7227 - Laserfiche WebLink

Home Browse Search

296 less than optimal, as they are in the upstream re- gions of the historic range of Colorado squawfish. Our observations suggest that the growth physi- ology and timing of spawning of the Colorado squawfish are not well adapted to the temperature regimes of upper-basin rivers (cf. Keast 1985). Nonetheless, the fish might have been common here under former, pristine conditions because early-life mortality then was relatively low and these life-history characteristics were not impor- tant impediments to population maintenance. Lack of strong directional selection then may have precluded evolution of phenotypes that grow more rapidly or spawn earlier in the year under upper- basin temperature regimes. Management recommendations ManagemenY~recommendations for this endan- gered species are based on the arguments that we have made and include possible tests of our hy- pothesis. Dams and reservoirs might be operated to in- crease both the length of the growing season and mx Appendix. Computation of vital statistics for simulated populations of fast(F)-and slow(s)-growing Colorado squawfish that mature at 3 and 6 years of age, respectively. It is assumed that, beginning in the first year of maturity and continuing through age 10, each slow-growing female produces 0.5 mature female offspring annually and each fast-growing female produces 6.5-based on survival with 99% annual mortality in the smallest length class; see Table l.a x !x (age in years) S F the availability of temperatures suitable for growth, and to stimulate earlier spawning. How- ever, such enhancement must becautiously consid- ered because it might also benefit undesirable spe- cies -perhaps to the ultimate detriment of Col- orado squawfish. Water-development programs that reduce available temperatures should of course be avoided. Investigations should be conducted to determine the relation between the size of age-0 Colorado squawfish and overwinter survival. If an important relation occurs, it would provide useful objectives for possible growth-enhancement efforts. Because elimination of introduced fishes is im- practicable in a river system as large as the Col- orado, concern must clearly be directed toward preventing introductions of additional, undesirable fishes to the already large non-native fauna. In- vestigations to determine which non-native fish species present problems for Colorado squawfish should be conducted. Perhaps ways can be found to reduce the negative effects of the important non- native fishes on the Colorado squawfish. lxmx %mxx S F S F S F 0-26 1.0 1.0 0 0 0 0 0 0 3 1.0 1.0 0 6.5 0 6.5 0 19.5 4 1.0 1.0 0 6.5 0 6.5 0 26.0 5 1.0 1.0 0 6.5 0 6.5 0 32.5 6 1.0 1.0 0.5 6.5 0.5 6.5 3.0 39.0 7 1.0 1.0 0.5 6.5 0.5 6.5 3.5 45.5 8 1.0 1.0 0.5 6.5 0.5 6.5 4.0 52.0 9 1.0 1.0 0.5 6.5 0.5 6.5 4.5 58.5 10 1.0 1.0 0.5 6.5 0.5 6.5 5.0 65.0 11 0.0 0.0 0 0 0 0 0 0 Sums 2.5 52 20 338 ., a For slow-growing population, G = (ElxmAx)/R° = 20!2.5 = 8 years; r = (IogeR°)/G = 0.916/8 = 0.115; and f = e' = 1.121. For fast-growing population, G = 338/52 = 6.5 years; r = (Ioge52)/G = 0.608; and f = 1.837. Values apply to ages 0, 1 and 2.