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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4.0 DISCUSSION

This study was designed to test three hypothesis associated with apparent overwinter decreases in age-O
Colorado squawfish. Theses hypotheses are:

Ho 1: Age-O Colorado squawfish use habitats unavailable to conventional seining methods.

Ho 2: Age-O Colorado squawfish move or are transported downstream into and from the study
region.

Ho 3: Age-O Colorado squawfish are lost to overwinter mortality from starvation, diseases,
parasites, etc.

In addition to addressing these hypotheses the data collected for this study were consistent with ISMP criteria
and can be used to assess cohort strength and long term trends in age-O squawfish numbers.

4.1 HYPOTHESIS

The hypotheses addressed by this study were developed to account for apparent decreases in age-O Colorado
squawfish catch rates observed over the 6 month winter period--September to March. Except for the 1990 year
class (7%), overwinter decreases in catch rates were similar; 1987 (77%), 1988 (62%), 1989 (56%), and 1991
(62%), even though catch rates from year to year varied dramatically (48.40/100 m2 in fall 1988 compared to
2.23/100 m2 in fall 1991). Consistent overwinter decreases in age-O squawfish suggest that the factors
influencing this decrease are density-independent.

4.1.1 Ho 1: A2e-O Colorado sQuawtish use habitats unavailable to conventional seinini! methods.

Hypothesis 1 addresses the idea that overwinter decreases in age-O squawfish are more a function of sampling
bias than actual decreases in numbers. This hypothesis implies a shift in habitat shift use from fall to spring.

Habitat preferences of age-O Colorado squawflSh are well documented. Holden and Stalnaker (1976) found
age-O squawfish in shallow backwaters and juveniles in backwaters and eddies 2 - 3 ft deep. Kidd (1976)
indicated that squawfish used backwaters almost exclusively in the Colorado River near Grand Junction (times
of capture were not noted). Recent research in Reach 3 (Valdez 1990) found that age-O squawfish were most
often found in backwaters and isolated pools, whereas juveniles moved between shorelines and backwaters.
These studies all show that age-O squawfish prefer backwater habitats, shifting to mainchannel habitats as
juveniles--or about 80 mm TL.

Age-O Colorado squawfish have been observed using habitats other than backwaters. Valdez et a!. (1987)
found age-O squawfish from the Colorado, Green and Yampa rivers in eddies, runs and along shorelines,
however 85% were captured in backwaters. Valdez (1990) sampled 95% of age-O squawfish in backwaters and
isolated pools. Tyus and Haines (1991) reported similar results with the majority (84%) of post-larval
Colorado squawfish found in backwaters. ISMP sampling criteria was designed to sample habitats most
commonly used by age-O squawfish. While these habitats are not used exclusively, research indicates the
majority of age-O Colorado squawflSh use these areas. Similarly there is no evidence to suggest that backwaters
are used more in fall than in spring.

It is unlikely that the overwinter decreases observed in this study were caused by a habitat shift in age-O
squawfish from fall to spring. The change from one habitat type to another is typically related to a subsequent
change in size--as shown in the studies above. Overwinter growth of age-O squawfish in the 5 years of sampling