Trends in Rainbow Trout Recruitment, Abundance, Survival, and Growth during a Boom-and-Bust Cycle in a Tailwater Fishery

Data from a large-scale mark–recapture study were used in an open-population model to determine the cause for long-term trends in growth and abundance of a Rainbow Trout Oncorhynchus mykiss population in the tailwater of Glen Canyon Dam, Arizona. Reduced growth affected multiple life stages and processes, causing negative feedbacks that regulated the abundance of the population, including higher mortality of larger fish; lower rates of recruitment (young of the year) during years when growth was reduced; and lower rates of sexual maturation in the following year. High and steady flows during spring and summer 2011 resulted in a very large recruitment event. The population had declined tenfold by 2016 due to a combination of lower recruitment and reduced survival of larger trout. Survival rates for 225-mm and larger Rainbow Trout in 2014, 2015, and 2016 were 11, 21, and 22% lower, respectively, than average survival rates between 2012 and 2013. Abundance at the end of the study would have been threefold to fivefold higher if survival rates for larger individuals had remained at the elevated levels estimated for 2012 and 2013. Growth declined between 2012 and 2014 owing to reduced prey availability, which led to very poor fish condition (~0.90–0.95) by fall 2014. Poor condition in turn resulted in low survival rates of larger fish during fall 2014 and winter 2015, which contributed to the population collapse. In Glen Canyon, large recruitment events driven by high flows can lead to population increases that cannot be sustained due to limitations in prey supply. When the ability to regulate prey supply is lacking, flows that reduce the probability of large recruitment events can be used to avoid boom-and-bust population cycles. Our study demonstrates that mark–recapture is a very informative approach for understanding the dynamics of tailwater trout populations.

Received December 6, 2016; accepted March 31, 2017 Published online August 2, 2017