Flow Related Responses of the Age-0 Population of Rainbow Trout in Lees Ferry

Authors: Luke A. Avery, Josh Korman, Adam Copp

Year: 2013 (xlv)

Abstract

The Rainbow Trout Early Life Stage Study (RTELSS) was initiated in 2003 to monitor the effects of experimental increased daily fluctuation of flows from Glen Canyon Dam on the Lees Ferry age-0 rainbow trout (Oncorhynchus mykiss) population. Though intended to be an isolated study, RTELSS has continued on as a monitoring effort that has provided valuable information on the response of age-0 trout to additional experimental flow events. The increased daily fluctuation of flows during the springs of 2003-2004 increased estimated redd mortality to 23% and 49% for years 2003 and 2004, respectively, up from 2-11% for years of normal fluctuation (2006-2012). However, a compensatory increase in early survival of larval rainbow trout nullified the effect at a population level. In 2008, a high flow event (HFE, ~42,000 cfs) occurred in March, which is during the peak of the spawning season in Glen Canyon. Though the flows decimated the redds and larvae of pre-HFE spawning rainbow trout, they also cleaned the spawning substrate, which lead to increased hatch success for post-HFE spawning fish. There was also a shift in dominate foodbase taxa from New Zealand mudsnail (Potamopyrgus antipodarum, non-digestible) to Simulids and Chironimids (highly palatable). This resulted in increased early survival and growth of trout. In July 2008 Lees Ferry age-0 rainbow trout population was estimated to be 883,000 fish (95% CI, 647,000-1,168,000), 5.5 times the July average for all previous years. Flows during 2011 were higher and less variable than other recent years (2006-2010, 2012). Increased wetted habitat is the likely reason for the record high redd estimate of 3,062 for 2011 (10-year average=1,805). Increased habitat quantity and decreased flow variability likely resulted in increased hatch success and month to month survival. The November age-0 rainbow trout population estimate for 2011 was 214,000 fish (95% CI: 175,000-253,000), 3.6 times the November average for all other years. High growth rates during a portion of the year were likely linked to higher than normal water temperatures during that same period. Population estimates for 2013 indicate some potential positive effects of a November 2012 HFE that may be explained by patterns in invertebrate drift rates.