8185

• UCREFRP Catalog Number: 8185
• Author: Valdez, R. A. and B. R. Cowdell.
• Title: Annual Summary Report-1992\
• USFW Year: 1993.
• Copyright Material: NO

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600

~ FALL SAMPLING. SPRING SAMPLING

500

~ 400
u:
LL
o
a:: 300
UJ
a:l
:::;;E
=> 200
z

100

o
20

90

100

30

40 50 60 70 80

TOTAL LENGTH (mm)

Figure 2. Length frequency histograms for all fall and spring age-O Colorado squawfish sampling of Reach
3, on the Green River, 1987-1992.

on the Green River was minimal, averaging only 4 mm TL. Thus, the same size modes were being sampled
(Figure 2), and the fish were still well below juvenile size, where habitat shifts are usually observed.

One interesting possibility suggested by Figure 2 is substantial mortality of age-O Colorado squawfish less than
35 mm TI... This would supports the findings of Thompson (1989) that age-O Colorado squawfish entering the
winter period at a length less than 35 mm TL tend to have low fat content and less chance of survival. If
mortality is higher in fish less than 35 mm TL, then overwinter growth reported in Table 3 may be due to loss
of smaller flSh--subsequently increasing mean TL for spring. This possibility is still under investigation.

4.1.2 Ho 2: A2e-O Colorado sQuawtish move or are transnorted downstream into and from the study rei!ion.

Hypothesis 2 addresses overwinter changes in age-O catch rates as a function of immigration or emigration of
squawfish. This hypothesis suggests that changes in age-O catch rates from fall to spring are due to movement
of fish into or out of the reach.

It is generally accepted that larval Colorado squawfish are dispersed downstream by river currents. Nesler
et a!. (1985) proposed that movement of larval fish shortly after hatching may be mostly involuntary since only
protolarvae and early meso larvae (size range 7.2 - 9.3 mm TL) were collected in drift nets, while larger
individuals were captured in seines (size range 8.1 - 29.0 mm TL). This suggests that following the larval stage,
squawfish probably remain in or near nursery backwaters, as long as these persisL Fish captured in this study
averaged 42 mm TL--substantially larger than those reported drifting. If fish were moving during the winter
period it was likely due to the abandonment of backwaters because of ice buildup, cold temperatures or varying

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