Bull trout numbers in northwest Montana’s Swan River drainage have dropped to their lowest level in 17 years, as introduced lake trout appear to be outcompeting — and eating — their native cousins.
Swan Lake is a stronghold for bull trout, one of a few remaining places in the nation where anglers can catch and keep the native fish. But their future here has been in doubt since 1998, when the first lake trout invader was recorded in the Swan.
For nearly 30 years, scientists have gone to the Swan to count “redds” – the underwater nests where bull trout lay their eggs. This year’s Swan drainage redd count – 366 – is the lowest since 1992.
Biologists, who spent much of October counting fish nests, predict that if lake trout continue to prosper, anglers should expect substantial changes to the species mix in Swan Lake.
State fisheries biologist Tom Weaver warns continued declines of bull trout are likely there if the non-natives spread further.
The high mark of 612 redds came, ironically, in 1998 — the same year bull trout were granted protections under the federal Endangered Species Act.
At the time the Swan was considered a stronghold, while native fish elsewhere in the Flathead River system were struggling.
Predatory lake trout were first introduced to Flathead Lake in the early 1900s, part of an attempt to produce a commercial fishery strong enough to encourage settlement in the area.
While the river system upstream of Flathead Lake seems to be stabilizing after bull trout numbers crashed there in the 1990s, biologists now are turning their attentions to the Swan drainage.
It took the invaders decades to get established in the Swan Lake drainage. Now that they are, it would be difficult to remove them. Biologists have tried for years to remove lake trout from Wyoming’s Yellowstone Lake, with mixed success.
The latest numbers for the Swan drainage, however, may not accurately reflect general population trends.
Bull trout remain for a few years in the small streams where they are born, before migrating down into larger lakes such as Flathead and Swan. They return to their birth streams at about age 6 or 7 years old to spawn the next generation.
That means this year’s redd count actually represents an echo from 2003 rather than a precipitous decline from the past few years.
Yet the recent monitoring there comes on the heels of more bad news from the Swan. In June, netting turned up the first invaders in the drainage’s Lindbergh Lake — four lake trout ranging from 16 to 19 inches long.
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