Historical and present day perspectives on fish populations

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fflSTORICAL AND PRESENT DAY
PERSPECTIVES ON FISH POPULATIONS
IN THE ROCK CREEK DRAINAGE IN
- WEST CENTRAL MONTANA, 1971 -2004

By: Rodney K. Berg
Fisheries Biologist
Montana Fish, Wildlife & Parks

Federal Aid

Project Number: F- 1 1 3-Rl

F-113-R2

Project Title: Statewide Fisheries
Management

Job Title: Rock Creek Drainage
Fisheries Management
Date: May, 2004

ABSTRACT

Rock Creek has historically provided one of the most important blue ribbon trout fisheries
in Montana. Rainbow trout provided the bulk of the fishery until recent years. Chronic low
flows, elevated water temperature and whirling disease have reduced rainbow trout density by
over 80 percent since 1993, while brown trout have increased substantially in density. Westslope
cutthroat and bull trout densities have also declined significantly since peaks observed in the early
1980's. Historic estimates for a 33 year period of record, 1971 through 2004, are presented.
Summer recruitment and winter mortality of rainbow trout and the status of whirling disease in
the drainage are discussed.

Bull trout redd counts conducted fi'om 1 993 to 2003 in six critical spawning tributaries
peaked in 1998 and have generally declined since then. Reduced redd counts appear to be
correlated with reduced stream flow in the spawning tributaries. Management recommendations
for an adfluvial bull trout population in East Fork Rock Creek Reservoir, imperiled by poor
spawning success and loss of fish to an irrigation diversion, are discussed.

-1-

INTRODUCTION

A basic inventory is essential in formulating management plans for maintaining and
utilizing a fishery. Seldom is this information complete for an entire area or drainage. Streams
and lakes in the Rock Creek drainage in west central Montana support one of the most significant
and heavily utilized sport fisheries in the region and state.

Inereasing human inhabitation and eneroachment on lands adjacent to streams, and
inereasing fishing and recreational use of streams in the Rock Creek drainage provide increasing
ehallenges to effective management of the blue ribbon trout fishery in this drainage.
Environmental changes in the 1 990's, including low stream flows associated with chronic drought
and the introduetion and spread of whirling disease, have also increased the need for better
information to meet management challenges.

DESCRIPTION OF STUDY AREA

The Rock Creek drainage lies in west eentral Montana, and is a principal tributary of the
Clark Fork River. It enters the Clark Fork River about 25 irdles upstream of Missoula, Montana.

TECHNIQUES

Electrofishing

Fish were sampled in streams in the drainage using direct current electrofishing. Boom-
suspended or mobile electrofishing apparatus carried in water craft were used on larger streams.
Baekpack electrofishing systems were used on smaller streams.

Gill Nets

Fish were sampled in lakes in the drainage using standard “Montana” experimental floating
and sinking nylon or monofilament gill nets 6 x 125 ft. with graduated mesh size from 3/4 to 2
inches square measure. Overnight stationary sets with these nets generally produced good
catches of a wide variety of fish species. The nets were equally distributed around the entire
perimeter of each lake to produce a representative catch. The locations of each gill net set were
marked on USGS topographic maps. In addition, GPS coordinates for each gill net set were
recorded for future reference.

Fish Sample Processing and Tagging

Sampled fish were measured to the nearest millimeter in total length, and weighed to the
nearest 1 0 grams. Sex and spawning condition (gravid, ripe or spawned) were recorded for fish
captured during their spawning period.

Fish population estimates were made using the log likelihood method ineorporated in
Version 4 of the Montana Fish, Wildlife and Parks program. Captured fish were marked with fin
cHps or punches and reeaptured after marked fish redistributed into the population for one to two
weeks.

-2-

Digitized by the Internet Archive
in 2017 with funding from
Montana State Library

https://archive.org/details/historicalpresen2004berg

FINDINGS, RESULTS AND DISCUSSION

Mainstem Rock Creek Fish Populations

Rock Creek has historically provided one of the most important blue ribbon trout fisheries
in Montana. Angler interest and fishing pressure has steadily increased over the past half century.

The highest angling interest and fishing pressure has always occurred on the lower 40 to
50 rmles of the mainstem of Rock Creek. This reach of stream is floatable by water craft during
high water. In addition, the famous salmon fly hatch occurs each year in this area during late
spring to early summer.

For these reasons, trout populations have been estimated by electrofishing in Rock Creek
for a longer period of record than in any other stream in west central Montana. Trout population
estimates were initiated by then Montana Fish and Game biologist, Ron Marcoux, in September,
1971, in the Fish and Game Section, 13.8 miles upstream from the mouth. The following fall,
Marcoux initiated estimates in the Hogback Section, 30.6 miles upstream from the mouth.

During this report period aU past trout population estimates in these sections were
standardized to a common estimator, the log likelihood estimator, incorporated in Version 4 of
the Montana Fish, Wildlife and Parks fish population estimate program. AU data were
keypunched and analyzed by a common observer to eliminate estimate variance due to a variety of
past estimator methods and observers.

Since the inception of electro fishing in 1971, 18 rainbow trout population estimates have
been made in the Hogback Section and 24 have been made in the Fish and Game Section (Tables
1 and 2). This includes the most recent estimates which were made in each section in Spring,
2004. Due to lower densities, brown, westslope cutthroat and buU trout have been successfully
estimated on fewer occasions.

Findings reveal rainbow trout have provided the bulk of the sport fishery in Rock Creek
until recent years. Hatchery rainbow trout were planted in Rock Creek through 1972. FoUowing
suspension of hatchery supplementation, wUd and total rainbow trout densities probably increased
significantly in Rock Creek. Even though baseline data prior to 1 972 are lacking for Rock Creek,
monitoring of other streams in Montana consistently showed significant increases in rainbow trout
densities foUowing suspension of hatchery planting.

Although trout population estimates obtained on Rock creek in the 1970's meet minimum
acceptable criteria, in terms of numbers of recaptured fish required to project a vahd estimate, the
1970's data should probably be viewed with skepticism due to primitive electro fishing gear
employed during that time period. Electro fishing currents used in the 1970's included alternating
and pulsed direct current outputs, which presently are known to cause fish injury, movement and
mortahty. If mortahty or movement of marked fish occurs between mark and recapture runs,
assumptions for a valid estimate are violated, and resulting estimates are invahd and inflated. It
appears several 1970's estimates in Rock Creek are invahd and inflated due to this factor.

-3-

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Table 1. Trout per mile estimates in the Fish and Game Section, Rock Creek, 1971 - 2004 (Catchable > 6.0", Large > 13.0" TL).

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157

187

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46

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33

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Catchable

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33

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196

248

211

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376

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80

48

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1

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59

135

383

383

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558

634

Large

299

90

52

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244

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57

82

98

233

500

230

555

344

330

186

57

Catchable

1,184

2,626

800

462

2,141

1,672

1,612

376

1,673

637

1,348

1,066

784

920

495

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1973

1978

1979

1979

1980

1980

1981

1981

1982

1982

1986

1986

1989

1993

1996

2001

2004

-5-

By the 1980's electro fishing methods began to employ continuous direct current reducing
electrofishing injuries, mortahty and movement. For this reason, trout population discussions in
this report are confined to estimates obtained after 1980.

Rainbow Trout

Most historic and all recent trout population estimates in Rock Creek have been made in
the spring. Between 1980 and 1993, catchable rainbow trout spring densities in the Fish and
Game Section of Rock Creek were remarkably consistent varying Ifom 1,314 to 1,548 fish per
mile (Figure 1). Between 1993 and 1996 and continuing through 2004, catchable rainbow trout
declined precipitously in the Fish and Game Section. Catchable rainbow declined tfom an average
density of about 1,500 fish per mile between 1980 and 1993 to 263 fish per mile in 2004, or by 82
percent.

In the Hogback Section, catchable rainbow trout spring densities varied from 1,672 to 376
fish per mile, averaging about 1,000 fish per mile, between 1980 and 1993 (Figure 2). Catchable
rainbow declined from a recent peak spring density of 1 ,066 fish per mile in 1 986 to 1 1 5 fish per
mile in 2004, or by 89 percent. Large rainbow declined from 555 fish per rmle in 1989 to 57 fish
per nfrle in 2004, or, also, by 89 percent. Explanations for these dramatic declines will be
presented later in this report.

Since 1980, spring and fall estimates for rainbow trout were obtained in four different
years, 1980, 1981, 1982 and 1986, in the Fish and Game Section. In each of these four years,
catchable rainbow trout population densities increased between spring and fall. Density increases
ranged form 4 percent in 1980 to 68 percent in 1986. In healthy trout populations which are not
over exploited by angling, catchable trout population increases should be expected between spring
and fall due to fish growth during this time period. This is often referred to as summer
recruitment.

Large rainbow trout, greater than or equal to 1 3 inches in total length, increased between
spring and fall by 158 percent in 1981 and 5 percent in 1986 but decreased by 27 percent in 1980
and 36 percent in 1982. This indicates large rainbow were over exploited or moved from the
section between spring and fall in 1980 and 1982.

Also, since 1980, spring and fall estimates for rainbow trout were obtained in four
different years, 1980, 1981, 1982 and 1986, in the Hogback Section. In 1980, catchable and large
rainbow densities decreased between spring and fall by 4 and 56 percent, respectively. In 1981,
catchable and large rainbow increased between spring and fall by 345 and 20 percent. In 1982,
catchable rainbow increased by 1 12 percent between spring and fall while large rainbow decreased
by 48 percent. In 1986, catchable and large rainbow decreased by 26 and 54 percent,
respectively, between spring and fall. These findings indicate catchable and large rainbow were
often over exploited or moved from the Hogback Section between spring and fall.

In aU trout populations, some decrease in catchable and large trout densities is expected
between fall and spring due to winter mortality. On two occasions, between fall and spring, 1980-
81 and 1981-82, estimates are available to measure winter trout mortahty on Rock Creek.

-6-

-7-

Figure 1. Catchable spring rainbow trout density estimates in the Fish and Game Section
Rock Creek, 1972 - 2004.

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Findings reveal catchable rainbow trout winter mortality of 2 percent in 1980-81 and 46 percent
in 1981-82 in the Fish and Game Section. In the Hogback Section catchable rainbow trout winter
mortality amounted to 77 percent in 1980-81 and 62 percent in 1981-82. Large rainbow densities
decreased between fall and spring by 6 percent in 1980-81 but increased by 3 percent in 1981-82
in the Fish and Game Section, indicating winter mortality was more than offset by movement of
spring spawning rainbows into the section. Large rainbow trout densities increased between fall
and spring by 44 percent in 1980-81 and 138 percent in 1981-82 in the Hogback Section
indicating a more significant movement of spawning fish into this section.

Implications of these observations with regard to validity of spring estimates for rainbow
in Rock Creek are significant. Since 1989, or for the past 15 years, only spring estimates have
been made on Rock Creek due to low water navigability difficulties and increased angler conflicts
(i.e.: increased fishing pressure) in the fall. Since fall estimates seem to be precluded for these
reasons and since rainbow trout, a spring spawner, have decreased significantly in density while
brown trout, a fall spawner, have increased, spring estimates should be continued. However,
marking and recapture electrofishing runs should be standardized to the same time each spring to
minimize estimate error and variability attributable to rainbow trout spring movement.

Brown Trout

In contrast to declining densities of rambow trout, brown trout have increased m density m
the Hogback and Fish and Game Sections. (Figure 3). Successful estimates of large brown trout
have been made smce 1989 in both the Hogback and Fish and Game Sections.

In the Hogback Section, large brown trout, greater than or equal to 1 3 inches in total
length, increased from 46 fish per mile in 1989 to 383 fish per mile in 2004, or by 733 percent. In
the Fish and Game Section, large brown trout increased from 76 to 342 fish per mile, or by 350
percent, between 1989 and 2004. By 2001 and 2004, large brown trout outnumbered large
rambow trout in the Hogback and Fish and Game Sections, respectively. Recent catchable brown
trout estimates could not be made until 2001 in the Hogback Section and 2004 in the Fish and
Game Section. In 2004, 634 catchable brown trout were observed in the Hogback Section
compared to 655 in the Fish and Game Section.

Total large trout density has essentially remained stable in the Hogback and Fish and
Game Sections due to replacement of large rainbow trout by large brown trout (Figure 3).
However, total catchable trout densities have declined from an average of about 1,000 fish per
mile between 1980 and 1993 to 634 fish per mile in 2004, or by 37 percent, in the Hogback
Section (Table 2). In the Fish and Game Section, total catchable trout densities declined from an
average of about 1,500 fish per mile between 1980 and 1993 to 918 fish per mile in 2004, or by
39 percent (Table 1).

Replacements of large rainbow trout by large brown trout indicates adult brown trout
have pioneered vacant space left due to declining densities of large rainbow trout in both sections.
Catchable brown trout, estimable in 2001 and 2004 in the Hogback Section and in 2004 in the
Fish and Game Section, will probably continue to increase in density. Earlier pioneering in the
Hogback Section, 30.6 miles upstream from the mouth, than in the Fish and Game Section, 13.8

-9-

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HOGBACK SECTION

FISH AND GAME SECTION

Figu re 3. Large rainbow and brown trout spring density estimates in the Hogback and
Fish and Game Sections, Rock Creek, 1977 - 2004.

-10-

miles upstream from the mouth , indicates pioneering of brown trout is probably occurring due to
downstream rather than upstream movement of fish.

Westslope Cutthroat Trout

Westslope cutthroat trout are a species of special concern in Montana due to declining
distribution and numbers in some parts of their range. In Rock Creek catchable westslope trout
were not found in estimable numbers untU after restrictive fishing regulations were implemented in
May, 1979. Following implementation of restrictive regulations, which, in part, reduced the daily
limit of trout from ten fish to three fish, westslope cutthroat trout estimates were obtained for the
first and only time in fall, 1981, in the Fish and Game Section at a density of 1 1 catchable fish per
mile (Table 1). In the Hogback Section, westslope cutthroat estimates have been obtained
sporadically, in seven different years between 1979 and 2004 (Table 2). Catchable cutthroat
densities in this section have ranged from 33 fish per mile in fall, 1979, to 376 fish per mile in fall,
1982. Explanations for the increase in cutthroat densities after 1979 and their demise after 1982
win be presented later in this report.

Bull Trout

Bull trout were generally found in estimable numbers from 1980 to 1986 in the Fish and
Game Section and from 1980 to 1993 in the Hogback Section (Tables 1 and 2). Estimates of bull
trout densities obtained prior to 1980, may not be valid due to primitive electro fishing gear which
probably caused fish movement and mortality, violating assumptions of a vahd estimate.
Explanations for the decline in bull trout densities in Rock Creek will be presented later in this
report. The highest densities of catchable bull trout since 1 980 were 317 fish per mile in spring,
1982, in the Fish and Game Section and 187 fish per mile in spring, 1981, in the Hogback Section.

Stream Flow

Stream flow has been monitored in Rock Creek near its confluence with the Clark Fork
River for a 31 year period of record from October 1, 1972, to September 30, 2003, by the U.S.
Geological Survey (USGS). USGS data indicate chronic low mean dally stream flows of less than
90 cubic ft. per sec. (cfs) have been observed in 10 of the past 17 years between 1987 and 2003
(Figure 4). Prior to 1987 mean daily stream flows of less than 90 cfs were observed in only two
of 14 years.

Water Temperature

Water temperature in Rock Creek near the mouth was monitored during a 3 year period.
May - August, 1972-1974, by U. S. Forest Service (USFS). USGS monitored water temperature
during a 3 year period. May - August, 1980-1982, and during a 7 year period. May - August,

1 995 - 200 1 . Although not conclusive due to some data uncertainties, this monitoring suggests a
warming trend in Rock Creek over the last three decades (Shane Hendrickson, USFS, 2003,
personal communication). The means of daily maximum water temperatures from May through

-11-

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DRILY HERN STRERHFLOH, IN CUBIC FT PER SEC

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1375 1980 1985 1990 1995 2000

DRIES: 10/01/1972 to 09/30/2003

EXPLRHRTIOH

DRILY HERN STRERHFLOH X HERSURED STRERHFLOH ESTIHRTED STRERHFLOH

Figu re 4. Daily mean stream flow measured at the USGS gage station, lower Rock Creek,
10/01/1972 - 9/30/2003.

-12-

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August have increased from about 15.5 degrees C (60 degrees F) in 1972 to about 20 degrees C
(68 degrees F) in 2001 (Figure 5). USFS regression analysis indicates this increase is statistically
highly significant.

Whirling Disease

Sentinel cage studies of rainbow trout at thirty locations in the Rock Creek Drainage,
1998-2003, indicate whirling disease was present in the drainage at the onset of testing in 1998
(Bill Granath, University of Montana, 2004, personal communication). By 2003, caged rainbow
tested positive for M. cerebralis. or whirling disease, at 24 of 30, or 80 percent, of the study sites
in the drainage (Figure 6). Only 6 of 30 sites tested negative (0% infection rates) for M
cerebralis. including three test sites on the Upper Middle Fork of Rock Creek and one test site
each on lower Spring, Ranch and Hogback Creeks.

In addition to being pervasive throughout the drainage, average grades of infection
(AGFs) are disturbingly high, averaging 2.5 or higher in 82 percent of the sites tested in 2002, the
most recent year for which fuU data analysis is available (Table 3). AGFs are based on the
original MacConnell-Baldwin Scale of 0-4. At an AGI of 2.5 or higher significant adverse
impacts on wild salmonid populations are likely to occur (Vincent, 2000).

Table 3. Average grades of infection of sentinel caged rainbow trout testing positive for
whirling disease in 2002.

Average Grade
of Infection

No. of Sites

Percent of Sites

Predicted Salmonid
Population Impact

0-1

1

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None

1 -2

0

0

Minor

2-2.5

1

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Minor

2.5 - 3

2

18%

Declines may occur

3-4

7

64%

Declines will occur

-13-

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I Sentinel Cage Loeations in the Rock Creek Drainage 1998-2003

Upper Middle Fork
Lower Middle Fork
Upper East Fork (at bridge)

Lower East Fork (Upper Forks Bridge

Ross Fork

West Fork

Forks Bridge

Watson’s Bridge

Bohrnsen’s Bridge

Gillie’s Bridge

Upper Willow Creek

Stony Creek

Windlass Bridge

Upper Puyear Ranch

Middle Puyear Ranch

Lower Puyear Ranch

Hogback Creek

Camp Siria (Hogback section)

Fish & Game
Valley of the Moon
Above Hogback Creek
Ranch Creek

Middle East Fork (Beaver Dam)
Clark’s Property

Middle Fork (Below Placer Creek)

Middle Fork USFS Bridge 5121

Butte Cabin Creek

Welcome Creek

Gilbert Creek

Spring Creek

Figure 6. Rainbow trout sentinel cage study locations in the Rock Creek drainage
1998 - 2003, depicting the locations testing positive for whirling disease.

-15-

CONCLUSIONS

The spread of whirling disease has undoubtedly been enhanced by chronic drought and
low stream flows in Rock Creek during the 17 year period from 1987 through 2003, and by the
significant warming of water temperatures in Rock Creek observed between 1972 and 2001.
Dramatic declines in catchable and large rainbow trout in the Fish and Game and Hogback
Sections are attributable to this combination of low stream flows, higher water temperature and
the spread of whirling disease.

Chronic drought and higher water temperatures have probably also been factors in the
declines of westslope cutthroat trout and bull trout in Rock Creek. It appears the recovery of
westslope cutthroat trout after restrictive regulations were implemented in May, 1979, was short
lived. Westslope cutthroat trout peaked in density in fall, 1981, in the Fish and Game Section and
in fall, 1 982, in the Hogback Section and have generally declined or been unestimable since that
time. Increased angling pressure since 1982 and the onset of whirling disease in the 1990's may
also have affected westslope cutthroat.

Bull trout have not been estimable in the Fish and Game Section since 1986 and in the
Hogback Section since 1993. Low flow and elevated water temperature have undoubtedly
impaired bull trout. Bull trout are generally recognized as being more vulnerable to drought and
increased water temperature than other trout species. Additional perspectives on the status of
bull trout in the Rock Creek drainage are available through redd counts obtained in spawning
areas. These findings are presented in the following section.

Bull Trout Red Counts

USFS biologists, Steve Gerdes and Shane Hendrickson, have spent considerable time and
effort m locating and counting bull trout redds in the Rock Creek drainage. These efforts have
helped to identify critical bull trout spawning areas in drainage. Since the counts have been made
for an 1 1 year period from 1993 through 2003, they are also useful in helping to determine trends
in bull trout status and abundance in the drainage.

In the Deer Lodge Forest segment of the drainage, redd surveys to date have identified six
important bull trout spawning tributaries. These tributaries are Stony, Copper, Little Stony, Ross
Fork, Carpp and Middle Fork Rock Creeks. Although redd counts have been variable from year
to year, redd counts in Stony, Carpp, Copper and Middle Fork Rock Creeks have usually been
relatively higher than redd counts in Little Stony and Ross Fork Creeks (Figure 7 and Appendix
1). Carpp and Copper Creeks are tributaries of Middle Fork Rock Creek while Little Stony
Creek is a tributary of Stony Creek. This emphasizes the importance of the Stony Creek and
Middle Fork Rock Creek drainages for buU trout spawning. Redd counts in Stony, Copper, Ross
Fork, Carpp and Middle Fork Roek Creeks peaked in 1998 and have generally declined since
then. Stream flow measurements in Middle Fork Rock Creek have declined by about a third since
1998, suggesting reduced redd counts may be correlated with reduced stream flow in the

-16-

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Rock Creek, 1993 - 2003.

year 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003

redds 25 30 36 48 60 112 69 72 63 55 38

flow 68 38 65 53 58 58 46 40 38 41 38

temp cop 8.8 11.1 10.5 11.1 6.7 10.5 9.2

tempmfk 6.6 7.8 8.4 10.8 10.3 6 9.2 7.2

-18-

Figure 8. The correlations of stream flow and water temperature with redd counts i
Middle Fork Rock Creek and two of its tributaries, 1993 - 2003.

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spawning tributaries (Figure 8).

East Fork Rock Creek Reservoir Fishery survey

East Fork Rock Creek Reservoir is the largest body of standing water in the Rock Creek
drainage. An important adfluvial population of bull trout is found in the reservoir along with a
few brook and rainbow trout.

Adfluvial bull trout survival is jeopardized in the reservoir due, in part, to aggradation of
the stream bed upstream of the reservoir which historically has been used by bull trout for
spawning. In addition. East Fork Reservoir is drawn down significantly each year during the
irrigation season resulting in the movement of some fish through the dam outlet and into East
Fork Rock Creek downstream of the reservoir. Since most of the water in the East Fork enters
an irrigation diversion during the irrigation season, many bull trout are lost to the diversion.
Ultimately, much of the water in the irrigation diversion is siphoned into an irrigation canal
transferring water to the Flint Creek drainage. This results in the transfer of at least some bull
trout from the Rock Creek drainage to the Fhnt Creek drainage where their fate is uncertain at
best.

A fishery survey of East Fork Rock Creek Reservoir was made in 2003 to evaluate present
day status of buU, brook and rainbow trout. Five overnight gill net sets were made each night
from June 27 to July 1, 2003. The 25 total overnight giU net catch produced a total of 48 bull, 28
rainbow and 7 brook trout. No other game or nongame fish were captured.

Six of 15 mature female bull trout examined, or 40 percent, were reabsorbing eggs from
the previous year, while forming new eggs in the abdominal cavity adjacent to the previous year’s
egg production. This finding confirms a high percentage of adfluvial buU trout in East Fork
Reservoir are unable to successfully spawn. Poor spawning success and the loss of fish to the
diversion ditch during the irrigation period have imperiled this bull trout population.

USFS intends to employ low tech habitat improvements in the East Fork of Rock Creek
upstream of the reservoir in an attempt to restore some surface water for bull trout spawning
(Steve Gerdes, USFS, 2004, personal communication). Since the bedload deposition is so
extensive, its uncertain whether these improvements will be successful without physical removal
or bypass of some of the bedload material.

The siphon in the irrigation canal downstream of East Fork Reservoir is in need of
replacement. Fish screens at the water outlet works in the reservoir and screens at the diversion
site downstream of the reservoir will be evaluated and probably employed as part of the siphon
replacement funding to mitigate and minimize bull trout loss to irrigation diversion.

East Fork Rock Creek Reservoir was planted annually with hatchery rainbow trout for 47
years from 1938 through 1984. For the first 24 years from 1938 through 1961 the plant consisted
of fingerling rainbow, one to six inches in length. For 23 years from 1962 through 1984 catchable
rainbow, 7 to 10 inches in length were planted. The plant was discontinued after 1984 due to the

-19-

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assumption that too many fish were flushed from the reservoir during irrigation drawdown.
However, this assumption was never validated by objective field observation.

Since rainbow trout still persist in the reservoir 20 years after planting ceased, it is clear
some self sustaining natural reproduction of rainbow occurs. However, since rainbow and brook
trout are scarce compared to bull trout, it is recommended that hatchery supplementation should
be resumed to provide enhancement of angler catch and to reduce angling pressure on bull trout.
Fingerling or catchable rainbow or cutthroat should be planted to evaluate escapement through
the dam outlet works and to determine their relative value in improving the fishery. Once it is
determined which species and size of fish are relatively less vulnerable to flushing, objective
judgements can be made with regard to whether hatchery supplementation should be continued as
a management tool.

Electro fishing studies should be conducted in East Fork Rock Creek upstream of the
reservoir to evaluate the status of bull trout, and to determine whether rainbow, cutthroat or
brook trout are present upstream of the reservoir. East Fork Rock Creek and East Fork
Reservoir provide considerable opportunity for habitat and trout population improvement.

-20-

51

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BIBLIOGRAPHY

Vincent, E. R. 2000. Whirling disease report 1997-98. Montana FWP. 33 pp.

-21-

APPENDIX 1

Bull Trout Redd Counts in Spawning
Tributaries of Rock Creek
1993-2003

-22-

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