8084

• UCREFRP Catalog Number: 8084
• Author: Gerhardt, D. R. and W. A. Hubert
• Title: Population Dynamics of a Lightly Exploited Channel Catfish Stock in the Powder River System, Wyoming-Montana
• USFW Year: 1991
• USFW - Doc Type: North American Journal of Fisheries Management
• Copyright Material: YES

6

DYNAMICS OF LIGHTLY EXPLOITED CHANNEL CATFISH

203

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4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
Age

FIGURE 3. -Catch curve for 599 channel catfish caught
in the Powder River system during 1987 (in = log,,).

were returned in 1987,6 in 1988, and 1 in 1989;
of 740 tagged in 1988, 10 were returned in 1988,
and 5 in 1989. Using fish for which tags were
returned within 1 year of their initial capture, we
estimated annual fishing mortality to be 1.9% for
fish marked in 1987 and 2.0% for fish marked in
1988, but this is considered a conservative esti-
mate because we did not adjust for tag loss or
nonreturn of tags by anglers.
Yield estimates based on the Ricker and
Thompson-Bell models (Figure 4) were similar.
When instantaneous natural mortality was held at
0.25, maximum yield was at instantaneous fishing
mortality rates of 0.20-0.25. These rates corre-
sponded to annual fishing mortalities of 18-22%.
For each level of exploitation, RSDs were com-

TABLE i.-Mean total lengths and mean weights of
channel catfish collected in the Powder River system
during 1987 (N = 599) and 1988 (N = 257).

l

Age Length (mm) Weight (kg)
(years) 1987 1988 1987 1988
3 241 251 0.04 0.20
4 265 307 0.07 0.23
5 293 308 0.13 0.27
6 339 342 0.32 0.40
7 390 420 0.57 0.79
8 440 435 0.89 0.93
9 513 540 1.41 1.75
10 580 572 2.12 2.07
11 568 616 1.97 2.80
12 595 653 2.28 3.04
13 619 640 2.47 3.22
14 619 666 2.39 3.33
15 653 648 2.93 3.20
16 643 677 2.70 3.53
17 660 690 3.08 3.13
18 638 690 2.80 3.54
19+" 635 3.08
"Age 19 years and older.

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Ricker

Thompson-Bell

0.01 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50
Instantaneous Rate of Fishing

FIGURE 4.- Yield estimates for channel catfish in the
Powder River system derived from the Ricker and
Thompson-Bell models. To obtain the actual biomass
per 1,000 kg of recruits with the Ricker model, multiply
depicted number by 1,000.

puted (Table 2). In addition to decreasing overall
abundance, a shift toward smaller size-classes was
indicated as exploitation increased. As the instan-
taneous rate of fishing mortality increased from
0.01 to 0.25, the proportions of stock-size fish in
the preferred, memorable, and trophy sizes were
estimated to decline from 50 to 22%. This would

TABLE 2. -Computed relative stock densities for chan-
nel catfish in the Powder River at various levels of ex-
ploitation.
Length category (length, cm)
Instanta- Pre- Memo-
neous rate Stock Quality ferred rab1e Trophy
of lishing (3~0) (41-50) (51-60) (61-70) (>70)
0.01 35 15 34 16 0
0.05 45 15 29 11 0
0.10 51 15 26 8 0
0.15 55 15 24 6 0
0.20 59 15 22 4 0
0.25 64 14 19 3 0
0.50 78 12 10 0 0