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

Age-O scales
Of205 age-O fish sampled for scales in fall of 1991, only 9 (4%) were excluded from
analysis, because scales were unreadable. Of the remaining 196 age-O Colorado pikeminnow
used in this analysis, 57 (29%) did not have scales in skin scrapes, and 139 (71 %) had readable
scales with circuli. We assumed that fish without scales in skin scrapes were too small to form
scales. Of the 57 fish for which no scales were present, total length ranged from 22 to 46 mm
with a mean of 32 mm, sd = 4.79. Total lengths of 139 Colorado pikeminnow with scales in fall
ranged from 29 to 65 mm with a mean of 42 mm, sd = 5.64 (Figure 2). Cl ofage-O fish with
scales ranged from 1 to 8 with a mean of 4.06, sd = 1.41 (Figure 3).

Formation of lst-yr growth check
To determine if Colorado pikeminnow formed a first year growth check, we followed an
analysis similar to that described in Lentsch and Griffith (1987). The relationship between
circuli formation and total length is shown in Figure 4 and described as follows: The total
number of circuli to the scale edge (C) of 154 Colorado pikeminnow from age-O up to 250 nun
TL (several larger fish were included to substantiate the upper end of this relationship) was
regressed against total length (TL):

(Equation 1)

C = 0.188(TL) - 3.753

where 0.188 is the slope of the regression line and -3.753is the y-intercept. The maximum
number of first year circuli (Cmax) was calculated from this equation using the greatest body
length achieved by age-O fish in the first growing season. We designated 70 nun as the
maximum length of age-O Colorado pikeminnow entering their first winter. We used this length,
because researchers consider it a maximum length for age-O fish going into their first winter and
because it is consistent with, if not greater than, maximum lengths reported by other studies
(Vanicek and Kramer 1969; Seethaler 1978; Valdez and Cowdell1996; R. Muth, personal
communication). This length was also the mean maximum for the eight year classes examined in
this study. Using Equation 1, we calculated the Cmax for age-O fish at 70 nun TL to be 9 (95%
CI = :f:0.2l).
Scales of133 Colorado pikeminnow (adult group) ranging in size from 210 to 774 nun
TL were used to distinguish between growth checks formed in the first winter and the first
growth check formed in following winters. To do this, we compared adult mean Cl to adult
mean C2 with the assumption that growth rates in the second year of life approximate those of
the first year. The mean C2 of this group was 11.0, sd 4.15 (Figure 5), which suggests that in a
full growing season (from April to October), Colorado pikeminnow formed approximately 11 C.
The growing season for age-O Colorado pikeminnow does not usually begin until July; hence, we
expected age-O Colorado pikeminnow to have formed less circuli in their first growing season
than in proceeding growing seasons. By adding 4 C (the average number of circuli formed by
age-O fish with scales by October) to 11 C (the average number of circuli formed in a full
growing season), we estimated the expected mean number ofC1 for adults to be near 15 if their
first growth check had not formed in the first year. However, mean number ofC1 for adults was

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