement programs are increasingly viewed as essential to meet global demand for protein.  However, empirical evidence is accumulating to demonstrate that 1) long-term sustainability of living aquatic resources depends on conservation of a diverse portfolio of naturally-reproducing populations, and 2) genetic interactions with domesticated individuals reduces the diversity and fitness of natural populations.  Two general strategies are possible to minimize adverse fitness consequences of unintentional (but not completely avoidable) genetic interactions between cultured and wild populations:  keep the cultured populations as genetically SIMILAR as possible to wild populations, or make them as DIFFERENT as possible.  For the first time, we quantitatively evaluated the inherent tradeoffs in the SIMILAR and DIFFERENT strategies, using a coupled quantitative genetics/population dynamics model.  Important findings include the following:  1) Although either strategy can in theory produce minimal fitness effects, in practice biological and logistical constraints will make it difficult to achieve extreme forms of either strategy.  The worst-case scenario, which occurs when the cultured population is genetically divergent but not different enough to prevent interbreeding, is not unrealistic for many practical situations.  2) The DIFFERENT strategy is a feasible alternative only if an opportunity for strong purifying selection occurs before escapees have a chance to reproduce.  3) Fitness effects of cultured individuals are larger if the cultured-wild interactions occur before density-dependent mortality in the wild.  4) Persistent, low-level leakage from aquaculture operations lead to at least as large fitness consequences as do the same number of individuals escaping in a large pulse.  5) Sterilization of cultured individuals is potentially a very effective strategy to reduce adverse fitness consequences for some species.