7970 - Laserfiche WebLink

Home Browse Search

persist in areas scattered throughout the basin where cold, hypolimnetic water is discharged from reservoirs (Vanicek, 1967; Vanicek and Franklin, 1970; Kaeding and Zimmerman, 1983; Holden, 1990). Even when natural conditions are considered, today's upstream distribution for these fishes in the upper basin is temperature-limited (in part, Kaeding and Osmundson, 1990). Water in these areas rarely achieves 20°C for any length of time in the summer, resulting in spawning a considerable distance downstream from their upstream distributional limits. Conditions for reproduction and survival are almost certainly less optimal .for peripheral populations than central ones, resulting in smaller effective population sizes toward the periphery. If the Lake Mohave region is neazer the focal point of the razorback's recent geographical distribution, upper basin populations are peripheral (as was similarly hypothesized for Colorado squawfish by Kaeding and Osmundson [1990)). Because genetic drift eliminates- variation more rapidly in smaller populations, those in the upper basin would be expected to exhibit fewer variants than more central ones. Since these populations are connected by considerable gene flow, the more common haplotypes in larger populations at the center of the range would eventually achieve the highest frequencies in peripheral areas. Available information is consistent with tl~e recolonization scenario outlined above. Lt addition to the progressive decline in diversity in populations further upstream, two of the more common haplotypes of Lake Mohave (e.g., ABADA'AAA, ACDCFBAB) become even more frequent in the upstream Lake Powell and Green River sample, with the most common haplotype (i.e., ABADA'AAA) fixed in our upper Colorado River sample. Known patterns of abundance indicate a cline in population size, as razorbacks appear to have been more numerous historically in the lower basin (Minckley et al., 1991); however, it is impossible to determine with available data if the system has achieved equilibrium. 14