Salmon Hatchery Management Given Selection In Captivity and Assortative Mating: Make Them Different or Keep Them Similar?

Captive environments such as hatcheries often incur artificial selection, both intentional and unintentional, that can lead to maladaptation in the wild.  Some traits under hatchery-based selection, such as run time, determine the likelihood of hatchery-wild interbreeding as well as fitness.  Given these dynamics in production hatcheries where the goal is an isolated program but some interbreeding inevitably occurs, should hatchery programs aim to keep cultured fish populations as similar as possible to natural populations they might interact with?  Or should the cultured fish populations be allowed (or encouraged) to diverge substantially from local wild populations?  By keeping them similar, each hatchery-wild interbreeding event might be relatively benign, but there could be a considerable number of such interbreeding events given assortative mating.  Alternatively, keeping them different should reduce the number of hatchery-wild interactions, but each event that does occur is likely to be more detrimental.  To quantitatively evaluate the inherent trade-offs, we develop a quantitative genetic model that follows the evolution of a trait that is under both natural and artificial selection and influences the likelihood of interbreeding.  We link the genetic dynamics to population dynamics for a generic salmon life cycle with interacting hatchery and wild populations.  Model results indicate that both similar and divergent selection in the hatchery lead to lower negative impacts on the wild spawner population size than an intermediate strategy throughout much of the parameter range.  Increasing the strength of assortative mating accentuates this bimodality in “optimal” hatchery strategy.  Increasing the similarity between hatchery and wild fish might be more difficult to achieve given unavoidable selection in captivity.  However, similar selection in the hatchery to the wild does more effectively mitigate hatchery-driven reductions in fitness as well as the ocean adult population size and therefore harvest, and it becomes the clear optimal strategy in terms of wild spawner population size under some assumptions and parameters (e.g., weak selection, small hatchery populations, hatcheries that target hatchery-reared fish, weak density-dependence, high ocean survivorship, and natural selection occurring at later life history stages).  These results indicate when the approach of selecting for a distinct population presents a viable alternative strategy to reducing unintended fitness consequences of hatcheries.