State of the Fishery – Big Hole & Rock Creek

Brad Liermann, FWP biologist for Rock Creek and Georgetown Lake.

After months of winter, local angling enthusiasts were eager to hear what might be in store for fishing this year, at last week’s annual State of the Fishery program presented by the George Grant Chapter of Trout Unlimited.

Leading off the program was David Brooks, Executive Director of Montana Trout Unlimited, announcing the hire of Chris Edgerton to be TU’s resident specialist on the Jefferson River and watershed.

Brad Liermann, Montana Fish, Wildlife & Parks fisheries biologist for Georgetown Lake and Rock Creek, presented information on a winter fish kill issue on Georgetown Lake.

Last year, as ice started melting, dead fish started turning up. Liermann estimated he saw as many as a thousand dead fish, mostly rainbow trout.

The culprit in the die-off was identified as low dissolved oxygen levels in the lake. A level of 5 milligrams per liter of water is considered a danger point for fish and at the end of winter, a year ago, there were levels of as low as 2.5 mg/liter. He has been monitoring oxygen levels this winter and it’s marginally better than a year ago.

Liermann said the main factor is snow depths covering the lake, which have been roughly double the long-term average these last two winters.

Fortunately, the lake remains a productive fishery and fall gill-netting surveys were in the normal range.

Liermann reported on some long-term trends on Rock Creek, Montana’s only blue ribbon trout stream west of the Continental Divide. Rock Creek, for years, was a renowned rainbow trout fishery until whirling disease came along in the mid-1990s. When rainbow trout numbers crashed, brown trout and westslope cutthroat trout numbers both increased to fill the ecological niche. In the last few years, rainbow trout have been making a comeback, though there is some concern that it might threaten the mostly pure westslope cutthroat trout population.

Jim Olsen, FWP biologist for the Big Hole River

Jim Olsen, FWP biologist for the Big Hole River, reported on a stable fishery on most stretches of southwest Montana’s premier trout stream.

He noted a temporary drop in brown trout numbers in the Melrose area following the 2015 outbreak of saprolegnia, an aquatic fungus that caused a significant die-off in the fall of that year. Numbers quickly bounced back to normal, likely from fish migrating in from other parts of the river.

Olsen highlighted a relatively lightly fished section, the area downstream from Pennington Bridge to the last takeout on the river at Twin Bridges. This stretch doesn’t have as many trout as other sections, something around 800 fish (browns and rainbows) per mile. “There are a lot of nice fish, well-fed and healthy.” He added that the trout density is low plus there is a lot of forage for larger fish, such as suckers, whitefish and dace.

This led to a discussion of some long-term impacts of the special regulations section of the Big Hole, from Divide to Melrose, originally, in 1981, and later extended to Jerry Creek.

Based on almost 40 years of data, a significant impact of the special regulations was an increase in fish numbers. Fish numbers almost doubled in the first five years. On the other hand, the numbers of fish over 20 inches dropped significantly. Higher densities of fish means less food for big fish to grow.

Olsen also reported on steady growth in fishing pressure on the Big Hole, with an estimated 80,285 angler days in 2017, almost double the numbers from around ten years earlier.

With a new fishing/floating season coming up, Olsen noted a change in river channels in the river about one mile upstream from the Glen Fishing Access Site. The river changed course so that almost the entire river is going down what was a smaller channel on river left. The river right channel is currently carrying a trickle of water. Floaters will find themselves dumped back in the main channel across the river and just downstream from the Glen FAS boat ramp.

The moral to the story is that rivers are a dynamic environment and hydraulics are powerful.

The next GGTU program meeting will be on May 8.

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