Bull trout on rise in Flathead drainages
October 03, 2010
by John Fraley
Field crews completed the 2010 bull trout redd (nest) counts between September 29 and October 29. The Flathead counts were about average for the past decade, the South Fork counts were slightly above average, while the Swan count was considerably below average.
Crews surveyed the eight index sections for the Flathead Lake population, the four reservoir index sections for the Hungry Horse/South Fork population and all ten spawning streams for a basin-wide count in the Swan Drainage. These same stream sections are counted each year and represent a known portion of the total bull trout spawning runs. Substantial precipitation during mid-late September resulted in increased stream flows, which created less than optimal conditions. It is quite possible observers may have missed some redds obscured by high flows and streambed scour.
Flathead Lake Population: This was the 31st year of index counts for the Flathead Lake bull trout population (Table 1). These fish spawn in tributaries to the North and Middle forks of the Flathead River. The 2010 index count of 190 redds in the eight standard stream sections is about equal to the average (193 redds) over the last twelve years, and three more than last year’s count of 187. The eight index reaches account for about 45% of the total Flathead Lake spawning run.
The Middle Fork index count of 136 this year is much greater than average (the highest since 1989) while counts in the North Fork index reaches were considerably below average. In addition to the index sections, we also surveyed Hallowat Creek, a tributary to Big Creek and the South Fork of Coal Creek to determine if the high stream flows shifted some of the spawning activity further upstream this year. We found 28 redds in the section of Hallowat Creek where a BPA funded habitat enhancement project provided large woody debris and spawning gravels lost after the Moose Fire in 2001. A similar result was found in South Fork Coal Creek with 19 redds observed, a portion of which were located in the BPA funded enhancement reach completed in 2007. We documented a total of 36 redds in the Big Creek Drainage, and 23 in the Coal Creek Drainage, although most of these were found upstream from the standard index reaches.
Trail Creek redd numbers declined from approximately 50 in 2007 and 2008 to 19 last year and 11 this year. We only observed 14 redds here in 2003, the progeny of which would have returned as six year old spawners this year, so low numbers were expected. However, it is possible that a research project, which involved substantial handling and PIT tagging of juveniles, may be a factor in the low redd numbers observed during the past two years. These activities have now been discontinued.
Hungry Horse/South Fork Population: This was the 18th year of index counts for Hungry Horse Reservoir index sections (Table 2). We had planned on completing a basin-wide count in the South Fork Drainage this 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 are more conducive to accurate counts. The 2010 reservoir index count of 95 redds is approximately 12% above the 17 year average of 84.5. The four reservoir index sections account for 23% of the eight stream index, or 19.5% of the South Fork Drainage basin-wide total.
Swan Lake Population: This is the 29th year of redd counts in the Swan Drainage (Table 3). Although Elk, Goat, Squeezer and Lion creeks comprise our annual index, assistance from the USFS and Plum Creek have allowed us to complete basin-wide counts annually since 1995. The basin-wide redd count has averaged 669 during this 15 year period. The 2010 count of 378 is approximately 43% below this 15 year average.
Redd numbers 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 carrying capacity, which would explain the 15% declines in 1999-201 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 graduate 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 possible 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. Additionally, the bull trout bycatch associated with the ongoing experimental lake trout suppression efforts could also affect bull trout numbers. Data being collected as part of this three-year removal will evaluate the potential effects of the bycatch. Bull trout redd numbers will continue to be monitored to determine if changes in fisheries management are warranted.
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