Turbidity effects on spatial dynamics of rainbow trout abundance and growth, Grand Canyon, AZ

Authors: Michael D. Yard, Josh Korman, Theodore Kennedy, Charles Yackulic, Scott VanderKooi

Year: 2013 (xlv)

Abstract

The Colorado River fish community in Grand Canyon has been altered by dam regulation and introductions of nonnative fish, particularly rainbow trout (Oncorhynchus mykiss). Rainbow trout are abundant and are likely to compete with and predate on juvenile native fish, including endangered humpback chub (Gila cypha), rearing near the Little Colorado River. We examined effects of seasonal changes in turbidity and invertebrate drift densities on trout abundance and vital rates. We implemented an extensive mark-recapture study, sampling five reaches (6 km/reach) quarterly (April, July, September and January) using electrofishing in a robust sampling design. Individual fish were marked with passive integrated transponder tags (about 20,000 fish per year) to estimate abundance, movement, growth, and survival of age-0 and older trout. Total rainbow trout abundance was estimated at approximately 500,000 individuals; however, trout densities were unevenly distributed between Glen Canyon Dam and the Little Colorado River (125 km), declining from 7,000 to 500 fish/km, respectively. In contrast, invertebrate drift densities increased with increasing distance downstream. Daily growth rates varied significantly among seasons and sites and were negatively correlated to turbidity levels. Spatial dynamics observed in rainbow trout metrics corresponded strongly with invertebrate drift availability and trout densities, and differences in turbidity among sites strongly mediated these relationships.