Bull trout populations hold steady in Flathead River drainage
November 14, 2011
BULL TROUT COUNTS STEADY IN THE FLATHEAD, UP IN THE SOUTH FORK, DOWN IN THE SWAN
Montana Fish, Wildlife and Parks biologists have completed counts of bull trout nests in eight sections for the Flathead Lake population. Bull trout spawn in the fall, building nests in tributaries during September and October. These counts serve as an index of how many migratory adult bull trout successfully spawn. Crews also conducted surveys for both the Hungry Horse Reservoir/South Fork of the Flathead Population, and the Swan River Population. The same stream sections are surveyed annually and represent a known portion of the total bull trout spawning runs in each drainage.
The Flathead count is an index of spawners migrating upstream from Flathead Lake. This partial count (8 Middle and North Fork tributaries combined) was 189 redds, which is average for the past decade.
The South Fork total (basin-wide) count (610 redds), was well above average. This count reflects bull trout migrating upstream from Hungry Horse Reservoir, and includes reservoir tributaries and upper South Fork tributaries.
The Swan bull trout redd count (312 redds) was 52 percent below average. This reflects bull trout migrating upstream from Swan Lake. The decline in bull trout redds in the Swan Drainage over the past several years led biologists to suggest a change in angling regulations. Swan Lake has been managed with a one bull trout per day fishing regulation and annual harvest has averaged about 180 bull trout. A shift to catch-and-release regulations (no harvest) was approved by the Montana Fish, Wildlife & Parks Commission at their meeting on November 10.
More detailed information and analysis follows:
Field crews completed the 2011 bull trout spawning surveys between September 26 and October 28. Overall, conditions were optimal for accurate counts this year.
Flathead Lake Population: This was the 32nd consecutive year of index counts for the Flathead Lake population (Table 1). These fish spawn in tributaries to the North and Middle forks of the Flathead River. The 2011 index count of 189 redds in the eight standard stream sections, is about equal to the average (186 redds) over the last decade and one less than last year’s count of 190. These eight index reaches account for 45% of the total spawning run out of Flathead Lake.
The Middle Fork index counts of 136 last year and 124 this year are much greater than the average of 86 over the previous 12 years (highest since 1989), while counts in the North Fork index reaches (54 and 65 in 2010 and 2011) are considerably below the previous 12 year average of 107 redds. There are no clear reasons why there appears to have been a shift in the distribution
of spawning to the Middle Fork index reaches, but biologists are sensitive to the possibility of increased research work and handling of juvenile bull trout in the North Fork drainage.
This year’s spawning cohort was largely progeny of the year classes which spawned in 2003, 2004 and 2005. These were some of the lowest counts on record in Big and Coal creeks, so the low numbers this year were not entirely unexpected. In addition to the annual index sections in these drainages, crews also surveyed Hallowat Creek, a Big Creek tributary and the South Fork of Coal Creek, observing 13 and 2 redds respectively. Including these additional counts, crews documented a total of 22 redds in the Big Creek Drainage and 8 in the Coal Creek Drainage. North Coal and Mathias creeks were not surveyed, so the Coal Drainage total of 8 redds may be low.
Trail Creek redds numbers declined from approximately 50 in 2007 and 2008 to 19 in 2009, 11 last year and 8 this year. We only observed 14 redds here in 2003, the progeny of which would have returned as six year old spawners in 2010, so low numbers were expected last year. However, fish spawning this year were generally progeny from considerably stronger year classes. All spawning in the Trail Creek Drainage gets quantified annually.
Crews counted portions of the mainstem North Fork in British Columbia documenting 143 redds, which is the highest count recorded for the river in Canada. However, without a complete survey of all spawning reaches (basin-wide count), it is not possible to determine the overall significance of this count. A complete basin-wide count for the Flathead Lake population is planned for 2012.
Hungry Horse Reservoir/South Fork Population:
This was the 19th consecutive year of index counts for the four Hungry Horse Reservoir index sections (Table 2), the 10th year for the four Wilderness sections (Table 2), and the 3rd year when a basin-wide count was completed (Table 3). A basin-wide (complete) count had been planned last year; however, the heavy mid-late September precipitation would likely have resulted in poor counts. Considering the time and expense involved in backcountry surveys, we decided to postpone the basin-wide count in the South Fork Drainage until conditions were more conducive to accurate counts, which was the case this year.
The 2011 reservoir index count of 124 redds is approximately 46% above the 18 year average of 85 redds. These four reservoir sections account for 23% of the eight stream index (n=10), or 20% of the South Fork Drainage basin-wide total (n=3). The 2011 basin-wide total of 610 is the highest basin-wide count on record; however, the reservoir index counts in 2006 and 2007 were higher.
Swan Lake Population: This is the 30th year of redd counts in the Swan Drainage (Table 4). Although Elk, Goat, Squeezer and Lion creeks comprise our annual index, assistance from the USFS and Plum Creek has allowed complete basin-wide counts annually since 1995. This year, the USFS assisted in the redd counts. The basin-wide redd count has averaged 651 during this 16 year period (1995-2010). The 2011 basin-wide count of 312 is 52% below this 16 year average.
Redd numbers in the Swan Drainage increased steadily through the 80’s and 90’s, peaking in 1997 and 1998 at over 800 redds. From 1999 to 2001 numbers declined by about 15%. We observed another 15% decline during the four year period from 2002 to 2005. Redd numbers rebounded in 2006 and 2007, but have steadily declined since. Mysis shrimp became well established in the 80’s and along with kokanee and more conservative fishing regulations helped fuel the observed increase in the bull trout population. The peak years of 1997 and 1998 may actually have been above the carrying capacity for bull trout, which would explain the 15% declines in 1999-2001 and 2002-2005.
Lake trout were first detected in the Swan River drainage in 1998 and juvenile fish were first captured in gillnets in 2003, indicating that reproduction was occurring. A study initiated in 2006 revealed that lake trout up to 16 years old were present in Swan Lake, and confirmed that the population has been steadily increasing since the early 1990’s. Although the exact mechanism causing the recent decline in bull trout redd numbers is unknown, it is likely that the increasing lake trout population could be a factor. Lake trout have lead to declines in bull trout populations in other waters similar to Swan Lake across the region. Biologists began an experimental lake trout removal in 2009 using various netting techniques. The bull trout bycatch (unintended take of bull trout) associated with this ongoing lake trout removal (netting) could also affect bull trout numbers. Based on current trends in redd numbers in the Swan Drainage, it appears that changes in fisheries management are warranted. A shift to catch-and-release regulations was approved by the Montana Fish, Wildlife & Parks Commission. Data being collected as part of the three-year removal experiment will be used to evaluate the potential effects of bycatch. Biologists will continue to monitor bull trout redd numbers to determine the effectiveness of potential changes in fisheries management.