Seasonal and Spatial Patterns of Growth of Rainbow Trout in the Colorado River in Grand Canyon

Rainbow Trout have been purposely introduced in many regulated rivers, with inadvertent consequences on nonnative fishes. We describe how trout growth rates and condition could be influencing trout population dynamics throughout Glen and Grand Canyons. This study is based on a large-scale mark-recapture program in the Colorado River (130 km-long study area) where over 8,200 PIT-tagged trout were recaptured from over 70,000 marked fish. We fit von Bertalanffy and bioenergetics models to observations of growth increments, and used model selection to compare extent of seasonal and spatial variation in growth. There was more evidence for seasonal variation in growth coefficient and annual variation in asymptotic length. Bioenergetic models, however, showed more variability for growth in weight across seasons and years than across reaches. This variability was more attributed to food consumption than metabolic losses. Growth in length and especially weight was highest during spring and lowest in fall and winter, and declined progressively over three years. These patterns were consistent with strong seasonal variation in invertebrate drift availability and effects of turbidity on foraging efficiency. These data provide the means to further evaluate factors regulating trout population dynamics, and the undesired consequences of their movement and persistence in downstream reaches.