Effects of Increased Discharge on Spawning and Age‐0 Recruitment of Rainbow Trout in the Colorado River at Lees Ferry, Arizona

Abstract

Negative interactions of Rainbow Trout Oncorhynchus mykiss with endangered Humpback Chub Gila cypha pose challenges to the operation of Glen Canyon Dam (GCD) to manage for both species in the Colorado River. Operations to enhance the Rainbow Trout tailwater fishery may lead to an increase in downstream movement of the trout to areas where they are likely to interact with Humpback Chub. We evaluated the effects of dam operations on age‐0 Rainbow Trout in the tailwater fishery to inform managers about how GCD operations could benefit a tailwater fishery for Rainbow Trout; although this could affect a Humpback Chub population farther downstream. A near year‐long increase in discharge at GCD in 2011 enabled us to evaluate whether high and stable flows led to increased spawning and production of age‐0 Rainbow Trout compared with other years. Rainbow Trout spawning was monitored by fitting a model to observed redd counts to estimate the number of redds created over a spawning season. Data collected during electrofishing trips in July–September and November were used to acquire age‐0 trout population and mortality rate estimates. We found that high and stable flows in 2011 resulted in 3,062 redds (1.7 times the mean of all survey years) and a population estimate of 686,000 age‐0 Rainbow Trout (second highest on record). Despite high initial abundance, mortality remained low through the year (0.0043%/d) resulting in significant recruitment with a record high November population estimate of 214,000 age‐0 Rainbow Trout. Recent monitoring indicates this recruitment event was followed by an increase in downstream migration, which may lead to increased interactions with downstream populations of Humpback Chub. Consequently, while our results indicate that manipulating flow at GCD can be used to manage Rainbow Trout spawning and recruitment, fisheries managers should use flow manipulation in moderation to minimize downstream migration in order to reduce negative interactions with other species in the Colorado River. Received September 8, 2014; accepted March 28, 2015

Trends in Rainbow Trout Recruitment, Abundance, Survival, and Growth during a Boom‐and‐Bust Cycle in a Tailwater Fishery

“…The results of our study indicate that these cycles may be avoided by (1) limiting flow conditions that result in large recruitments; (2) decreasing survival rates for young-of-the-year fish during years in which large recruitment events occur by implementing stranding flows; or (3) creating conditions that increase or stabilize prey availability. This study and other studies (Korman et al 2011(Korman et al , 2012Avery et al 2015) have demonstrated that high flows during spring and summer can result in large recruitments, ultimately leading to increases in population size. However, avoiding these flows is difficult owing to the operating rules that govern annual and monthly release volumes from Glen Canyon Dam.…”

Factors controlling the abundance of rainbow trout in the Colorado River in Grand Canyon in a reach utilized by endangered humpback chub

“…Second, using flow fluctuations to limit survival rates of recently emerged trout in Glen Canyon (BOR 2011) is a logical alternative as our study indicates that the majority of emigrants from Glen Canyon left within their first year, and possibly within 6 months from emergence. Our results also indicate that the fraction of trout emigrating from Glen Canyon was higher in a year when a large cohort was produced, thus to minimize impacts to the tailwater fishery, flow-control may only be needed in years where large cohorts are anticipated, such as in equalization years (Avery et al 2015) or following controlled floods in spring (Korman et al 2011). Third, it appears that mechanical removal efforts near the LCR will have to be part of any trout control strategy to account for the large population of trout in upper and middle Marble…”

“…Canyon (Avery et al 2015) and increase catch rates in the tailwater fishery (Rogowski et al 2015), unfortunately also lead to higher trout abundance near the LCR. Quantifying the trade-off between the Glen Canyon trout fishery and the status of humpback chub will require continued D r a f t monitoring of both species at the LCR to better define the relationship between rainbow trout abundance and the productivity of the humpback chub population.…”

Can data from disparate long-term fish monitoring programs be used to increase our understanding of regional and continental trends in large river assemblages?

“…In river systems where improvements in water quality are noted, our results suggest positive changes in fish assemblages have occurred over time (e.g., the Illinois and Mississippi rivers); while in river systems where water quality conditions have not improved significantly (e.g., the Tallapoosa River) there is evidence to suggest negative effects (6,22,27). Spatial and temporal changes in Colorado River water temperatures caused by the construction and operation of Glen Canyon Dam and drought ( Fig 2B ) have differentially affected native and non-native fishes [ 31 , 66 , 67 , 68 , 69 , 70 ]. Similarly, the construction of the FCRPS has altered the water temperature regime in the Columbia River and affected native and non-native fishes in different ways [ 71 ].…”