8175

• UCREFRP Catalog Number: 8175
• Author: Converse, Y. K., L. D. Lentsch and R. Valdez.
• Title: Evaluation of Size-dependent Overwinter Growth and Mortality of Age-0 Colorado Pikeminnow.
• USFW Year: 1999.
• USFW - Doc Type: Salt Lake City, Utah.
• Copyright Material: NO

assumptions of past researchers (Vanicek and Kramer 1969; Seethaler 1978; Hawkins 1992).
Second, the association between length of age-O Colorado pikeminnow in fall and survival in
spring may provide a management avenue for increasing recruitment of early life-stages of
Colorado pikeminnow.

Previous scale analyses on Colorado pikeminnow

Scale analysis is a common, non-lethal method used to study relationships between age,
growth and size offish (e.g. Summerfelt and Hall 1987; Jearld 1983) and to understand
population recruitment at critical life stages. For the Colorado pikeminnow, several studies in
the past 30 years have included scale analyses relevant to formation of first year growth check
for Colorado pikeminnow.
Vanicek (1967) and Vanicek and Kramer (1969) found that age-O fish began forming
scales at approximately 35 to 40 mm TL and that the mean size of fish at the time they formed
their first growth check was 44 nun TL. In their study, growth checks appeared on scales of
Colorado pikeminnow that were over one year old by July suggesting that somatic growth
resumed after winter in Mayor June. Vanicek (1967) did not determine what percentage of
Colorado pikeminnow formed first year annuli.
Seethaler (1978) reported that most Colorado pikeminnow did not exhibit what he
considered a first year 'mark', although his criterion for this is not stated. Seethaler did not catch
age-O Colorado pikeminnow during his study. Because Vanicek and Kramer (1969) stated that
scale formation begins at approximately 35 to 40 mm TL, and because Seethaler attributed
Vanicek (1967) with claiming that Colorado pikeminnow do not form scales in their first year, he
assumed that the first year growth check was at the scale focus or center and added one year to
ages estimated by scales.
Hawkins (1992) examined scales from 272 Colorado pikeminnow 81 to 896 mm TL.
Two fish aged at nine years were discovered by identification tags to be hatchery reared 10 year
olds that did not form a first year growth check. Because of these results and assumptions in
earlier studies about the lack of a first year growth check, Hawkins (1992) assumed that most
Colorado pikeminnow do not form a [lIst-year growth check and added one year to ages
estimated by scales.
Valdez et al. (1995) used scales of2l Colorado pikeminnow between the lengths of60 to
119 mm TL to distinguish between age-O and juvenile fish caught in 1994. They found that fish
greater than 80 mm TL had two growth checks while those less than or equal to 70 nun TL had
one. Although providing some information on size of fish when growth checks first appear and
that some fish do not have first year growth checks, overall these studies were inconclusive about
whether all age-O Colorado pikeminnow form growth checks in their first or second year. Past
assumptions that the Colorado pikeminnow does not form a [lIst-year growth check appear to
have been sustained through a misleading rationale that because most age-O Colorado
pikeminnow are not large enough in the fall to form scales, most adults do not have first year
growth check. This assumption may have lead to overestimation of age by one year in past
studies. A better criterion for scale aging might be that more than 9 circuli indicates no first year

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