Fisheries
Applied Research and Technical Assistance
Kalispell Field Office
Experimental
suppression of lake trout to support bull trout recovery in western Montana
lakes
Reproducing lake trout populations,
established either by natural invasion (i.e., migration from other connected
waters) or illegal introduction (i.e., “bucket biology”), have increasingly
presented a major obstacle to bull trout recovery efforts in western Montana.
Bull trout (Salvelinus confluentus) and lake trout (Salvelinus
namaycush) are closely related species (same genus in the char complex) and
while hybridization between the two is unlikely in the wild, there are strong
competitive and predatory interactions between them (Martinez et al. 2009).
In the Flathead River drainage over two dozen natural lakes are home to native
bull trout populations that have existed since the Wisconsonian glaciation,
about 10,000 years ago. Today, bull trout persistence is threatened in fully
half of these waters, including all of the largest lakes (i.e., eight over
1,000 acres in size) and most of the medium sized lakes as well. Less than 3%
of the historic bull trout habitat (roughly 147,000 surface acres) in the
Flathead watershed is positioned upstream of barriers where bull trout
populations are considered stable, secure and isolated from invasion.
|
Photo Credit: Wade Fredenberg/USFWS
Large male bull trout removed from gillnet
and released alive back into Swan Lake |
Photo Credit: Wade Fredenberg/USFWS
Graduate Research Assistant Ben Cox holding a large male
lake
trout in pre-spawning condition, captured from Swan Lake |
Lake trout have several competitive
advantages over bull trout. These advantages include longevity - lake trout
may live upwards of 30 years while bull trout typically live less than 15;
annual spawning producing large numbers of eggs, which exhibit high rates of
survival, especially in lakes where nonnative Mysis shrimp and kokanee
were also introduced; and the ability to compete with bull trout for food and
space in the lake habitat where the lake trout spend nearly all their time.
Bull trout, on the other hand, must undergo arduous and sometimes long (up to
150 mile) spawning migrations and bull trout egg and juvenile survival is
dependent to a large extent on the quantity and quality of available spawning
and rearing habitat; some of which has been blocked off or impaired by human
activities in many of the intensively managed watersheds.
|
Photo Credit: Daily Inter Lake
Juvenile lake trout entangled in a gillnet
and being removed from Swan Lake |
Photo Credit: Wade Fredenberg/USFWS
Contract gillnetting crew removing small lake trout from
Swan Lake gillnet for transfer to the Flathead Food Bank
|
The Service has identified
competitive interaction with nonnative fish as a very high priority for bull
trout recovery and research initiatives and is actively supporting several
experimental lake trout suppression projects out of our Kalispell suboffice.
The general goal is to determine whether we can selectively remove lake trout
by targeted gillnetting at a high enough rate to keep lake trout populations
in check. Keys to success of the program are to remove a large proportion of
the subadult lake trout (so that limited numbers will reach maturity) and then
systematically target adult lake trout to cause a longer term reduction in the
ongoing rate of reproduction. All this must be accomplished while limiting
inadvertent bycatch of bull trout in the nets.
Ongoing projects where
the Service and our partners are attempting to suppress lake trout include
Swan Lake and Quartz Lake.
|
Photo Credit: Wade Fredenberg/USFWS
Lake trout captured alive in Swan Lake and about
to be prepped for surgical implant of a sonic tag
|
Photo Credit: Wade Fredenberg/USFWS
Incision being made in body cavity of large lake trout
for insertion of sonic tag to facilitate tracking. The incision
was subsequently sutured and the fish was
released back into Swan Lake and tracked to its spawning
grounds
|
In
Swan Lake, a broad coalition of Federal, State, academic and private partners
have funded and guided efforts to remove nearly 10,000 lake trout by
commercial scale gillnetting in 2008 and 2009. The resulting catch is
believed to constitute over half the lake trout population in Swan Lake (based
on a depletion estimate), which has grown dramatically since lake trout were
first detected only a decade ago. Sonic transmitters implanted in adult male
lake trout have been used to track the spawners and then deploy targeted
netting to remove both mature males and egg-bearing females from the spawning
grounds, which are concentrated in select areas of coarse lakebed rubble. To
date, bull trout bycatch has been acceptably low. Progress reports on this
effort are available.
|
Photo Credit:
C. Guy/USGS
Wade Fredenberg with subadult bull trout captured by
angling for
purposes of genetic sampling from Quartz Lake in Glacier
National Park |
Photo Credit:
C. Guy/USGS
Adult
bull trout captured by angling for purposes of
genetic
sampling from Quartz Lake in Glacier National Park |
A second ongoing project involves a
very similar study approach in Quartz Lake in Glacier National Park. Lake
trout were first detected in Quartz Lake in 2005, jeopardizing one of the last
intact native aquatic species complexes in large lakes in the Flathead (Fredenberg
et al. 2007). In this case, the challenge is made even more difficult because
the lake is accessible only by several miles of hiking trail. A partnership
between USGS, Glacier National Park, and the Service resulted in gill net
removal of over 500 lake trout in 2009, after a motorboat was helicoptered in
to facilitate the netting. Because the lake is manageably sized (869 acres),
it is hoped that early intervention will stave off the decline of bull trout
that has occurred in similar waters (Fredenberg 2002).
Funding and Collaboration:
U.S. Fish and Wildlife Service, Region 6; Montana Fish Wildlife and Parks;
Montana Department of Natural Resources and Conservation; Montana State
University; Montana Trout Unlimited; U.S. Forest Service; Confederated Salish
and Kootenai Tribes; Glacier National Park; U.S. Geological Survey.
Related Publications:
Fredenberg, W. 2002. Further evidence that lake trout
displace bull trout in mountain lakes. Intermountain Journal of Sciences 8
(3): 143-152. (pdf).
Fredenberg, W.A., M.H. Meeuwig and C.S. Guy. 2007.
Action Plan to Conserve Bull Trout in Glacier National Park, Montana. U.S.
Fish and Wildlife Service, Kalispell, Montana. (pdf).
Martinez, P.J., P.E. Bigelow, M.A. Deleray, W.A.
Fredenberg, B.S. Hansen, N.J. Horner, S.K. Lehr, R.W. Shneidervin, S.A.
Tolentino and A.E. Viola. 2009. Western Lake Trout Woes. Fisheries 34(9),
September 2009. (pdf).
Swan Valley Bull Trout Working Group. 2007 progress
report. (pdf).
Swan Valley Bull Trout Working Group. 2008 progress
report. (pdf).
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