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PRocEED=s OF THE DESERT FISHES CouNciL, 1995 SYMr oslum populations had 7 to 28% survival (Table 9). The exceptional population, East Sandia Spring, has a survival rate twice that of the next highest (Toe Nail). It also has the highest survival when males are used as predators. East Sandia Spring has the smallest water volume and presumably the lowest population numbers. On my visit, G. geiseri was relatively rare. Conspecific predation by G. geiseri also varies widely but has little correlation with predation on G. af'inis. Variation in predation by G. geiseri on G. affinis young is substantially less than predation by G. affinis when more than 100 young are used: 7% to 28% versus 0% to 69% survival. Presumably the consistency of predation by G. geiseri reflects the recent transfer of the fish from San Marcos. Predation rates of females from East Sandia Spring may reflect evolutionary changes in the 50 years since they were released. Predation by female G. nobilis on G. affinis young again varies significantly. The five samples with sample sizes more than 100 vary between 0 and 14% survival. The small sample sizes have survival rates between those for large samples. Although the range of survival of G. affinis young preyed on by G. nobilis is similar to that of young preyed on by G. geiser, 14% versus 21%, the relative variation is infinity versus three-fold. If few young survive, a high upper figure cannot occur. Birth weight: Average birth weights of Gambusia affinis also vary extensively from less than 10 milligrams to more than 20 milligrams (Table 10). The two New Mexico populations (Lost River and Bitter Lakes #3) have tiny babies (6.0 and 8.0 milligrams). The other small young are Contrabando Canyon, Fairy, Clymer Meadow, Hanks Bull, Patty's Ranch, Big Brown, Hi Island, and Uncertain. Many of the small young are from east Texas, yet other east Texas fish (Village Creek and Egg Nog) have relatively large young. Only Fairy is stenothermal, but commonly G. affinis is absent at stenothermal locations (Hubbs, 1995). All sites have low salinities. The heaviest young are from Heart of the Hills and San Marcos. Fish from Heart of the VOLLTW XXVII - PUBLISHED 1996 Hills (eurythermal) have larger young than those from the nearby Fessenden Spring (stenothermal). The sample location at San Marcos is under the I-35 bridge at a site that is relatively eurythermal for the San Marcos River (Hubbs and Peden, 1969). Again, the variation has little apparent association with geographic or environmental circumstances. Average birth weights of Gambusia geiseri range between 17.1 and 23.3 milligrams. These weights are of the heavier end of those for G. affinis but clearly vary far less. No birth weight data are available for East Sandia Spring. Average birth weights for G. nobilis vary but only three populations have data, so comparisons are not readily available. In general, G. affinis and G. speciosa birth weights are about 16 milligrams, G. geiseri about 20 milligrams, Poedlia about 30 milligrams, G. longispinis about 25 milligrams, G. gaigei about 30 milligrams, G. heterochir about 30 milligrams, and G. nobilis about 40 milligrams. Poiedha formosa from San Marcos has heavier young than P. latipinna from San Marcos; sailfin molly young from Comal Springs are heaviest of all. Heart of the Hills sailfin Molly young are about the same weight as Comal amazon Molly. Aransas County P. latipinna young are much lighter than those from the San Marcos River system. Interbrood intervals: Average interbrood intervals for G. affluis females varies between 27 and 41 days (Table 11). The geographically proximate Phantom and Carpenter Hill populations have no overlap (Hubbs, 1996). Again there is no association of the results with environmental or geographic factors. Average interbrood intervals for G. geiseri females is at the long end of the G. affinis range, but the variation is less than for G. affinis (8 days vs. 14). The population extreme in predation rate (East Sandia) has no data on interbrood interval. The interbrood intervals for G. nobilis females are similar, but small sample sizes make any interpretations inconclusive. In general, interbrood intervals are about 35 days for G. afinis, 40 days for G. geiseri, 44 days 5