Date of Award:

8-2026

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Wildland Resources

Committee Chair(s)

Phaedra Budy

Committee

Phaedra Budy

Committee

Chad Teal

Committee

Daniel MacNulty

Abstract

The first year of life is a vulnerable period for most fish species. Whether young fish survive this early stage determines how many individuals eventually survive and grow into reproductive adults, a process called recruitment. In desert rivers like the Colorado River Basin, native fish evolved alongside extreme natural conditions but now face additional pressure from drought and water storage infrastructure, which alter the shoreline habitats on which young fish depend. Long-lived, migratory native species with complex life cycles are especially vulnerable. We used four decades of monitoring data from the Upper Colorado River Basin to evaluate whether young-of-year (YOY, fish in their first year of life) Colorado Pikeminnow (Ptychocheilus lucius) survival limits adult population persistence. We found that late-summer catch of young fish and late-summer river flow were the strongest predictors of autumn YOY cohort strength. Strong autumn cohorts were linked to higher adult abundance 7 to 8 years later in the Colorado River. However, young-of-year catches in the Green River have steadily declined over the past 40 years, with no detectable link to adult abundance. These findings show that young Colorado Pikeminnow face a recruitment bottleneck that differs between the two remaining wild populations. A cohort-informed approach identifies strong year classes early which could make conservation investment more effective for this imperiled species, particularly in the Green River where young fish declines have been most severe.

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