Montana Bull Trout Spawning Surveys Completed in the Flathead, Results are Encouraging
Montana Fish, Wildlife & Parks 10.17.12
Bull trout spawning surveys in the Flathead Basin were completed by FWP between September 24 and October 9. The results indicate a secure bull trout population, as defined through the Flathead Lake Co-management process.
Basin-wide counts of redds: Surveys in 31 tributaries of the North and Middle forks of the Flathead reflect the number of adult spawning bull trout migrating upstream from Flathead Lake. This year, 500 spawning sites or “redds” were counted, indicating that several million bull trout eggs have been deposited in these nests.
The count of 500 redds is 66 percent above the minimum secure level of 300 redds calculated for the Flathead Lake population. This count is nearly identical to 2008, when 503 redds were found in the basin-wide count.
This was the 11th basin-wide count since 1980. These basin-wide counts (covering all known bull trout spawning areas) are conducted every three to five years as a check to insure our index counts adequately capture overall trends.
Index counts of redds: This was the 33rd consecutive year of index counts for the Flathead Lake bull trout population. These counts take place in portions of eight tributaries to the North and Middle Forks of the Flathead River. Identical sections are counted annually and represent a known portion (about 45 percent) of the total bull trout spawning in the drainage.
The 2012 results for index streams are positive. The 2012 count of 229 redds in the eight standard stream sections is somewhat higher (~20%) than the average of 193 redds (range 130 – 251) over the last 14 years and the highest since the 2001 count of 230. This current level is about 60 percent of the 1980’s average, yet twice as high as the average in the 1990s. This rebound is encouraging and indicates the current bull trout population is stable.
According to Fisheries Biologist Tom Weaver, estimates of total spawning using the annual index counts can be made with a high level of confidence. Comparison of the 2012 index count of 229 redds with the basin-wide total of 500 redds calculates out to 45.8%, showing no change in the correlation for this relationship with a sample size of 11 years. Weaver says this indicates that annual index counts adequately capture basin-wide trends even though there is some shifting between individual streams.
