Quantifying the contributions of tributaries to large-river fish populations through mark-recapture modeling

Abstract Objective Tributaries may play a vital role in maintaining populations of large river fishes, although the specific contributions of tributaries toward recruitment of river-wide populations are not often understood. Tributaries may experience fewer cumulative anthropogenic impacts relative to main-stem rivers and may offer more natural conditions supportive of native fish populations, which may provide opportunities for fish population restoration. Thus, an improved understanding of tributary–main-stem population dynamics may inform targeted conservation actions for spatially structured populations of large-river fishes. Colorado River tributaries in the Grand Canyon, Arizona, USA, are a focus of imperiled Humpback Chub Gila cypha conservation, which includes translocations to enhance population redundancy and to expand the overall population. However, the fate of fish dispersed to the main stem has not been thoroughly quantified. Methods Using open population mark–recapture models, we quantified the relative contribution of three groups of Humpback Chub, including fish of confirmed tributary origin that were either translocated or produced in situ, and others presumed to be Colorado River main-stem-origin fish, to three main-stem populations. Our specific study objectives were to (1) estimate Colorado River abundances of tributary and main-stem-origin fish over time, (2) compare relative group-specific contributions to three main-stem populations, and (3) compare group-specific survival rates of Humpback Chub in the Colorado River and in a tributary where a recent translocation has occurred. Result Tributaries contributed 26% and 43% of the overall abundance in two tributary inflow reach populations and zero in a third, which we attributed to uncharacteristically low tributary survival immediately following translocation. In the main stem, survival of tributary-origin fish was higher compared to main-stem-origin fish, suggesting an advantage of tributary residence. Conclusion Our contrasting results from three different tributary inflow populations highlight the potential role for tributaries in sustaining large-river fish populations, which may have important implications for long-term maintenance of river metapopulations.