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PRoCEEDIIVGS OF THE DESERT FISHES COUNCIL, 1995 SYMPOSIUM <br />is presumed that most of the adult mortality was <br />from natural causes. It is likely that some natural <br />mortality occurred with young as well. I presume <br />that 90% survival is close to the maximum <br />survival rate possible in these types of <br />experiments. At the end of an experiment, the <br />adult was returned to the stock tank and may <br />have been used again. As ten or fewer young <br />were used in most experiments, the number of <br />experiments is about 10% of the number of young <br />reported. The data reported here are survival <br />percentages. Predation would be the inverse. <br />Young used in predation experiments were <br />tested with available adults. For example, <br />Woman Hollering young might be tested with <br />Egg Nog adults on one date and Cow Creek <br />adults the next Similarly, Cost adults could prey <br />on Too Much Pond young and then on Fairy <br />young at a later date, or Cut 'n' Shoot combined <br />with Hog Eye and then Uncertain. <br />Birth weight: Females were isolated in <br />floating breeding chambers that were checked <br />daily. Most of the young were used in predation <br />studies, but some young (at random) were placed <br />in a 40°C drying oven for at least two days and <br />weighed. The birth weight of some individuals in <br />a brood might vary by as much as 800%, but <br />variation among individuals was usually about <br />1096. The average weight of individual young was <br />recorded. Sample sizes are listed in Table 1. <br />Interbrood interval: Isolated females were <br />continuously maintained in the breeding <br />chambers. The date of birth of the first <br />individuals in a brood was recorded. The <br />occasional young that were found on the next day <br />were considered part of that brood. A female <br />would then have another brood after an interval <br />of more than three weeks. That was considered <br />a second (third, etc.) brood, and the difference <br />between the dates is reported as the interbrood <br />interval. Occasionally, a female captured during <br />a nonbreeding interval in nature would have a <br />second brood shortly (5 - 10 days) after the first. <br />That interbrood interval was excluded from the <br />data. If a population had more than 10 <br />interbrood intervals, the longest and shortest <br />were deleted, if more than 20, the two longest <br />VOLUME XXVII - PUBLISHED 1996 <br />and shortest were deleted, etc. Sample sizes are <br />listed in Table 1. <br />Results <br />Predation: Although many authors (Seale, <br />1917; Krumholz, 1948; Koster, 1957; Myers, <br />1965; Axelrod and Schultz, 1971 and 1983; <br />Minckley, 1973; Walters and Legner, 1980; <br />Schoenherr, 1981; Harrington and Harrington, <br />1982; Meffe and Snelson, 1989) have reported <br />that Gambusia adults avidly prey on their own <br />young, Hubbs (1992) reported that half of the <br />experimental young survived a week Additional <br />data provide a similar result (Table 3). Similarly, <br />I have shown that young with male predators had <br />higher survival than those with females (70% <br />versus 30%) (Table 4). I also contrasted the <br />individual predator-predation comparisons and <br />got the same results (Table 5). This excluded the <br />possibility that tests with one sex as predator <br />came from a more predaceous population than <br />those of the other. Many more individual tests <br />had chi-squared values favoring males having a <br />lower predation rate. It should not be surprising <br />that two of more than 800 contrasts were <br />statistically (p > 0.01) the converse of the <br />majority. The same statistical level favoring the <br />conclusion that females were more predaceous <br />was obtained by 394 contrasts. Consequently, <br />predation by adults on young is primarily by <br />Gambusia females. <br />Predation by males on poeciliid young had <br />results similar to but more extensive than those <br />reported previously (Table 6). Survivorship of <br />Pmdha young exposed to G. affinis predation was <br />the lowest of any with a sample size of over 100 <br />young. Similarly, survivorship of Poecilia young <br />was lowest in one of the other three comparisons. <br />Predation rate may be affected by feeding by the <br />predator or escape by the prey (Fuiman and <br />Magurran, 1994). Although Poecilia young are <br />relatively large, they are less active in aquaria <br />than are Gambusia young. The second-lowest <br />survivorship with male G. minis as predator was <br />conspecific. Predation by G. nobilis males was <br />relatively high as young survival with G. nobilis as <br />predator was lowest in all but three (G. nobilis <br />3