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Last modified
7/14/2009 5:02:28 PM
Creation date
5/22/2009 4:44:29 PM
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UCREFRP
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
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<br />5 <br /> <br />where a and b are constants from the weight-length relationship. This equation is similar to <br />relative condition (Kn) described by Le Cren (1951), and relative weight (Wr) that Wege and <br />Anderson (1978) calculated by pooling data from several different fish populations in .North <br />America. For valid comparison, slopes and intercepts of any population to be compared must be <br />equal to the ideal population. Cone (1989) illustrated how different slopes or intercepts would <br />adversely affect the outcome of Wr (or Kn). Kn values repr~sented the deviation of each fish <br />from the average weight at a given length. Kn provided a simpler statistic for comparison than a <br />or b from the weight-length equation (2). Kn is comparable between fish of different lengths or <br />species, and it does not change with different measurement units (Le Cren 1951). Average fish <br />of any species or length will have a Kn of 100. Properties and associated problems of condition <br />factors were discussed in detail by Le Cren (1951), Bolger and Connolly (1989) and Cone (1989). <br />Monthly averages of Kn were calculated for each river. . <br /> <br />Age and growth <br /> <br />Scales were collected from tagged fish between 1979 and 1989, and from smaIl untagged <br />fish collected from the Green River in 1990. Scales were removed between the lateral line and <br />insertion of the dorsal fin. Scale samples were placed between wax paper within small envelopes <br />labeled with: collection date, river and river mile, species, sex, Carlin tag number, fish TL (mm), <br />fish weight (g), and if the fish was a recapture. <br /> <br />Scales were soaked in water and cleaned with a soft artist's brush. Each scale sample was <br />mounted wet between microscope slides, taped closed, and dried. Species, tag number, and <br />capture date were recorded on each slide. Scales were magnified 39 times original size with a <br />Bausch and Lomb scale projector. Each scale was projected onto a she~t of paper that contained <br />orientation axes (Figure I). Scale margin and annuli were traced onto the paper as a record of <br />each scale. Measurements for back-calculation were made along either of the posterior lines A-B <br />or A-B' (Figure I). The computer program DISBCAL was used to calculate the fish-length to <br />scale-radius regression and back-calculate lengths at each annulus (Fry 1982; Anonymous 1989). <br /> <br />Scales were measured from the focus to each annulus and to the scale margin. Total <br />length was plotted as a function of scale radius and fitted with least-squares linear regression of <br />the form: <br /> <br />L = a + b * SR, <br /> <br />(6) <br /> <br />where L is fish length (mm), SR is scale radius (mm), b is the slope, and a is the y-intercept. <br />The intercept (a) estimates fish length at scale formation (Tesch 1968). However, Carlander <br />(1969) warned this may not always be true because early scale growth is not proportional to <br />growth of the fish. <br /> <br />Predicted length at each annulus (L.) was back-calculated by the Fraser-Lee method using <br />. I <br />equatIOn: <br /> <br />L. =!4=ill * S. + a (7) <br />I S " <br />c <br /> <br />where Lc is fish length (mm) at capture, S. is scale radius at the ith annulus, Sc is scale radius at <br />capture, and a is the intercept from equati~n (6). Lengths were calculated for all annuli from <br />each fish and averaged by age group in a summary table. <br /> <br />Ages were validated by two known-age, hatchery-reared fish that were stocked and later <br />recaptured. Ages were also partiaIly validated by ageing tagged fish that were recaptured. The <br />time interval between tagging and recapture was compared to the estimated interval obtained by <br />subtracting age at recapture from age at tagging. <br />
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