7835 - Laserfiche WebLink

Home Browse Search

DISCUSSION Growth, Age and Size Our estimates of growth rates and mean lengths by age class were similar to those reported by Seethaler (1978) and Hawkins (1992) up to about age 8-10 (Fig. 3 a); these similarities were expected given that our analysis through age 7 was also based on scales. Mean lengths at a given age were closest to those reported by Seethaler (1978), though his estimates were slightly less than ours. This may reflect differences in the annual reference point: winter for Seethaler and mid- to late spring for this study. A 13 ° C threshold water temperature for Colorado squawfish growth (see Osmundson 1987, Kaeding and Osmundson 1988) is generally attained in the study area during March or April. This provides 1-3 months of additional growth for fish measured during mid-April to late June compared to those with estimated lengths at annulus formation the previous winter. Results of scale analysis and additive recapture increments diverge after fish are 10 years old. Seethaler's (1978) and Hawkins' (1992) scale-based Colorado River results end at ages 11 and 12, respectively; however, Hawkins' (1992) pooled data from four upper basin rivers continues through age 18. All three of the earlier growth curves indicate high and relatively constant growth rates through the last year examined whereas our growth rates are much lower after the fish attain about 550 mm TL (Fig. 3a). We concur with Hawkins (1992) who cautioned that scale-based estimated ages are probably not reliable for Colorado squawfish beyond about age 10. Our estimates of mean annual growth increments from recaptured adults in the Colorado River are similar to those reported by other investigators. Tyus (1988) reported that recaptured adult Colorado squawfish (N = 59; 482-770 mm TL) from the Green-Yampa rivers averaged a gain of 10.2 mm (SD = 11.3 mm) in length per year, and Hawkins (1992) reported 10-15 mm for fish > 500 mm TL from various rivers. Direct comparisons with our data are difficult because annual increments vary with fish size and the proportion of these earlier samples consisting of the faster growing small adults is unknown. However, when we pooled increments from all fish 2! 450 mm TL (N = 121), mean annual growth was 14.4 mm (SE _ 1.2 mm); from fish z 500 mm TL (N = 104), 11.9 mm (SE = 1.0 mm). Thus, growth estimates from earlier studies using Carlin-tagged fish were very similar to those of our PIT- tagged fish, and suspicions of potential negative effects on growth from dangler-tags are probably unfounded. The average annual growth increment of 30 mm determined from back- calculation of scale radii as reported by Hawkins (1992) appears too high except for fish < 500 mm TL, and suggests an upper size limit for aging Colorado squawfish using scales. Using the additive increment method to calculate mean length at each age, we noted a decline in growth rate after Colorado squawfish reached z 550 mm TL, followed by a rate increase after they reached approximately 650 mm TL (Fig. 3a). Although this difference in rate was not significant, the lack of significance may have resulted from small numbers of larger (> 650 mm TL), recaptured fish. Hawkins (1992) also noted an increase in growth rate at around 650 mm TL, evidenced by both back-calculated scale data and recapture data. If this rate A-16