Georgetown Lake management survey

MONTANA DEPARTMENT OF FISH AND GAME
FISHERIES DIVISION

JOB PROGRESS REPORT

State Montana Titl e Investigation of More

Project No. F-12-R-2A Important Waters

Job No . Il-b Titl e Georgetown Lake Management

Period Covered; July 1> 1977 - Survey

June 30, 1978

ABSTRACT

Summer fishermen averaged 0.6 fish per hour, and ice fishermen, 2.5 fish
per hour during the 1977-78 fishing season on Georgetown Lake. Rainbows
comprised 71.6% of the summer catch while kokanee made up 58.1% of the
winter catch.

Rainbows averaged 11.1 inches and 10.0 inches in the summer and winter
respectively while kokanee averaged 11.6 inches and 9.3 inches in summer and
winter. Younger fish generally predominate in the winter catch. Spawning
kokanee averaged 12.0 inches while spawning brook trout averaged 10.5
inches in length. The kokanee spawners are comparable to spawners in past
years but the brook trout spawners declined approximately 1.5 inches.

Age 0+ and 1+ rainbows predominated in the catch. The majority of
the rainbows are captured by anglers before they reach age II. Flourescent
pigments were used to mark 46.8% of the rainbows planted in May, 1977, but
only 11.3% of the recaptured fish were marked. All the fish showed an
apparent scale planting check, indicating that they were all hatchery trout
and that mark loss was substantial. Natural reproduction appears to be
nominal.

Winter dissolved oxygen levels suffered only moderate declines even
though maximum snow depths reached 20.0 inches and ice depths reached 26.5
inches.

BACKGROUND

Georgetown Lake is a shallow, 2,768 acre lake at an elevation of 6,400
feet, approximately 18 miles west of Anaconda. Georgetown is rated as one
of the most heavily fished lakes in the state. Sub-catchable rainbows are
planted each spring, while the kokanee and brook trout are self-sustaining.
Rainbows are the dominant gamefish but kokanee have steadily increased in the
catch in recent years.

FINDINGS

1977 Summer Creel Census

Thirteen days of creel census were conducted during the 1977 summer
fishing season. The 255 shore fishermen interviewed fished 802 hours to creel
353 fish for an average of 0.4 fish per hour (Table 1). Boat fishermen (66)
fished 201 hours to catch 284 fish at a rate of 1.4 fish per hour. Collectively,
the 321 anglers averaged 0.6 fish per hour (Table 1). This rate is at the lower
end of the range of catch rates (0.6-1.4) for the last decade. Since not all of
the censuses have been conducted on a random basis the statistics should be
viewed only on a trend basis. A majority of the anglers interviewed (65%)
resided within 40 miles of the lake while another 18% were state residents
from beyond 40 miles and 17% were non-residents.

Of the game fish checked, rainbows (456) comprised 71.6% of the catch,
while kokanee (175) and brook trout (53) contributed 27.5% and 0.9%, respectively.
Rainbows averaged 11.1 inches in total length and kokanee averaged 11.6 inches
(Table 2). While slightly larger, these lengths are not statistically different
from the mean lengths of fish checked in the 1970 and 1974 summer creel censuses.

Table 1. Angler effort and success during the summer, 1977 and winter, 1977-78
fishing seasons on Georgetown Lake

Season

Angler
type

No.
Interviewed

Fish/hour

Fish/day^''

1/
Trip length""

Sununer

shore

255

0.4

2.0 (22)

3.7 (22)

boat

66

1.4

5.4 (34)

3.1 (34)

all

321

0.6

4.1 (56)

3.3 (56)

Winter

all

237

2.5

14.1 (42)

5.5 (42)

ll Sample size in parentheses indicates anglers completing trip

Table 2.

Composition of summer, 1977 and
Georgetown Lake (sample size in

winter, 1977-78 angl
I parentheses)

ing catcl

les m

Season

Species

Mean
length

Mean
weight

Percent of
catch

Slimmer

Rainbow

11.1

(251)

0.57

(202)

71.6

(456)

Kokanee

11.6

(23)

0.49

(5)

27.5

(175)

Brook trout

11.4

(4)

0.72

(3)

0.9

(6)

Winter

Rainbow

10.0

(171)

0.42

(29)

38.7

(851)

Kokanee

9.3

(194)

0.27

(37)

58.1

(1276)

Brook trout

10.5

(12)

-

3.2

(71)

- -^ -

FUWT CPEEK PHILLfPS3lJr7G- 16 Ml.

^DTE:

^MAX. DEPTH IN CHANNEL IS 38 FT

Lie ACCESS

HIGHWAY 10 A.

N.FFLfvrrCRt'£K

/GEORGETOWN LAKE

STUART MILL CRFEK

T5N-RI3J4W

GRANTE - DEER LODGE COUNTIES

SURFACE ACRES 27S8

CONTOUR INTERVAL- 5FT
SCALE
EACT FORK RES - 6^1 „ ^

FEET

^rD^JTANA fish ANr:> game DtPT-ige^
^ Fip:ure 1, Dissolved oxygen sampling sites on Georgetovm Lake, winter 1977-78

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Age and Growth

Research was initiated in December, 1976, to determine age and growth
relationships for rainbow trout and kokanee in Georgetown Lake.

Scales and lengths were taken from rainbows captured by angling and
gillnetting. Although sample sizes are small, the two capture methods appear
to produce similar results for fish over 8 inches (Table 4). Comparisons of
angling and gillnet catches will be expanded to determine if the creel census
will supply unbiased estimates of growth.

Table 4. Comparison of mean lengths of rainbows captured by angling and

gillnets in Georgetown Lake, summer, 1977. (Sample size in parentheses)

Time period

Age

Mean total length

Gillnet

Creel census

June 1-15

July 1-15

July 16- August 31

1+

11+

111+

0+

1+

11+

111+

0+

1+

11+

111+

10.5 (14)
12.7 (3)
14.9 (1)

8.2 (1)
11.4 (16)
13.7 (4)

14.6 (1)

8.3 (14)
11.6 (15)
13.1 (4)

11.0 (25)
13.6 (12)

11.3 (17)

14.1 (4)

13.9 (1)

8.0 (4)

11.0 (29)
13.6 (6)

15.1 (4)

Anglers appear to select for fish over 8 inches. Trout planted at 5.1
inches on May 23, 1977, reached 8 inches by mid- July (Figure 2) and were
recruited into the fishery. Age 1+ fish predominate in the catch in early
summer (Figure 3) while 0+ rainbows increased in the proportion of the catch
in late summer and predominated in the winter season. Very few of the aged
fish were age III or older.

Aging of rainbow scales was confused by an apparent anomaly in the scale
growth pattern. Examination of scales from marked fish confirmed that a
false annulus was present. Back-calculation indicated that normal growth
resumed at 6.6 inches, indicating a growth check occurs shortly after planting.

Fish marked with flourescent pigments were examined prior to planting
and showed that 93.7% or 140,404 of the processed fish retained their mark.
This represents 46.8% of the total plant of 300,171. Initial recapture
samples showed a much lower proportion of marked fish, 11.3%, indicating
that either natural reproduction or mark loss is substantial. All scales
examined showed an apparent planting check indicating that they were from

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TOTAL
(inches)

12 .,

10 .

•»*

♦ -'

«o

^o — * **<* t o— o»#

•♦(spawned)

-T"
J

A

-r-

■♦■

SON

dath;

YEAR CLASS

197'+
1975
1976

D

Figure ^, Average length of kokanee caught in Georgetown La.ke, June, 1977 to
Febitiary,1978,

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planted trout and therefore that mark loss was the major cause of the low
proportion of marked fish. Presence/absence of the scale planting check
may be a more reliable estimator of wild trout /hatchery trout ratio.

Age 1+ kokanee averaged 6.7 inches in June, 1977 while 11+ and III4- fish
averaged 9.1 and 11.1 inches respectively (Figure 4). Age 14- and 11+
salmon grew 1.1 and 0.7 inches respectively by January, 1978, while III+
fish spawned and died at 11.9 inches in November and December, 1977. Salmon
are recruited into the fishery at the end of the second year (age II) at
approximately 8 inches.

RECOMMENDATIONS

An intensive creel census should be conducted during the winter and
summer 1978-79 fishing seasons to determine changes in angler effort and success.
Collection of age and growth data on game fish during the census will provide
additional information on key characteristics of the fishery. A survey should
be initiated to determine angler preference in management of the fishery.

The angler harvest of kokanee should continue to be monitored to
determine the effects of liberalized daily limits (35 salmon per day).

The brook trout spawning run should continue to be monitored to determine
the effect of heavy angling pressure at the mouth of Flint Creek.

Dissolved oxygen should be monitored during periods of ice-cover
to delineate oxygen depletions.

Prepared by: James E. Vashro
Date: October, 1978

Water referred to: 2-06-7961-5 Georgetown Lake

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