7019

• UCREFRP Catalog Number: 7019
• Author: Hawkins, J. A.
• Title: Age and Growth Of Colorado Squawfish From the Upper Colorado River Basin, 1978-1990.
• USFW Year: 1991.
• USFW - Doc Type: Fort Collins, CO.
• Copyright Material: NO

23

Age and growth

Scales were removed from 272 tagged fish collected between 1979 and 1989; 64,additional
scale samples were collected from smaller untagged fish collected from the Green River in 1990.
Regenerated scales comprised about 25% of each scale sample. A minimum of six scales were
usually needed to obtain at least one useable scale. False che.cks were distinguished from tr!le
annuli because they did not form a complete ring around the scale. Scales often showed erosion
along the posterior margin but seldom were eroded along the anterior or lateral margins. A few
were eroded along one lateral margin but usually not both lateral margins. When several scales
were available from one fish, the scale whose shape was consistent with previous scale shapes was
chosen for ageing. The first three to four annuli were often difficult to distinguish compared to
older annuli that were more discrete and obvious. The first annulus was identified as the
boundary between slow (close) and fast (spaced) growing circuli. Other annuli were distinguished
by a discrete dark band composed of closely spaced circuli and usually by patterns of cross-over.

A total of 336 scales were used to plot fish-length and scale-radius relationship
(Figure 12). It was described best by the linear equation:

L = 185.6358 SR + 45.1507 (r?:: 91%),

where L is fish total length (mm) and SR is scale radius (mm). Of the 336 fish used in the
body-scale relationship, ten were removed from further aging analysis due to indistinct annuli.
The remaining 326 fish were aged and used in back-calculating length at each annulus.

.
Apparently the first annulus was not observed and measured dur.ing ageing. Seethaler
(1978) and Musker (1981) also failed to detect and measure the first annulus. Although Vanicek
(1969) reported measurements for the first annulus, it may not form on most Colorado sQuawfish.
Lack of first-year annuli is apparently common in salmonids within the Intermountain West
(Lentsch and Griffith 1987). Length measured at first annulus should be similar to lengths of
age-O fish collected at the end of their first growing season or at the beginning of their second
growing season. In the Green River between 1964 and 1966, age-O fish averaged less than 25 mm
TL in September (Vanicek and Kramer 1969). Valdez (1990) observed average lengths of 30.6,
34.0, and 38.0 mm for age-O fish from 1986, 1987, and 1988 year classes, respectively, collected
in October from Cataract Canyon. With slight growth over winter, these year classes averaged
33.2, 42.7, and 42.0 mm by late March or early April. These lengths represent expected length of
the first annulus and confirm the apparent lack of an observable annulus on most scales. Vanicek
and Kramer (1969) reported that scale formation occurred in fish between 35 and 45 mm TL;
therefore, most of these fish would not have had scales during annulus formation. Other fish
may have been sufficiently large to have scales, but the annulus would probably be undiscemible
from the scale focus. I observed annulus formation in late May and Vanicek and Kramer (1969)
identified it in early June. Time of annulus formation is significant because it identifies when
growth resumes after the winter period. Resumption of growth is caused by a combination of
increased food and temperature. Time of annulus formation probably varies with habitat used.
If a fish can exploit a warmer habitat such as a shallow backwater in early spring, it is likely to
start growing sooner than a fish using the colder main-channel.

Scale ages were validated with scales coUected from two hatchery-reared fish that were
stocked near Green River, Utah, and later recaptured. These fish were born at Dexter National
Fish Hatchery in June of 1974, stocked in April of 1980, and recaptured and killed in May of
1984. They were 415 and 460 mm TL at capture in 1984. These fish were 10 years old at death.
The two scale samples were aged without knowledge of their origin or age, and they contained
nine annuli. Adding the undetected first annulus resulted in a correct age of 10 years. These
fish were stocked close to time of annulus formation. In addition, back-calculated lengths at
stocking (233 and 322 mm TL) were very close to actual reported lengths (200-300 mm TL) for
fish stocked in 1980 (C. McAda USFWS, Grand Junction, personal communication), validating
back-calculated lengths.