This year’s basin-wide estimate of 500 redds indicates a stable bull trout population as reflected by the number of successfully spawning adult bull trout migrating upstream from Flathead Lake. The counts have been relatively stable over the past 15 years (about 3 generations of bull trout), averaging 434 redds. This year’s estimate of 500 redds is 66 percent above the minimum secure level of 300 redds calculated for the Flathead Lake bull trout population under the Flathead Lake and River Fisheries Co-Management Plan. Based on the average number of eggs per female, this translates to several million eggs deposited in the gravels of North and Middle Fork Flathead tributaries.
Mature bull trout migrate upstream from Flathead Lake in the summer, spawn in September, and then return to Flathead Lake in October after spawning. The young fish hatch and then live in the tributaries from 1-3 years before migrating downstream to Flathead Lake to grow to adults and complete the life cycle. Biologists estimate that about half the adult bull trout in Flathead Lake spawn each year.
This year’s estimate of 500 redds is 57 percent of the 1980’s average, yet twice as high as the low level reached in the mid-1990s. This rebound is encouraging and indicates the current bull trout population is relatively stable. This was the 33rd consecutive year of index counts for the Flathead Lake bull trout population.