Assessing variation through space and time in the vital rates of humpback chub in the Little Colorado River

Authors: Maria C. Dzul, Charles Yackulic

Year: 2013 (xlv)

Abstract

Managers are interested in the factors that affect humpback chub, Gila cypha, demography in the Little Colorado River and whether this portion of the humpback chub population is nearing its carrying capacity. Understanding how vital rates (growth, survival, etc.) vary in space and time can serve as a useful first step for identifying factors that control population abundance. Here, we examine spatiotemporal variation in growth, movement, survival and abundance of age-one humpback chub living in the Little Colorado River, Grand Canyon, AZ from 2009-2012 using data collected by US Fish and Wildlife Service. To evaluate variation in survival and movement, we divide the Little Colorado River into segments (i.e., sites) according to river kilometer and use a multi-strata mark-recapture model to determine rates of movement and differences in survival between sites. We estimate abundance of age-one fish in different locations and periods using the Horvitz-Thompson estimator. We assess spatiotemporal variation in growth using a linear model with cohort, site, and initial length as fixed effects on growth. Results indicate that summer growth was higher for upstream sites compared to downstream sites, but that the effect of site on winter growth varies from year to year. Apparent survival is estimated to be lower at the most downstream site compared to the upper sites; however, this could be in part due increased emigration into the Colorado River at downstream sites. Furthermore, year had a large effect on annual apparent survival, with greater apparent survival occurring in 2010 compared to other years. While spatial patterns are inconsistent with density dependence, temporal patterns are roughly consistent. One explanation is that the carrying capacity in upstream portions of the Little Colorado River. Obtaining a better understanding of spatiotemporal effects on age-one humpback chub survival can help inform current management efforts to translocate humpback chub into other tributaries of the Colorado River and give us a better understanding of the factors that may limit the carrying capacity of this tributary.