Flow spikes reduce Green River smallmouth bass reproductive success

Authors: Kevin Bestgen, Edward Kluender, Matthew Haworth, Ben Applegate, Tildon Jones

Year: 2024

Abstract

Smallmouth bass Micropterus dolomieu is considered the most problematic non-native fish species in the upper Green River basin because of negative impacts on native fish populations and endangered fish recovery. Although mechanical removal is an effective, short-term management strategy, other approaches that target early life stages, coupled with removal of older life stages, may be longer-lasting and ultimately, further reduce the negative impacts of smallmouth bass. We used short-term, high magnitude, and relatively cold flow releases—flow spikes—from Flaming Gorge Dam, timed with the estimated smallmouth bass reproduction and nest-guarding period, to reduce their reproductive success in the downstream Green River. Observations of physical habitat change, downstream transport of larvae, abundance of young bass measured by seining and electrofishing, and distributions of hatching dates from otolith daily increment analysis were used to assess effects of recent flow spikes. We observed substantially increased depth and water velocity during flow spikes in many potential or observed spawning locations as well as reduced water temperature, especially when low Yampa River flows had a reduced Green River dilution effect. Seining and electrofishing showed reduced age-0 smallmouth bass abundance, the desired effect, compared to observations from other warm and low flow years. Distributions of hatching dates showed predictions of timing of smallmouth bass spawning were adequate, and demonstrated reduced reproductive success based on lower survival of young bass, and reduced spawning activity of adults. These preliminary results indicated flow spikes may be a viable tool to reduce smallmouth bass reproductive success in established populations and may improve the status of endangered fishes in the upper Colorado River basin. Suppression of bass in newly invaded reaches may require additional tools to prevent their establishment.