Pure Cutthroats Found In Glacier

Fisheries experts, concerned about dwindling westslope cutthroat populations, have been seeking out native trout populations in Glacier National Park. Their hope is to identify genetically pure strains, which later could be used in reintroduction efforts if westslope cutthroats are listed as threatened or endangered.

According to Leo Marnell, senior park scientist at Glacier, fisheries specialists and aquatic ecologists agree that, based on the current rate of decline, the species likely will be listed within the next five to 10 years.

“When you put it all together, the natural range of westslope cutthroat has been reduced by 90 percent from its historic size,” Marnell said, blaming predation by non-native lake trout and habitat destruction as the main culprits.

Westslope cutthroats previously were listed as threatened in the late 1960s, but that designation was lifted in 1973 when scientists realized that westslope and Yellowstone cutthroats were distinct subspecies.

As the taxonomic turmoil was sorted out, the fish were delisted, although numbers of westslope cutts continued to drop across their entire range from the eastern Cascades of the Pacific Northwest to the Missouri River and from the Idaho Panhandle south to Yellowstone National Park.

In Glacier, however, the population remained relatively stable, sheltered by the protective umbrella of the National Park Service for most of the past century.

Waters like Avalanche Lake continue to provide dandy fishing for westslope cutts, leading scientists to look at those park populations as possible genetic stockpiles for any future recovery efforts.

The question, then, is which lakes contain indigenous populations and which ones have stocked populations, placed there by man during the last 100 years or so?

Fish indigenous to a region, Marnell said, are more suited for use in a recovery project because they have evolved and adapted over the centuries to thrive in that region.

Working in waders rather than gumshoes and keeping an eye to a microscope rather than a magnifying glass, Marnell set out to deduce the nature of Avalanche Lake’s trout.

If the lake turns out to be home to stocked populations, fisheries managers would be less likely to reintroduce those fish into nearby waters. If, as Marnell’s detective work indicates, the fish are indigenous, then the matter becomes a more mysterious but less imperative “howdunit.”

Avalanche Lake is nestled in a mountain cirque, ringed on three sides by steep rock walls and fed by high-elevation snowmelt. If the fish were indigenous, Marnell assumed, they would have had to swim upstream from the valley below. A deep, waterfall-filled gorge, however, links Avalanche Lake with the waters below, creating a natural barrier to any upstream migration.

“We had some real red flags that told us the fish were stocked,” he said. “First of all, we knew parts of the park had a strong history of transplanted fish. We also knew that if the westslope cutthroat were there, other species like bull trout and Rocky Mountain whitefish should have been there as well, but they weren’t.

“And then we had this huge physical obstacle, the gorge, which is a natural barrier. Everything pointed to a stocked population.”

But, as any good detective knows, matters are not always as elementary as they appear.

Loading his equipment onto a platform raft, Marnell and partner Dirk Verschuren took to the 57-acre lake, boring core samples of sediment and plankton from Avalanche’s silty bottom. Using lead isotope dating, they aged each layer to within three years, moving back through several centuries as the probes went deeper.

“A lake with fish will be home to a different aquatic flora than the same lake without fish,” Marnell said. “If the lake was stocked by man, we would see the change over time; we would see the fingerprints of the food web trapped in the sediment.”

Marnell and Verschuren keyed on eggs of water fleas, trapped in the sediment over the centuries, to reconstruct a 300-year living history of Avalanche Lake. It was the first time such technology had been used to chronicle a fishery, making the success of the methods almost as important as the final answer.

That work, coupled with genetic analysis of the lake’s westslope cutthroats, finally has resolved the issue once and for all. According to Marnell, it’s good news for anglers and biologists alike: The fish are genetically pure, indigenous westslope cutthroats.

“If and when the park is in a position to consider a recovery of native westslope cutthroat populations, especially in the Lake McDonald basin, we have the original ancestral strain right here in this lake,” Marnell said.

“This is the seed, genetically speaking. It’s a genetic treasure chest that holds a rare and valuable wealth, not just for fishermen but for everyone who wants their children to grow up in a world with these native species.”

Several theories have emerged on the origin of the fishery. Perhaps the last mini-Ice Age could have created temporary ice dams in the gorge, allowing the fish to move up incrementally over time as the ice receded. Another theory is that the fish made their way to Avalanche Lake long before the gorge was cut to such an extreme depth.

“There’s a chance we’ll never know for sure how they got there,” Marnell said. “But what’s important is that we know they did get there on their own and they’ve been isolated from other species for centuries.”