Dynamics in rainbow trout abundance and growth in an intermittently turbid river, Grand Canyon, AZ

Background/Question/Methods

Nonnative fish, including rainbow trout (Oncorhynchus mykiss), have been purposely introduced in many regulated rivers below dams that favorably alter flow, temperature, and water clarity. In some rivers the abundance of these nonnative fishes may decline further downstream as flow, temperature, and water clarity begin to more closely resemble natural conditions. In the Colorado River below Glen Canyon Dam, rainbow trout are the most abundant fish. Although there numbers decrease downstream, they still remain sufficiently abundant during some years to negatively impact the survival of juvenile native fish, including endangered humpback chub (Gila cypha), at locations over 130 river kilometers below the dam. Here, we ask how spatiotemporal variation in turbidity and invertebrate drift densities (#•m^-3) affects rainbow trout survival, growth and condition and link these patterns to both movement and the distribution of trout densities throughout the study area. We answer these questions using a mark-recapture study based on quarterly sampling of five reaches (6 km/reach) between April 2012 and January 2014. Reaches were spread between Glen Canyon Dam and the Little Colorado River tributary (total distance of 130 km) and individual fish were marked with passive integrated transponder tags (over 20,000 fish per year).

Results/Conclusions

Over the study period, rainbow trout abundance declined system-wide from 1,200,000 to 620,000 fish. Trout densities decreased in reaches further from the dam; however, the greatest declines through time occurred in the most upstream reach (mean: 19,078 fish/km in April 2012 to 8,890 fish/km in January 2014), while density increases in the most downstream reach (206 fish/km to 600 fish/km), which serves as a rearing area for humpback chub. Monthly growth rates and condition varied significantly among seasons and reaches and were negatively correlated to trout densities and turbidity. Lowest growth and condition occurs in the most upstream and downstream reaches. Drift densities for amphipods (Gammarus lacustris) were seasonally variable, with highest densities during summer (0.4 m^-3) and lowest for winter (0.05 m^-3). Under decreasing trout densities, summer drift densities increased cumulatively with distance downstream. Spatial and seasonal dynamics in invertebrate drift correspond with trout growth except under increases in turbidity when visual prey detection is less even though drift availability remains high. Results from this study suggest that turbidity and trout densities may partly control individual trout growth rates; however, it remains uncertain whether local trout densities and movement among reaches are governed at a population level by growth.