Dennis
Workman
Montana Department of Fish, Wildlife and Parks
Missoula, MT 59801
INTRODUCTION
"In no part of the world is
the water more limpid or pure, for whatever may be the depth of the river the
bottom is seen as if there were nothing to intercept the view." These words
were written by Father deSmet describing the upper Clark Fork River he observed
on a trip through the upper basin in 1841. Obviously, the days of such purity
are sadly gone for the Clark Fork. But by the same token, gone hopefully forever
are the days when the Clark Fork was "Western Montana's sewer to the
ocean." This was the headline of an article in the Daily Missoulian,
July 10, 1960, in which the reporter described vividly the disgusting mess,
which flowed through the area carrying a heavy load of toxic metals, trash of
all descriptions, and sewage from nearly every town and industry in the valley.
In those days fish kills were documented from the headwaters down the river as
far as Superior, and clean-water aquatic insect life was at a nadir. Tremendous
improvements in the fishery have occurred since 1960, but there is still a long
way to go to restore the full potential of the sport fishery and the amenities
it provides.
The sport fishery of the Clark Fork upstream from Milltown
Dam consists predominantly of brown trout (Salmo trutta) with small
numbers of rainbow trout (Salmo gairdneri), cutthroat trout (Salmo
alarki lewisi), bull trout (Salvelinus aonfluentus), and brook trout (Salvelinus
fontinalis). Downstream from Milltown Dam rainbow trout dominate the species
composition with smaller numbers of the other species. In our present-day
management of these species we strive to provide fishermen a good opportunity to
catch a trout 14 inches or larger and we work to improve the environment for
self-sustaining wild trout populations.
METHODS
Quantitative fish
population data have been collected on the Clark Fork using electrofishing to
sample trout populations and statistical estimators to derive population
statistics. Electrofishing is conducted using a boat equipped with stationary
negative and either mobile or stationary positive electrodes. Alternating
current produced by a gasoline-powered alternator passes through a rectifier
changing it to direct current and then into the water via the electrode system.
Fish in the electrical field generated around the boat are compelled to swim to
the positive electrode and are immobilized to the extent they can be dip-netted
and placed in a live box on the boat. Fish are weighed, measured, and marked, a
scale sample is taken for aging, and the fish are released into the river near
where they were captured. Marks placed on the fish are either a small notch cut
in the edge of a fin (which will grow back) or a numbered plastic tag inserted
behind the dorsal fin. These marks allow identification of individual fish,
which have been captured previously. By sampling the population and marking fish
in this way we can see the Chapman modification of the Peterson mark-recapture
estimator to determine total numbers of fish in a section of river (7,8).
Our fish sampling equipment has improved over the years. In
the beginning we were limited to sampling sections of river, which we could
wade. Our present-day equipment gives us the capability to sample fish in the
river regardless of its size. As equipment has evolved allowing us to sample
bigger waters we have worked our way down the river making population estimates
and generating population statistics at nine different locations as far
downstream as Superior (fig.1).
TROUT
POPULATIONS
The highest trout population
numbers estimated in the river to date have been found in the pH Shack section
immediately downstream from the Anaconda Minerals Company (AMC) settling ponds
near Warm Springs (fig. 1). Fishery biologists began studying that population in
1969 (5). In that year, no trout were found there, and only one brown trout was
captured in the Williams- Tavenner (W-T) section immediately downstream from
Deer Lodge. In the early 1970's, AMC began treating their wastewater at Butte.
and in 1975 they began liming Silver Bow Creek water as it entered the settling
pond system, to maintain pH levels high enough to keep toxic metals out of
solution (4). Brown trout responded and populations increased steadily until
1979 when the spring population estimate in the pH Shack section peaked at
approximately 1,500 brown trout/mile (table
1). A steep decline in numbers
followed the peak (fig.2). The 41-percent decline in brown trout numbers
between 1979 and 1982 was believed to be related to excessive fisherman harvest.
For this reason, the Fish and Game Commission implemented more restrictive
fishing regulations in 1982. Creel limits were changed from 10 trout any size to
a "slot limit" of five trout under 14 inches or four under 14 inches
with one over 18 inches.
The brown trout population responded favorably to this regulation change.
Between spring 1982 and spring 1984 total brown trout numbers increased 116%,
and brown trout 14 inches and larger increased 213% (fig. 2 and
table 1). In
1984 the large-trout population was at an all-time high for a 16-year period of
record. The Williams-Tavenner section, 30 river miles downstream, was also
studied during the same 16-year period. Populations in this section responded to
MIC wastewater treatment, but numbers leveled off at a much lower point than in
the pH Shack section, and the area did not attract nearly as many fishermen as
the pH Shack section (table
1). A statistically significant trend in numbers was
not present in the W-T section after 1979.
Curious about the dramatic decline in trout numbers between
the pH Shack and W-T sections, biologists conducted studies to determine if the
difference was related strictly to habitat or if water quality deteriorated
progressively with increasing distance downstream from the treatment system at
Warm Springs. The Sager Lane section lies about midway between pH Shack and
Williams- Tavenner (fig.1). Figure 2 indicates populations in 1981 and 1982 at
Sager Lane were intermediate between the pH Shack and W-T sections (6). The
theory of decreasing water quality was strengthened by these results since
habitat differences between Sager Lane and pH Shack appeared to be
insignificant.
A review of trout population estimates on the Clark Fork
suggests that clean-water tributaries are a major influence on water quality and
trout populations in the main river. The Little Blackfoot River increases the
average flow of the Clark Fork by 50% at Garrison. At Phosphate, a short
distance downstream, brown trout are generally in greater abundance than above
the Little Blackfoot at W-T (fig.2). After the clean-water influence of the Little
Blackfoot dissipates, trout populations in the Bearmouth and Bonita sections
reach the lowest number found anywhere in the river. There are no significant
sources of clean water to the Clark Fork in the 64 miles of river between the
Little Blackfoot and Rock Creek near Clinton (fig.1). Trout numbers drop to
fewer than 50 per mile in the Bearmouth and Bonita sections just upstream from
Rock Creek (2). Sampling in these sections in 1984 indicated numbers may
presently be-too low to estimate (W. Hadley, Fishery Biologist, Montana
Department of Fish, Wildlife and Parks, Deer Lodge, 1984, personal
communication).
Rock Creek nearly doubles the flow of the Clark Fork with
clean water. and trout populations in the Clark Fork downstream from Rock Creek
improve significantly. In 1980, brown trout numbers in the Turah section, down
stream from Rock Creek were approximately 350 per mile of river compared to 47
brown trout per mile in the Bonita section just upstream from Rock Creek.
Although the number of trout in the Turah section is lower than the number
expected from a large river like the Clark Fork, the beneficial clean-water
influence of Rock Creek on the trout population in the Turah section is
significant.
Seventeen miles downstream from Rock Creek the Blackfoot
enters the Clark Fork River through Milltown Reservoir. The clean-water
influence of the Blackfoot is dampened by toxic metals and accumulation of fine
sediment in the reservoir. In the fall of 1980, when population estimates were
initiated below Milltown Dam, the trout population in the Milltown section below
the dam was estimated at approximately one-half the population at Turah just
upstream from the dam (fig.1). Interestingly, species composition also changed
at Milltown from 70% brown trout above the reservoir to 84% rainbow trout below (1).
Based on these tests it was suspected the sediment and toxic
metals released downstream during annual drawdowns and occasional deep drawdowns
was suppressing trout populations in the river downstream from the dam. The
influence of fine sediment and toxic metals began to be documented in 1970 with
live-caged fish tests during a major reservoir drawdown by Montana Power Company
(1). Mortality rates of 100% of 2- to 4-inch rainbow trout and up to 80% of
7--to 9-inch trout at stations below the dam while only one trout died at the
control station indicated clearly adverse effects to trout from the release of
reservoir sediments into the river below.
In 1982 a new plan of operation was implemented by Montana
Power Company, which resulted in reducing the amount of material flushed from
the reservoir. Since then trout populations have apparently increased; however,
because of a change in the time of year the population was sampled and a change
in fishing regulations, further study is needed to evaluate the 400 trout per
mile increase in numbers observed between 1980 and 1984 below the reservoir (1).
Downstream
from Missoula the Clark Fork nearly doubles in size again when the Bitterroot
River enters (fig. 1). Our only trout population work on the river downstream
from Missoula has been at Superior. There, a 13-mile-long section was sampled in
1983 and 1984 indicating an average population over the 2 years of 515 rainbows
per mile (3). Annual population statistics will be gathered from this section
for the next several years to determine trout response to the lower Clark Fork
River environment.
DISCUSSION
Toxic metals in the upper Clark
Fork River flood plain present difficult cleanup problems. In addition, AMC's
treatment system of settling ponds and liming is not as effective during the
winter in removing toxic metals from solution as during summer. There is also
the problem of bypass flows around the settling ponds during spring runoff and
possibly during heavy summer storms. How effectively we deal with these and
other as yet unknown problems will determine the future of the Clark Fork River
water quality and sport fishery.
ACKNOWLEDGMENTS
Fish population data were collected using Federal Aid to Fish and Wild- life
Restoration money in project F-12-R. Special thanks go to the dedicated
biologists who collected and reported the data and to Rod Berg for reviewing and
editing the manuscript.
LITERATURE
CITED
1.
Marcoux, R.G. 1970. Western Montana fishery study. Inventory of waters of the
project area. Montana D-J Job Progress Report F-12-R-17, Job I-a,multilith. 8 p.
2.
Peters, D.J. 1981. Western Montana fishery investigation. Montana Job
Progress Report F-12-R-27, Job I-b, multilith. 10 p.
3.
Peters. D.J. 1985. Western Montana fishery investigation. Montana D-J Job
Progress Report F-12-R-31. Job I-b.
4.
Phillips, G. 1983. A
Clark Fork Prognosis. Montana
Outdoors, Nov./Dec.
5.
Spence, Liter. 1970. Western Montana fishery study. Progress Report F-12-R-17.
Montana D-J Job l-a. multilith. 8 p
6.
Vashro. James. 1983. Western Montana fishery investigation. D-J Progress Report
F-12-R-24 to 28. Job l-a. multilith. Montana 23 p.
7.
Vincent. E.R. 1971. River electrofishing and fish population estimates. Prog.
Fish Cult. 33(3): 163-169.
8.
Vincent, E.R. 1974. tion estimates. Addendum to river electrofishing and fish
popula- Frog. Fish Cult. 36(3): 182.