Spatial and temporal variation in vital rates of an endangered fish in a desert stream

Background/Question/Methods

Humpback chub (Gila cypha) persist in six populations in the Colorado River Basin, one of which is found below the Glen Canyon Dam. In this population, almost all spawning and much rearing of juvenile chub occurs in a tributary to the Colorado River, the Little Colorado River (hereafter LCR). The LCR is a perennial spring-fed river that also receives variable, and at times quite substantial, inputs from a large arid drainage following spring snowmelts and late summer monsoons. Population dynamics of chub within the LCR are themselves believed to be fairly variable both in space and time; however growth, survival and movement (hereafter referred to as vital rates) of sub-adults have not been examined throughout the LCR. Previous studies, instead, reported that abundances were higher at locations further upstream and speculated that survival may be negatively correlated with cohort size. Here we studied vital rates of humpback chub between their first and second years of life in five separate years (2001-2002, 2009-2013), to determine whether there were consistent spatial patterns in vital rates and to examine interannual variability. We are interested in the questions because both density-dependence and spatial heterogeneity can counteract temporal variability and help stabilize population dynamics.

Results/Conclusions

Summer growth was higher for upstream sites compared to downstream sites, but there were no consistent spatial patterns in winter growth. Apparent survival was lower at the most downstream site compared to the upstream sites; however, this could be due to increased emigration into the Colorado River at downstream sites. Interannual variability in apparent survival was much greater than spatial variation, with the 2010 age1 cohort (the smallest cohort) exhibiting much greater apparent survival (and slightly higher growth) than in other years. Consistent with past studies we found abundance to be greatest at sites further upstream in most years. Interestingly, the smallest cohort (2010) was relatively evenly distributed throughout the river to begin with, but exhibited high rates of upstream movement, suggesting that chub may have been redistributing themselves to take advantage of increased resource availability upstream. We also present information suggesting higher potential invertebrate and small fish food-base occurs in the upper portions of the LCR. We end by discussing the implications of our findings for current efforts to translocate fish from the LCR to other locations in the Grand Canyon.