Objective:The Colorado River ecosystem has experienced habitat alterations and nonnative species invasions; as a result, many of its native species have had extirpations, abundance declines, and range constrictions. Despite these pitfalls, the Humpback Chub Gila cypha has persisted and in the last 10-15 years has expanded its range to become abundant in western Grand Canyon, a river segment in which it had been rare for the prior three decades. The goal of this study was to evaluate the population dynamics (i.e., growth, survival, movement, abundance) for this "new" group of Humpback Chub. Methods:We analyzed a 6-year mark-recapture study from a fixed monitoring reach in western Grand Canyon, and we provide the first estimates of survival and growth (vital rates) for these fish. We compared vital rates in western Grand Canyon to those of two life history forms (residents and migrants, representing fast and slow life history trajectories, respectively) from the more established Little Colorado River (LCR) aggregation in eastern Grand Canyon. Result: Compared to LCR migrants and LCR residents, Humpback Chub in westernGrand Canyon had intermediate values for apparent survival, growth, and asymptotic length. Relatively high survival of subadults coupled with fast growth allowed for rapid population growth in western Grand Canyon. However, a large cohort in 2017 failed to lead to noticeable increases in adults. Seasonal survival patterns were distinct in all three groups, and apparent survival was lowest in western Grand Canyon during spring months. Adult Humpback Chub in western Grand Canyon were mobile and had a high probability of transience (i.e., just passing through the reach) and temporary emigration, demonstrating the need for future movement studies in western Grand Canyon to better distinguish emigration from survival. Conclusion:There is considerable spatial variability in viral rates for Humpback Chub in Grand Canyon. We discuss how observations are related to disparate temperature regimes experienced by the three groups and whether (how) the relationship between metabolism and temperature influences vital rates within the river network.

Flows released from reservoirs are often modified to mitigate the negative ecosystem effects of dams. We estimated the effects of two experimental flows, fall-timed floods and elimination of sub-daily variation in flows on weekends, on growth rates of rainbow trout (Oncorhynchus mykiss) in the Colorado River downstream from Glen Canyon Dam. Experimental flow effects were compared to effects of water temperature, phosphorous concentration, solar insolation, and competition, by fitting mixed effect von Bertalanffy models to ~ 10,000 observations of growth from mark-recapture between 2012 and 2021. There was strong support for models predicting faster growth during intervals with higher solar insolation, and lower water temperature and competition for prey. Effects of phosphorus and experimental flows were small and uncertain. Drought-related increases in dam release temperatures during summer and fall were predicted to result in severe weight loss for larger trout and could eventually threaten the viability of the population and the fishery it supports. The effects of water temperature and competition on fish growth substantially exceeded the effects of controlled floods and steadier flows.