The Apache trout Oncorhynchus apache has become threatened through hybridization with introduced nonnative trouts, among other reasons. We used 10 isozyme locus polymorphisms, which were in the aggregate diagnostic for discrimination of alleles of Apache trout, rainbow trout Oncorhynchus mykiss, and cutthroat trout Oncorhynchus clarki, to assess extent and directionality of interspecific hybridization in 645 individuals from 31 wild populations within the historical range of Apache trout. Only 11 potentially unhybridized populations of Apache trout were found. Rainbow trout introgression was documented in 19 of the 31 populations, including at least two in which all individuals sampled were hybrids. In four of these introgressed populations, hybridization between cutthroat trout and Apache trout was detected; at two of these localities individuals with alleles from all three species were sampled. Apache‐cutthroat hybrids were found at one locality where no rainbow trout alleles were sampled. In 19 of the 20 hybridized populations sampled, a trend of backcrossing toward Apache trout was evidenced. No pure rainbow trout or cutthroat trout were found in the population samples. Because of the extensive hybridization present, it was not possible to estimate the genetic variability extant in the Apache trout genome; only one of seven alleles detected exclusively in hybridized populations could be confidently assigned to a species of origin (rainbow trout). Apache trout recovery efforts will be confounded due to the variable conditions among populations with respect to introgression, habitat deterioration, and barriers to rainbow trout immigration.
THE ALCHESAY-WILLIAMS CREEK National Fish Hatchery is operated by the Bureau of Sport Fisheries and Wildlife of the U.S. Department of the Interior. The Alchesay Unit of this twohatchery complex is located on the North Fork of White River 8 miles north of Whiteriver, Arizona. The extensive watershed above A1chesay is typical of terrain found in southwestern United States above 5,000-feet elevation. Although heavily wooded, the ground cover is generally sparse. Extended periods of drought and occasional heavy rains are conducive to soil erosion. Portions of the area have been clear dut by logging operations which increases the amount of top soil washed into the drainage. Alchesay Spring, primarily a natural underground aqueduct of the White River, can supply up to 9,000-gallon-per-minute waterflow to hatchery operations. Runoff in the upper watershed produces turbid water in the river which then flows through the hillside and into the rearing facilities. It is this occasional turbidity that causes many problems in the hatchery (fig . 1). At Alchesay when turbidity exceeds 70 Jackson Turbidity Units (J.T.U.) the fish feeding activity drops sharply. They are unable to see the food thrown into the water under these conditions and feeding stops. Under normal conditions the feed .conversion ratio (pounds of feed fed per pound of weight gain) approaches one. However, when water is turbid the conversion ratio increases and 2.5 or more pounds of feed may be required for each pound of fish produced.Many man-hours have been spent on cleaning silt from raceways and ponds since hatchery 66
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