FITNESS IN THE ENDANGERED GILA TOPMINNOW

The Gila topminnow (Poeciliopsis o. occidentalis) is a small, live-bea ring, endangered fish extant in a maximum of nine locales in four sepa rate watersheds in the United States To determine if these populations differed in their fitness, we obtained samples from the four watershe ds and examined them for four fitness correlates: survival growth rate , fecundity, and bilateral asymmetry. Earlier research found that one population, Sharp Spring, had higher allozyme heterozygosity than the other three and had higher survival, growth rate, and fecundity and lo wer bilateral asymmetry than a sample from one of the other population s with no polymorphic allozyme loci, Monkey Spring. We also verified t hat Sharp Spring fish were polymorphic for the same allozyme loci wher eas the Monkey Spring population was not. We did not, however find pos itive associations of allozyme heterozygosity with the four fitness co rrelates for the four samples. Because the earlier study had much lowe r survival, it is likely that the differences resulted from differenti al response of the two populations to a stressful laboratory environme nt. Whether this unknown stress occurs in natural environments or its effect is predictive of other stresses remains unresolved. As a result , we concur with suggestions in the draft recovery plan that topminnow s from nearby sources be used for reintroductions and that the Sharp S pring stock not be used outside the upper Santa Cruz River drainage.