Special river, special fish: The South Fork of the Flathead grows some incredible cutthroats

The South Fork of the Flathead has long been renowned for its westslope cutthroat trout fishery. But results of a multi-year study of the watershed may suggest it’s getting even better.

The South Fork above the Hungry Horse Dam drains the Bob Marshall Wilderness. The North and Middle forks of the river, while also a valuable fishery, are increasingly under pressure by non-native fish. Rainbow trout, a non-native species to the watershed, can interbreed with cutthroat, creating hybrid offspring. Those hybrids are generally not as a robust fish as the native trout.

But the cutthroats of the South Fork are isolated from non-native rainbows by the Hungry Horse Dam. 

Results of DNA analysis done this summer found that of 32 trout that were sampled in the South Fork, only pure westslope cutthroat trout genes were found, said Montana Fish, Wildlife and Parks biologist John Fraley.

Fraley and other FWP biologists have been sampling the genetics of cutthroats in the upper South Fork on and off for 30 years now. The first study Fraley did ran from 1985 to 1996. The purpose was to document genetics, growth rates, size and age of fish in the headwaters area of the drainage. He found that the genetically pure fish grew faster and lived longer than trout in the other stems of the river.

Why this is the case isn’t entirely understood, but there are likely several factors in play. For one, it’s excellent fish habitat. The river drains the Bob Marshall Wilderness and isn’t impacted by roads and other development. Secondly, there’s a length limit regulation on the fishery. Anglers can keep three fish a day and they have to be under 12 inches in length in the wilderness. Fraley’s 1980s and 1990s study, where 1,349 fish were caught and released, found that cutthroats from the South Fork grew faster than the rest of the basin. Trout in the South Fork grew to about a foot in length in less than five years, while trout in other areas of the basin it took five or six years.

More recent studies have found that the trout may be getting even bigger, on average.

“We found that the mean length of cutthroats in 2015 (334 mm) was greater than the mean length of cutthroat in 2010 (310 mm), and greater than the combined mean length of WCT in 1985-1996 (265 mm),” Fraley wrote. “The mean length in 2015 was also greater than in any of the individual years during the 1985-1996 period.  This apparent increase in growth or longevity could be related to fires in a large proportion of the watershed resulting in release of nutrients or warmer water temperatures.  The increase could also be a result of our very small sample size or other factors and should be viewed with caution.  At any rate, the news is encouraging.”

Three hundred millimeters is just under a foot in length.

Some of the trout in the South Fork live to be quite old, for a fish. Fraley found that some of the trout he caught were about 11 years old and one was probably 12.  For example, he caught and tagged one cutthroat in 1988 when it was 15 inches and 6 years of age, then caught it again in 1990 and again in 1994, when it was 17.5 inches in length.

Those are the oldest documented stream living cutthroat trout across its range, Fraley noted.