Stream flows affect number of bull trout spawning sites

Bull trout populations appear mostly stable in Northwest Montana, although Montana Fish, Wildlife and Parks’ annual inventory of spawning sites showed an overall drop from its 10-year average.

Fisheries officials attributed the declines this year to low stream flows, following the driest summer on record. The valley only received 1.09 inches of rain from June through August.

Bull trout spawning sites in Northwest Montana’s Region One include portions of the Flathead, Clark Fork and Kootenai river drainages.

During the annual count, state and federal biologists and other observers walk hundreds of miles of streams each fall, searching out “redds,” or the depressions female bull trout create in stream beds to lay their eggs.

Once the eggs are laid, they are immediately fertilized by a male, after which the female covers the fertilized eggs with gravel. The total number of nests observed each year are used as a proxy for overall bull trout populations in the region.

In a news release, state bull trout specialist Tom Weaver noted the number of redds was lower than expected.

“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 Weaver.

One drainage where the population appeared to drop significantly was in the Kootenai River drainage below Libby Dam.

Biologists rely on 10-year averages to assess redd count trends, but the Kootenai drainage experienced a more than 50 percent drop in 2015 from its 10-year average, registering only two thirds of the lowest count recorded in the past 10 years.

Observers counted just 74 redds this fall, compared with a 199-redd average and a 10-year low of 113 redds. Counts in the past 10 years have inventoried as many as 379 of the spawning sites below the dam.

The agency attributed the drop in suitable spawning habitat — and the corresponding drop in redds — to extremely low stream flows resulting in partial and complete debris barriers, higher impacts from beaver dams and the loss of suitable gravel for the spawning sites.

It also noted that 2015 was the lowest count on record for the Keeler Creek Drainage, which the report noted is “likely influenced by an expanding northern pike population in Bull Lake.”

That portion of the Kootenai drainage includes the West Fisher, Quartz, Pipe and Bear Creek drainages

Above the dam, the bull trout population in the Lake Koocanusa drainage is the most robust in the region, and Canadian biologists helped to identify 2,272 redds, a 16 percent increase over the 10-year average.

The reservoir’s drainage includes the Grave Creek, Wigwam River, Skookumchuck River and White River drainages.

The state has been counting redds in the North and Middle Forks of the Flathead River for 36 years, and 2015 reflected a modest overall drop from the last 10 years’ average; from 474 to 404.

The North Fork’s drop from an 86-redd average to 50 was consistent with the past few years and did not include 20 spawning sites located below a beaver dam blockage in Big Creek.

The totals from the Middle Fork were more encouraging, with 132 redds representing a 16 percent increase over the 10-year average, and aligned with numbers recorded during the 1980s.

In the North and Middle forks, biologists count all 33 streams sections known to support bull trout spawning once every three to five years. This year’s partial count — representing about 45 percent of all spawning habitat — was extrapolated to an estimated total of 404 redds.

The South Fork drainage, one of the region’s strongholds for bull trout, also resulted in a slightly blow-average inventory of redds this year.

The Hungry Horse Reservoir drainage was well below average — 52 redds, compared with 95 — and the sample of wilderness spawning habitat resulted in a decrease from the average of 304 redds to 265.

Swan Lake’s bull trout population was similarly below average but stable. Observers counted 421 of the nests this fall, nearly identical to the 428 counted last year but down from the 10-year average of 494.

The agency’s news release noted that the Swan Lake population appears to have recently stabilized at this slightly lower level, and believe it is likely the result of lake trout competition and predation, as well as by-catch of bull trout during lake trout suppression efforts.

In the Upper Stillwater Lake drainage, observers counted a 10-year high of 45 redds, well above the average of 27. Eleven redds were counted upstream of Whitefish Lake in the Swift Creek drainage, which was identical to the average.

Bull trout in the lower Clark Fork drainage spawned at a rate well below the average of 106 redds, with 70 of the nests identified this year. That drainage includes bull trout inhabiting the reservoirs behind the Thompson Falls, Noxon Rapids and Cabinet Gorge dams.

However, the state agency considers the numbers sufficient to reflect a stable population, and some redds identified below the stream sections monitored each year were not included in the total.