Does the Brood Abide? The Restoration of Native Westslope Cutthroat Trout in Montana

Sometimes things don’t turn out like you expect. That cliché can be applied to the Southfork West Fork of the Gallatin River near Big Sky, Montana. In recent years, the genetic status of this isolated population of Westslope cutthroat trout (WCT) has been uncertain. Protected to a large degree from an invasion of non-native trout by Ousel Falls, the WCT of the Southfork will always need active protection and monitoring. Over the course of several decades there have been impacts to the watershed from logging as well as threats from resort development, residential subdivisions, and private ponds. A variety of genetic assessments have been conducted, some of which showed a potential influx of rainbow trout. If confirmed this could be devastating for native fish in the Southfork. Hybridization is a significant factor leading the regional decline of WCT.

Westslope cutthroat trout in a Greater Yellowstone river. (Photo Jonny Armstrong/USGS.)

Westslope cutthroat trout in a Greater Yellowstone river. (Photo Jonny Armstrong/USGS.)

In the summer of 2018 Greater Yellowstone Coalition, with project partners Montana Fish, Wildlife, and Parks (FWP) and Custer Gallatin National Forest, undertook a genetic sampling effort to determine how well the WCT population was doing. Are they spawning? Are they losing a battle with Rainbow trout? Does something need to be done to conserve these fish?

With some coveted field time, we were able to collect small clips off fish from several locations on the Southfork and its tributaries. At each location we netted 10-20 fish, most within a stretch of 100 feet. This roughly equates to 1,000 to 1,200 fish per mile! They were healthy and with a good variation in size (2 – 12 inches). 

It never ceases to amaze me how quick and clever trout are. They can find the deepest little crease and with a flick of the tail be well hidden. While it was good to see healthy fish, were they still WCT?  The samples were sent off the University of Montana genetics lab and all we could do was wait for the results.

The report from the University of Montana lab, funded in part with a grant from the National Fish and Wildlife Foundation (NFWF), came back with mixed news.  Do we have a thriving group of fish?  The fish in the Southfork have far less rainbow trout hybridization than expected! Most fish had no rainbow genes. What was a mild surprise was the mixture of Yellowstone cutthroat trout in the population which ranged between 3 and 5%.  This means the fish in the Southfork are a conservation population (less than 98% and greater than 90% pure) of Westslope.  Based on Montana Fish, Wildlife, and Parks’ standards this means no action is warranted and we simply need to make sure things don’t change. The Brood Abides!

Habitat degradation and fragmentation, as well as competition and hybridization with non-native trout have greatly imperiled these beautiful fish. Greater Yellowstone Coalition is committed to restoring native trout populations where possible, and ensuring both wild and native trout have clean, healthy waters and habitats. Happily, now we know the cutthroat trout of the Southfork West Fork of the Gallatin River are thriving – and we aim to keep it that way.

-Bob Zimmer, Waters Program Coordinator