Vital rates of a burgeoning population of Humpback Chub in western Grand Canyon

Abstract

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.

Why new scientific information is important for native fish conservation: A case study from the humpback chub (Gila cypha) in the Grand Canyon, U.S.A.

“…No humpback chub beyond the Little Colorado River, or beyond the critical habitat designation, are considered in these conservation triggers. Although the humpback chub population in the Grand Canyon has increased dramatically (Dzul et al, 2023), the numbers within the Little Colorado River have declined below the trigger levels, resulting in costly conservation measures being enacted (e.g. translocating fish).…”

“…This apparent plasticity of habitat needs is advantageous, as managers have the ability to modify physical habitat and temperature through modifications to dam operations (Dibble et al, 2020). (Dzul et al, 2023), the numbers within the Little Colorado River have declined below the trigger levels, resulting in costly conservation measures being enacted (e.g. translocating fish).…”

“…In recent years, humpback chub in the Grand Canyon have expanded their contemporary range into the far western Grand Canyon (Rogowski et al, 2018; Van Haverbeke et al, 2017), where a large portion of the river habitat is characterized by a single wide shallow channel, silt banks and fine sediment substrate. Recent population estimates have documented a robust aggregation of humpback chub in the western Grand Canyon (Dzul et al, 2023). In addition, long‐term standardized monitoring surveys have shown that hoop net catch per unit effort is higher in the western Grand Canyon than near the Little Colorado River (Van Haverbeke et al, 2017), the contemporary location of the largest humpback chub population.…”

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

“…Only the tributary mouths and the lower ~100-200 m of Shinumo and Havasu creeks were accessible to Humpback Chub from the Colorado River, due to the presence of impassable barriers to upstream movement evident under postdam flows (Healy et al 2022a). While limited reproduction likely occurred in the main stem until recently (Van Haverbeke et al 2017;Gilbert et al 2022;Dzul et al 2023), we suspect that the areas below these impassible barriers and the lower reaches of Bright Angel Creek provide spawning habitat for Humpback Chub. For example, Humpback Chub spawning behavior was observed downstream of the Shinumo Creek barrier (B. Healy, National Park Service [NPS], personal observation, July 2020), and some tagged, nontranslocated Humpback Chub were detected on antennas entering Bright Angel Creek during time periods when temperatures were likely suitable for spawning in early summer (Healy et al 2020c).…”

“…For mark-recapture modeling of main-stem aggregation population dynamics, we used Humpback Chub recapture data collected during hoop-net (50 × 100 cm, 6-33 mesh, single 10-cm throat) monitoring events targeting the Bright Angel Creek, Shinumo Creek, and Havasu Creek Inflows of the Colorado River, as defined above, by NPS, USFWS, and USGS Grand Canyon Monitoring and Research Center (USGS-GCMRC) biologists. These defined inflows were consistently monitored during our study, in contrast to the annual riverwide (Colorado River from Glen Canyon Dam to Lake Mead) monitoring in randomly selected sites occurring since 2000 (Fonkin et al 2023) or numerous fisheries research projects conducted in support of the Glen Canyon Dam Adaptive Management Program distributed throughout the Grand Canyon (e.g., Yackulic et al 2014;Van Haverbeke et al 2017;Dzul et al 2023). Hoop nets were baited (Purina Aquamax fish feed, St. Louis, Missouri, USA) and placed along the shoreline overnight.…”