Hatchery Supplementation Increases Potential Spawning Stock of Rio Grande Silvery Minnow after Population Bottlenecks

Objective: Supplementation of imperiled wild fish stocks with captively raised fish is a commonly used conservation tool. Programs designed to maintain or improve fish populations through supplementation should be evaluated to determine whether they are meeting conservation objectives. The Rio Grande Silvery Minnow Hybognathus amarus is a small-bodied, endangered minnow endemic to the Rio Grande basin of the southwestern United States. The wild population of Rio Grande Silvery Minnow has been supplemented with captive-reared fish since 2002. Our objective was to determine whether supplementation measurably increases the number of spawning fish after years of population bottlenecks; this objective was simplified because nearly all hatchery-released fish have been given identifying markings.Methods: We leveraged a long-term fish sampling data set (18 years) covering the species' contemporary range and a single-season, high spatial coverage fish sampling data set to determine whether the number of potential spawners was increased by the addition of hatchery fish.Result: Hatchery-reared fish increased the catch rates of spawners by up to an order of magnitude in some years. We also observed that most hatchery-reared fish were recaptured near the point of release. We were able to determine that supplementation with hatchery fish increased the abundance and distribution of Rio Grande Silvery Minnow and provided the desired demographic boost after severe population bottlenecks.Conclusion: Releasing hatchery-reared fish may be a useful tool for conservation of imperiled freshwater fishes. However, there is a need for spawning and rearing infrastructure that matches the life-history of the species and sufficient post release monitoring to evaluate the effectiveness of hatchery supplementation. Use of hatchery-reared fish has been beneficial for improving population resiliency of Rio Grande Silvery Minnow following recruitment bottlenecks; however, natural recruitment can greatly exceed hatchery supplementation and conditions promoting natural recruitment should be restored.