Survival and Dispersal of Hatchery-Raised Rainbow Trout in a River Basin Undergoing Urbanization

Abstract

We evaluated the effect of urbanization on water temperature and the survival of rainbow trout Oncorhynchus mykiss that support a popular coldwater fishery in the Chattahoochee River near Atlanta, Georgia. Using multistratum tag recovery models, we estimated the monthly survival, dispersal, and angler harvest reporting rates of stocked trout in two study reaches from March to October 2006. The best-approximating models indicated that the monthly survival of stocked trout was negatively related to angler effort and the amount of time that water temperature exceeded 20°C. We found that trout survival was more sensitive to high water temperature than to angler effort. Dispersal rates from a warmer downstream to a cooler upstream reach were 6–7 times those from upstream to downstream. Empirical temperature–discharge models indicated that water temperatures in the river downstream of Morgan Falls Dam have increased from 1976 to 2006, urbanization being a likely contributing factor. We estimate that on average 60 m³/s of additional discharge in summer are needed under current (2006) conditions to maintain water temperatures similar to those prior to urbanization (1976). Increased water use from the river accounted for less than one-fourth of the 60 m³/s, suggesting that other factors, such as a higher percentage of impervious surfaces, were primarily responsible for the higher temperatures. Using these models, we estimate that on average 20% of the stocked trout were lost during the summer months owing to angling and other sources (e.g., high temperature) under preurbanized conditions, whereas 70% of the fish are lost under current conditions. Our models suggest that increasing the hypolimnetic releases from an upstream dam could mitigate the effects of urbanization on water temperature during the summer.