Bull trout spawning surveys in eight North and Middle Fork Flathead River tributaries were completed by FWP between October 1 and October 10 and the results are encouraging. Biologists count the number of “redds” or nests built in streambed gravels by adult bull trout from Flathead Lake. Identical sections are counted annually and represent a known portion (about 45 percent) of the total bull trout spawning in the drainage. The entire drainage is counted every three to five years. This is the best estimate of the mature bull trout spawning run from Flathead Lake.
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.
Photo caption: A bull trout redd or nest in Granite Creek, tributary of the Middle Fork Flathead River, October 1 2013. A female bull trout migrated 120 stream miles upstream from Flathead Lake to build this redd.