Habitat alterations and introduction of non-native fishes have reduced the distributions of Flannelmouth Sucker Catostomus latipinnis, Bluehead Sucker Catostomus discobolus, and Roundtail Chub Gila robusta to less than 50% of their historic ranges in the Colorado River basin. Main-stem Colorado River habitat has been widely altered by construction of dams and is a main driver in the decline of native fishes. Tributaries are sometimes less altered than main-stem habitat, and may be important areas for life history processes such as spawning, but their role in population maintenance of Flannelmouth and Bluehead Sucker populations is poorly understood. Using mark-recapture techniques, I evaluated the use of tributaries as spawning areas for Flannelmouth and Bluehead suckers.
Suckers were captured with fyke nets as they migrated upstream into Coal Creek, a tributary of the White River, presumably to spawn, and were PIT tagged and released. The PIT tagged fish were subsequently detected using two remote PIT tag antenna arrays placed in Coal Creek, one near the mouth and the other 2 km upstream. Suckers began to migrate into Coal Creek, in late May of 2012 and 2013 when water temperatures reached 9–12°C. Water temperature likely cued spawning movements because fish captures and PIT tag detections in Coal Creek occurred at similar temperatures in both years, despite different flow patterns. However, within 24 hours of capture and handling, the majority of suckers abandoned Coal Creek, and returned to the main-stem White River. Suckers not handled and detected only by PIT tag detector arrays remained resident in Coal Creek for an average of 10–12 days (1–32 days) depending on year and species. Handling-induced movement rendered abundance estimation of the Coal Creek population difficult and such post-handling movements should be considered when designing mark-recapture studies for these species.
Tagged Flannelmouth and Bluehead suckers returned to Coal Creek in subsequent years, demonstrating >40% spawning site fidelity. High levels of site fidelity indicated that small tributaries may be important spawning habitat for these species. Finally, remote PIT tag detections also showed that 81% of tagged fish returning to Coal Creek were not also captured in fyke nets. Hence, remote PIT tag antenna arrays allowed stronger inference regarding movement and tributary use by these species than could be achieved using just fyke nets. This research shows that tributaries are an important part of Flannelmouth and Bluehead suckers life history and should be considered when developing management strategies for these species.
Climate change may also alter flow and water temperature regimes in the upper Colorado River basin and impact native fish communities. Climate change models generally predict decreased stream flow and increased water temperatures. A better understanding of the impacts of flow and water temperature on the life histories of these species should lead to more informed management and allow assessments of how climate change will impact extant populations.
Fish community sampling showed that the upper White River was pristine, as native fishes were > 90% of those captured. Timing of reproduction for Flannelmouth Sucker, Bluehead Sucker, and Roundtail Chub along a 150 river kilometer longitudinal gradient of a main-stem White River reach and two tributaries revealed that water temperature was the dominant environmental factor cueing reproduction for all three species. Flannelmouth Sucker, Bluehead Sucker, and Roundtail Chub reproduction began earlier in spring at warmer, lower-elevation, downstream locations and progressed longitudinally upriver in later spring or early summer as water temperatures increased. In tributaries, warmer water temperatures initiated reproduction by suckers earlier than in the adjacent main-stem White River. Timing of reproduction (based on detection of larvae) was 20-29 days earlier in 2012 than 2013 at the same locations due to earlier warming in the low flow year 2012. Daily growth microincrements in otoliths of larvae showed hatching dates were also earlier in 2012 due to warmer water temperatures and first hatching dates were progressively later in the year upstream.
Presence of larvae and otolith-derived hatching dates demonstrated a distinct and predictable progression of first reproduction in an upstream direction associated with warming water. There was also a clear upstream limit to reproduction for all three species. The upstream limit for Roundtail Chub was at a lower elevation than for either sucker species. A regression model was used to predict water temperature during fish spawning seasons as a function of covariates air temperature, date, elevation, and flow. Using published climate change projections for water temperature and flow, the regression model showed that both years of this study were warmer than average and 2012 conditions approximated those for potential climate change scenarios modeled. Lower flows and warmer air temperatures as a result of climate change will result in earlier reproduction and may expand the upstream limits of thermally suitable habitat for reproduction by these three fishes, while reducing habitat size.
Expansion of habitat for reproduction upstream does not ensure population persistence, as most juvenile recruitment for Flannelmouth Sucker and Bluehead Sucker occurred only in downstream reaches of the White River study area, and recruitment for Roundtail Chub was limited everywhere. Thus, populations of these species may require a network of suitable riverine habitat, including tributaries, along a relatively extensive longitudinal continuum, to promote survival of life stages needed for population persistence. The long-term implications of climate change are unknown and managers should strive to perpetuate this valuable and relatively pristine native fish community in the upper White River drainage as a vestige of the native fish communities that formerly existed throughout the Colorado River basin.