Discovery of Grass Carp larvae in the Colorado River arm of Lake Powell.
Grass Carp Ctenopharyngodon idella stocking in the upper Colorado River basin is statutorily limited to triploid, i.e., functionally sterile, individuals. Systematic fish sampling in the upper Colorado River basin indicates adult Grass Carp remain rare but have been increasing in abundance since 2007. Grass Carp larvae collected in the Colorado River arm of Lake Powell, about 26 km downstream from the inflow, in 2015 and 2016, are evidence of the presence of a spawning population. These larvae were the first documentation of Grass Carp spawning in a river basin of the western United States. While Grass Carp reproductive biology suggests spawning occurred in the Colorado River or its tributaries upstream from Lake Powell, the sample location, age, and number of larvae collected pose the possibility of reservoir spawning. Addition of another, reproductively viable, nonnative, fish species to the already heavily invaded upper Colorado River basin could further impact conservation of the federally endangered native fishes.