Low flows and stream blockages may have contributed to some lower-than-usual spawning results in the annual northwest Montana bull trout survey this fall.
“In some streams, our annual index sections were not accessible to fish due to debris jams, beaver dams or other flow-related conditions, resulting in lower-than-expected counts,” said Tom Weaver, Montana Fish, Wildlife and Parks bull trout specialist in Kalispell. However, he said the overall count indicates numbers of the federally threatened species remain stable in the region.
Bull trout grow to maturity in large rivers or lakes, but return to small mountain streams to reproduce. Female bull trout furrow gravel nests, called redds, in the streams 4 to 6 feet long and up to 3 feet wide to lay their eggs. Males immediately fertilize them and then cover the redds with clean gravel.
Biologists and volunteers from FWP as well as the state Department of Natural Resources and Conservation, Bonneville Power Administration, Avista Corp., Glacier National Park, U.S. Forest Service and Plum Creek Timber Co. all participate in the fall creek surveys.
Results varied widely in the fall counts. The North Fork of the Flathead count of 50 redds was lower than any of the previous 10 years, which range from 51 to 144. But the Middle Fork of the Flathead produced 132 redds, compared with the 10-year average of 114. The Middle Fork’s range was 56 to 171.
Region 1 fisheries program manager Mark Delray said extremely low counts in the Big Creek tributary system of the Flathead prompted some extra searching for answers.
“Since the Moose and Roberts (forest) fires, a log debris accumulation has formed approximately one mile downstream from the lower end of our index section,” Delray noted in an email. “An additional survey of this area found new gravel deposition and channel braiding, with extensive beaver activity which blocked most upstream navigation. We observed 20 redds in the area below the blockage, which has not contained suitable spawning habitat prior to the recent channel changes.”
Overall, the rivers feeding into Flathead Lake produced a redd count under the 10-year average and the second lowest in the time period. However, Delray said the year’s low water flows, blockages and habitat shifts indicate the bull trout activity might have been closer to average than it appears.
Redds in the Swan River drainage appear to be stabilizing at a lower figure than the 10-year average of 494. Delray said the 421 redds counted this year in the basin “are likely the result of competition/predation from lake trout and bycatch bull trout mortality from the interagency lake trout suppression experiment.”
Commercial net fishermen have been removing lake trout from Swan Lake for the past several years. This fall, they found two illegally introduced walleye in their nets – raising concern that a new invasive predator also could be damaging the bull trout fishery. Montana Trout Unlimited has offered a $20,000 reward for evidence leading to the conviction of those responsible.
On the plus side, a redd count of 45 in the Stillwater River was the highest in the past 10 years for a basin separate from either the Flathead or Swan drainages.
Lower Clark Fork bull trout counts from reservoirs behind the Thompson Falls, Noxon Rapids and Cabinet Gorge dams came in lower than average with 70 in 2015. Counts in the Koocanusa Reservoir were around average and slightly higher than the past several years, but downstream of Libby Dam the results were considerably below average. Delray said a combination of low flows, beaver impacts and overall reduction of spawning habitat were responsible. Increases of northern pike populations in Bull Lake are also a possibility.