Leveraging public harvest to reduce invasive hybridization in Yellowstone National Park: field identification and harvest of cutthroat × rainbow trout hybrids

Abstract

Leveraging public harvest can be a cost-effective invasive species management tool, but target taxa must be correctly identified and removed at rates that achieve biological objectives. We explored the potential role of recreational anglers to curtail expanding hybridization between invasive rainbow trout (Oncorhynchus mykiss; RT) and native Yellowstone cutthroat trout (O. clarkii bouvieri; YCT) in the Lamar River watershed in Yellowstone National Park. We sought to (1) develop a hybrid identification key that could be used by anglers and (2) estimate angler participation, catch, and potential exploitation rates. We assessed seven morphological features of trout in the field (n = 251, 15 locations) and collected fin clips to estimate RT ancestry proportion using genetic analysis. An identification key was built using recursive partitioning to objectively distinguish YCT from RT and hybrids. A single-choice dichotomous key (white pelvic fin tip present/absent) correctly classified 93% of fish as native (YCT) or containing RT ancestry (RT or hybrid). Success increased to 97% when a second criterion was added (head spot count ≥ 6). Using angler surveys (2013–2017), we estimated that 10,000 anglers catch 50,000 trout annually. In a popular road-accessible area, most trout are probably caught and released ~ 5 times each year. The combination of high angler participation, substantial annual catch, and an accurate and easy to use identification method indicate that leveraging public harvest is a promising management tool. Invasive hybridization is a global conservation issue threatening many native taxa; this case study highlights some factors for resource managers to consider prior to implementing public harvest regulations and the benefits of standardized keys to distinguish hybrids in the field.