Zachary Hooley-Underwood, CPW; Kevin Thompson, CPW - retired
Gunnison River Basin
- Determine whether CPW biologists can effectively use exclusion devices such as weirs to prevent non-native and hybridized suckers from participating in native sucker spawning events in intermittent tributary streams.
Flannelmouth Sucker (Catostomus latipinnis), Bluehead Sucker (Catostomus discobolus), and Roundtail Chub (Gila robusta) are often referred to as the “three species” because the typically coexist with each other, and have similar life cycles. Natives of the Colorado River basin, they each occupy only 45 – 55% of their historic native range in the upper Colorado River basin. All three have experienced declines due to habitat loss and invasions of non-native predatory and competitory fishes. In addition, Bluehead and Flannelmouth suckers are prone to hybridization with non-native sucker species (primarily white sucker and longnose sucker) that were unintentionally introduced to the Western Slope. The range and relative numbers of these non-native suckers have expanded in western Colorado over the last 30 years, in some areas dramatically. Continued hybridization and introgression could result in the eventual extinction of the native suckers.
Unlike native Cutthroat Trout or other coldwater species, these native suckers rely on big rivers and low elevation tributaries for much of the year. Therefore, CPW can’t effectively conserve the genetics of these species by creating isolated populations of these suckers above barriers in small streams as they do with Cutthroat Trout. Instead, CPW initiated a study to investigate the effects of excluding non-native suckers and their hybrids from an important spawning tributary of the Gunnison River. CPW conducted a three-year study of excluding non-native and hybridized suckers from the spawning run using a picket weir and fish trap to answer this question, comparing the selected stream to another in which no fish control was attempted. To test the effectiveness of the exclusion, larval suckers were collected in both streams, and were genetically analyzed to identify the species, or hybrid composition, of each larvae. We found that picket weirs cannot always accommodate the volume of water and debris associated with spring runoff in the streams these fish spawn in, and unfortunately, there were periods of time when the weir was compromised during each year of the study. This resulted in the entry of at least some non-native suckers every year, and many of the resulting larvae had non-native genetics. Additionally, we found that the ratio of non-native to native larvae was much higher than the ratio of non-native to native adults in the exclusion stream, but these ratios matched up in the unmanipulated stream. The unmanipulated stream hosted a greater proportion of just the two native sucker species compared to the manipulated stream. This finding was concerning as it indicated that a few non-natives participating in the spawning run can have a large, disproportional effect on the population.
A new study is underway to use a better weir design – a resistance board weir – in a larger stream to exclude greater numbers of White Suckers and hybrids. Resistance board weirs can be more easily kept clean, and if overwhelmed they submerge, allowing debris to pass over. The new weir was deployed in March 2020, but precautions surrounding the Covid-19 pandemic resulted in an early termination of the work. The study resumed in 2021 and was replicated in 2022. The weir was operated during the full spawning period in 2021, and we expect that non-native suckers were unable to gain entry to the stream that year. A heavy snowpack and cool spring followed by a week of rapid warming in 2022 resulted in far-above-average flows in the creek that overwhelmed the abilities of the weir. Non-native suckers were able to access the creek for approximately half of the spawning season. Larvae collected in 2019-2023 will be genetically analyzed to determine the effectiveness of the weir.
If this study determines that non-natives can be successfully repressed to the advantage of native suckers, progeny produced in a manipulated stream would result in more pure fish in the Gunnison River. While this approach would not result in the disappearance of non-native suckers from the entire Gunnison basin, it may provide an avenue toward ensuring that the native species persist in the Gunnison Basin. If successful, this strategy could be implemented in other river basins on appropriate tributaries as well.
Video: Western Slope Native Suckers Leaving Spawning Grounds
These Colorado River basin native Bluehead Suckers and Flannelmouth Suckers are leaving a spawning tributary in the Roubideau Creek drainage near Delta, Colorado. This video was captured in late May 2016, and PIT tag antenna data indicate that this type of scene was common for about 4 days as the tremendous numbers of fish that had ascended the stream to spawn made their exit as the water began to drop and clear up.