Population dynamics of Yellowstone Cutthroat Trout in Henrys Lake, Idaho

Yellowstone Cutthroat Trout (YCT) Oncorhynchus clarkii bouvieri is a species with significant ecological and recreational value.  In many YCT fisheries, managers are tasked with balancing angler expectations and fish conservation.  Henrys Lake supports a popular trophy trout fishery, but the increase of nonnative Utah Chub Gila atraria has caused concern for YCT.  Long-term trends in abundance, length structure, body condition, and growth of YCT were summarized to evaluate the effect of Utah Chub.  Additionally, we investigated abiotic and biotic factors influencing YCT.  Archived hard structures were examined to provide a comprehensive evaluation of changes in age and growth of YCT in the system.  Stocking records and catch rates of Utah Chub and trout in Henrys Lake were used as covariates to explain changes in YCT catch rates and growth.  Catch rates varied from 1.5 – 15.4 YCT per net night during the 2002 – 2020 sampling period, but no consistent patterns were identified.  Length structure was consistently dominated by stock- to quality-length fish and few fish > 600 mm TL were captured.  Relative weight of YCT has decreased from a mean ( SD) of 115.916.5 in 2004 to 93.2 8.2 in 2020.  Age of YCT varied between 1 and 11 years; fish captured during 2010 to 2020 were the oldest.  The majority of fish sampled were age-4 and younger.  Total annual mortality of age-2 and older YCT was higher than other Cutthroat Trout populations (i.e., 0.70 during 2002 to 2010 and 0.60 during 2011 to 2020).  Based on regression models, we identified positive relationships between catch rates of YCT, Brook Trout Salvelinus fontinalis, and Rainbow Trout Oncorhynchus mykiss × YCT hybrid trout.  Negative relationships were observed between growth of YCT and abundance of Utah Chub and Brook Trout.  Although negative relationships were identified, YCT growth in recent decades is as fast as or faster than earlier time periods.  Results from this research suggest that major changes in YCT population dynamics are not evident over the last 20 years.  This study provides insight into the factors influencing an adfluvial trout population.  In particular, results from this research may be useful for managers of systems where Utah Chub have been introduced.