Water Wisdom: Good Genes

Native fish thrive in Big Sky

By Stephanie Lynn EBS CONTRIBUTOR

A study performed by the Greater Yellowstone Coalition in collaboration with Montana Fish, Wildlife and Parks and the Custer Gallatin National Forest confirmed westslope cutthroat trout in the South Fork of the West Fork above Ousel Falls have a low percentage of hybridization.

Silvery green with a slash of red across their lower jaws, cutthroat trout are striking reminders of the West as Lewis and Clark saw it. They also occupy a vital place in complex food webs.

Habitat loss, introduced species and overfishing have precipitated the decline of these native fish resulting in their designation as a species of concern by the state of Montana. Foreign trout species, including brook and brown trout, outcompete juvenile cutthroat for food and also prey on cutthroat trout, while rainbow trout engulf endemic genes through interbreeding.

Hoping to protect threatened trout, GYC and their agency partners surveyed three streams that flow into the Gallatin River for westslope cutthroat trout in 2018. Promising preliminary findings in the South Fork led to formal fish genetics analyses at seven locations on this creek and its tributaries, funded in part by the National Fish and Wildlife Foundation. After sampling, GYC planned to ensure native trout persist in the South Fork, either by removing hybrid “cutbows” or by using a technique known as “swamping” to infuse the population with genetically-pure westslope stock.

“We expected to find a hybridized westslope cutthroat population above Ousel Falls, but discovered a conservation population with on average nearly 95 percent westslope cutthroat genes,” said Bob Zimmer, GYC waters program coordinator.

The study estimated that the South Fork shelters approximately 1,000 to 1,200 fish per mile with genes that are 93 to 95 percent westslope cutthroat and five percent Yellowstone cutthroat. In addition, some fish had one to three percent rainbow genes. With these unexpected results in hand, the partners doubled-down on other efforts to protect the endemic population.

The headwaters of the South Fork are located within the Yellowstone Club, where the YC manages guided trips through stream rotation and closures in sensitive areas while educating their members about fish handling and stream etiquette.

“The genetic testing last summer provided similar results to the previous two samplings that have been done and further proved that there is an incredible fisheries resource in the upper South Fork watershed,” said Rich Chandler, environmental manager at the Yellowstone Club. “We intend to maintain all the programs at YC that help promote the westslope cutthroat trout’s important role in the ecosystem.”

Ousel Falls creates a natural barrier that harbors one of six conservation populations of westslope cutthroat trout in the Upper Gallatin watershed. Protecting these vulnerable fish through habitat conservation and fisheries management sustains their vital ecological niche and our natural heritage.

Stephanie Lynn is the education and communications coordinator for the Gallatin River Task Force.