This year’s survey of bull trout spawning turned up average results in Montana's Flathead River Basin, but indicate a significant decline in the Swan River drainage.
Every fall, Montana Fish, Wildlife and Parks crews survey spawning streams, looking for bull trout spawning beds called “redds.” The surveys focus on the same stream sections every year in the South Fork, North Fork and Middle Fork of the Flathead River system, as well as the Swan River, which also feeds into Flathead Lake.
Because of substantial rainfall in September, there were higher stream flows and less-than-optimal survey conditions, raising the possibility some redds were missed.
Surveys have been conducted in the Swan River Basin for the last 29 years, with a basin-wide average of 669 redds for the last 15 years. This year’s count of 378 redds is less than half of that average.
Counts peaked at around 800 in 1997 and 1998 but declined in the following years until a brief rebound 2006 and 2007. They have declined ever since.
Biologists believe a reason for the decline may be non-native lake trout, which were first detected in Swan Lake in 1998. Subsequent studies indicate that lake trout have been present in the Swan River system since the early 1990s, with the population steadily growing since then.
Lake trout have led to declines in bull trout populations in other waters throughout the region, most notably the Flathead Lake bull trout population that depends on spawning in the upper Flathead River system.
Bull trout counts in the North and Middle forks of the Flathead focus on eight stream sections, and this year’s count of 190 redds is on par with the 193 average over the last 12 years. The eight stream sections account for an estimated 45 percent of the total Flathead Lake spawning run.
This year’s count of 136 redds in Middle Fork stream sections is the highest since 1989, but counts in the standard North Fork stream sections were down considerably, as they have been for years. To determine if bull trout spawning patterns have shifted, crews looked upstream from the standard stream sections and found that may be the case. In the upper reaches of the Big Creek drainage, there were 36 redds, and 23 were counted in the upper Coal Creek drainage.