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

Abstract

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

Changes in prey, turbidity, and competition reduce somatic growth and cause the collapse of a fish population

“…Very few tagged fish were recaptured in reaches other than the ones they were released in, and these fish were excluded from the analysis. We also conducted sampling trips in October of each year in reach I only, and use length and mass data from these trips to evaluate the effects of condition factor on rates of maturation during the spawning season (January and April trips) for trout large enough to reproduce (≥225 mm, Korman et al 2017). Trout were designated as mature if gametes were expressed when the body cavity was lightly squeezed.…”

“…To demonstrate the effect of growth on potential recruitment, we fit a linear relationship between condition factor of trout in Glen Canyon in the fall (October trips) for fish large enough to reach maturity (≥225 mm; Korman et al 2017), and the proportion that matured the following winter (January) or spring (April), when the majority of spawning occurs (Korman et al 2011 a ). The analysis was based on 2,681 trout sampled during October trips and 2,736 trout whose maturation state was evaluated on January or April trips the following year.…”

Multiple challenges confront a high-effort inland recreational fishery in decline

“…Previous studies that have assessed the impact of severe flood events on trout abundances have found responses ranging from dramatic decreases in trout density (Jowett and Richardson 1989;Kitanishi and Yamamoto 2015) to no change between pre-and postflood measurement periods (George et al 2015). High flow events leading to food supply limitations and subsequent population collapse were responsible for boom-and-bust population cycles in a tailwater rainbow trout fishery below the Glen Canyon Dam in Arizona (Korman et al 2017). However, this mechanism seems unlikely in the LBR, as substantial wastewater inputs have increased biological productivity (Sosiak 2002;Askey et al 2007), and these inputs have remained relatively constant throughout the study period (see Taube et al 2016).…”

“…However, it is unclear how changes in fish behavior along the periphery of these relatively large study sites could result in the consistent population decline observed across the entire study period. Similarly, the decision to use a multisession modeling framework ignored the possibility that an individual fish occurred in more than 1 year (Royle et al 2014), and we acknowledge that these models feature less ecological resolution than the outputs that would be generated via open population capture-recapture models (see Korman et al 2017). The approach used here, however, was warranted because rainbow trout are short-lived (i.e., maximum age of 7 years; Rhodes 2005) and have high instantaneous total mortality (van Poorten and Post 2005), which indicated that there was likely high population turnover among years.…”

Size- and stage-dependence in cause-specific mortality of migratory brown trout

“…Food availability is another factor deserving consideration in future studies. Korman et al (2017), for example, found prey biomass to be a key driver of growth and survival in a strongly fluctuating population of rainbow trout Oncorhynchus mykiss . Unfortunately, no time-series data exists on the abundance of prey fish species in our system, which made investigations impossible in this study.…”