Bull Trout Spawning Numbers Affected by Low Stream Flows

In its 36th year, annual survey shows mostly stable populations in Northwest Montana

By Tristan Scott

An annual stream survey of native bull trout in Northwest Montana revealed mostly stable populations, with a decline in reproduction due to low stream flows and blockages at some spawning sites, according to Montana Fish, Wildlife and Parks officials.

Following the driest summer in the Flathead Valley on record, fisheries biologists attributed the declines to low stream flows, with the most significant drop 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.

In the agency’s 36th year of the fall surveys, biologists counted just 74 redds, 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 North Fork of the Flathead River 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, and represented a 16 percent increase over the 10-year average. The Middle Fork’s range was 56 to 171.

The South Fork drainage, a regional stronghold for bull trout, also resulted in a below-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, according to FWP. 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.

Most biologists agree that the decline is due in large part to predation by non-native lake trout, which exploded in numbers beginning in 1992 and have dramatically changed the dynamic of the aquatic food web throughout the entire Flathead River basin.

The agency 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 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 FWP bull trout specialist Tom Weaver.

FWP Region One Fisheries Manager Mark Deleray noted that bull trout redd counts provide a means to assess the status of bull trout populations over time, and that one year’s count alone is not indicative of a population trend. Rather, the redd counts provide an annual basis for bull trout conservation discussions among fisheries professionals and angler groups.

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