Gunnison River Basin
To determine whether populations of pure Flannelmouth and Bluehead suckers can be maintained in drainages where expansion of non-native sucker species is likely to occur in the future.
To deploy PIT tags in fish and maximize future detections of those fish using stationary and mobile antennas, allowing estimation of fish survival rates for Flannelmouth Sucker and Bluehead Sucker. We also assessed short-term retention through the spawning season.
Flannelmouth Sucker (Catostomus latipinnis), Bluehead Sucker (Catostomus discobolus), and Roundtail Chub (Gila robusta) are often referred to as the “three species”. Natives of the Colorado River basin, they each occupy only 45 – 55% of their historic native range in the upper Colorado River basin. In addition, Bluehead and Flannelmouth suckers are prone to hybridization with sucker species not native to the Colorado River basin (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 even result in the eventual extinction of the native species as we know them today.
Unlike native cutthroat trout or other coldwater species, these native suckers inhabit big rivers and low elevation tributaries, leading to a lack of opportunities for segregation from non-native suckers and hybrids via in-stream barriers. Therefore, 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. Would preventing access to the tributary by the unwanted fish result in detectable changes in the proportion of pure native sucker progeny drifting to the Gunnison River from the tributary? 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. Unfortunately, there were periods of time when the picket weir was compromised in each year of the study, resulting in the entry of unwanted spawners as determined by species composition among outmigrating fish as the conclusion of the spawning season. Additionally, we found that the results of genetic tests on larval fishes poorly reflected the morphological identity of spawning adults in the manipulated stream, but accurately in the unmanipulated stream. The unmanipulated stream hosted a greater proportion of just the two native sucker species compared to the manipulated stream.
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. We now anticipate conducting the study during spring of 2021 and 2022.
If non-natives can be successfully repressed to the advantage of native suckers, progeny produced in that 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.
Researchers also conduct tagging operations using passive integrated transponder (PIT) tags in spawning tributaries of the Gunnison River to determine the timing of arrival for each species and the degree of overlap in their spawn timing. A stationary PIT tag antenna (passive interrogation array; PIA) was installed in Roubideau Creek to increase future tag detections and refine arrival timing estimates. Detections on this PIA will also allow future estimation of survival rates for these Colorado natives.
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 ascended the stream to spawn made their exit as the water began to drop and clear up.