Cooperative Fish and Wildlife Research Units Program: all
Education, Research and Technical Assistance for Managing Our Natural Resources

Understanding the relative strengths of intrinsic and extrinsic factors regulating populations is a longstanding focus of ecology and critical to advancing conservation programs for imperiled species. Conservation could benefit from an increased understanding of factors influencing vital rates (somatic growth, recruitment, survival) in small, translocated populations, which is lacking owing to difficulties in long-term monitoring of rare species. Translocations, herein defined as the transfer of wild-captured individuals from source populations to new habitats, are widely employed for species conservation, but outcomes are often minimally monitored, and translocations that are monitored often fail. To improve our understanding of how translocated populations respond to environmental variation, we developed and tested hypotheses related to intrinsic (density-dependent) and extrinsic (introduced rainbow trout Oncorhynchus mykiss, streamflow and temperature regime) causes of vital rate variation in endangered humpback chub (Gila cypha) populations translocated to Colorado River tributaries in Grand Canyon (GC), USA. Using biannual re-capture data from translocated populations over 10 years, we tested hypotheses related to seasonal somatic growth, and recruitment and population growth rates with linear mixed-effects models and temporal symmetry (TSM) mark-recapture models. We combined data from re-captures and re-sights of dispersed fish (both physical captures and continuously recorded antenna detections) from throughout GC to test survival hypotheses, while accounting for site fidelity, using joint live-recapture/live-resight (JLRR) models. While recruitment only occurred in one site, which also drove population growth (relative to survival), evidence supported hypotheses related to density-dependence in growth, survival, and recruitment, and somatic growth and recruitment were further limited by introduced trout. Mixed-effects models explained between 67 – 86% of the variation in somatic growth, which showed increased growth rates with greater flood pulse frequency during monsoon season. Monthly survival was 0.56 – 0.99 and 0.80 – 0.99 in the two populations, with lower survival during periods of higher intra-specific abundance and low flood-frequency. Our results suggest translocations can contribute toward the recovery of large-river fishes, but continued suppression of invasive fishes to enhance recruitment may be required to ensure population resilience. Further, we demonstrate the importance of flooding to population demographics in food-depauperate, dynamic, invaded systems.