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Fin damage analysis.-A characteristic result of interaction of these two poenhids is damax
<br />to topminnuw caudal fins
<br />a direct result of mos
<br />uitofish a
<br />res
<br />i
<br />M
<br />ff
<br />l r
<br />,
<br />q
<br />gg
<br />s
<br />ou (
<br />e
<br />e et a
<br />., 198;3). 7 hn
<br />was quantified in a laboratory experiment and at Bylas. 1Hhe null hypothesis was that their
<br />would be no difference in fin damage frequency in allopatry or sympatry. Caudal fins wer,
<br />examined and scored for no damage (intact, rounded fin) or damage (shredded fin, missing
<br />tissue). Laboratory fish were held allopatrically or sympatrically in 37.5 or 75.0 1 aquaria at
<br />constant densities for fit days before examination. The field sympatric sample was compared with
<br />a 1968 collection (ASU museum) made prior to mosquitofish colonization.
<br />Stress analysis.-To test the physiological stress hypothesis of Schoenherr (1981), 1 establish-
<br />[
<br />,
<br />37.5 and 75.0 1 aquaria in April 1982 and stocked 15 females and 5 males of either or bosh
<br />species, respectively. After 12 weeks, all surviving P. occidentalis were preserved in 10% formaliu
<br />for three days, then transferred to 70% ethyl alcohol for a week, Standard lengths (nearest 0.1 mnu
<br />and wet weights (nearest 0.1 µg) of the ovary and somatic tissue were measured for all females
<br />Ovaries were dissected and numbers of embryos and mature ova counted. This experiment %",
<br />repeated in August 1982. Additionally, wild topminnows were collected from two pools of Sharp
<br />Spring, Santa Cruz County, Arizona (Meffe et al., 1982) in June 1980, one sympatric with, and
<br />one allopatric to, mosquitofish. Fish were frozen on dry ice, transferred to a laboratory freezer,
<br />and analyzed as above within a month of collection.
<br />RESULTS.-Laboratory Coexistence Study. -All opa tric populations of /'.
<br />occidentalis successfully reproduced and grew in laboratory aquaria,
<br />whereas those sympatric with G. affinis had no net recruitment and steadily
<br />declined (Fig. 1). Population growth in simple and complex sympatric
<br />tanks was statistically indistinguishable, and these were combined in
<br />analysis.
<br />In allopatry, P. occidentalis populations grew rapidly, stabilized, then
<br />began a slow, long-term decline (Fig. 1). The decline is attributed to toxic
<br />waste accumulations in aquaria; in particular, soluble reactive phosphate
<br />concentration toward the end of the experiment was 3.43 mg11, compared
<br />with values in topminnow natural habitats ranging between 0.00 and 0.10
<br />mg/l (Meffe et al., 1983). Dead and moribund fish, particularly juveniles,
<br />were common in the latter half of the experiment in allopatric tanks.
<br />Despite these toxicity problems, topminnow populations-all grew in the
<br />absence of mosquitofish. In contrast, there was no successful recruitment by
<br />P. occidentalis in presence of the introduced predator. Newborn fish were
<br />repeatedly observed in aquaria, but never survived more than a day; several
<br />were seen being eaten by female mosquitofish. Analysis of all P. occidentalis
<br />females that died before the decline phase revealed that most were gravid
<br />(42/51 allopatric [82.4%]; 36/42 sympatric [85.7%1, and that there was no
<br />significant difference in fertility between the two groups (allopatric: X =
<br />6.66, SD = 8.02; sympatric X = 5.95, SD = 4.36; t = 0.517, P = .6063.
<br />Topminnows in sympatry with mosquitofish therefore reproduced
<br />normally, but all offspring were eaten. There was no difference in mean
<br />number of topminnow_ adults surviving at the end of the 63 week
<br />experiment (allopatric X = 0.83, SD = 0.84; sympatric: X = 2.33, SD =
<br />2.39; t = 2.055, P = .0606), indicating their survival was not reduced by
<br />mosquitofish.
<br />There was similarly no recruitment of G. affinis in any experimental
<br />unit. Females were fertile (allopatric: X = 24.4, SD = 19.0, n = 18;
<br />Sy171pattic: X = 31.1, SD = 31.1, n = 23), but cannibalism was so severe
<br />that no juveniles escaped in these restricted aquarium habitats, even with
<br />vegetation as a refugium. In nature, refugia for the neonates of both species
<br />Sympatric
<br />n.12
<br />60
<br />5 10 15 20 25 30 35 40 45 50 55 60
<br />TIME (WEEKS)
<br />Ftc. 1.-Results of laboratory coexistence study. Plotted is mean number of P. occidentalis, plus
<br />?r minus two standard errors (solid lines) with range indicated (broken lines), versus time.
<br />11hpatric I is a complex habitat and allopatric 11 is a simple habitat; both habitats were
<br />vnnbined for P. occidentalis sympatric with G. affinis.
<br />are quantitatively and qualitatively different, especially with the presence of
<br />,hallow areas inaccessible to adults. Since mosquitofish obviously recruit
<br />Kell in nature, predation rates in this experiment were probably artifactual.
<br />However, it does illustrate the potential extent to which mosquitofish
<br />predation may limit population growth, and suggests that this effect is
<br />partly a function of habitat size and structure.
<br />Field Coexistence Study.-There was no recruitment of topminnows in
<br />ympatric field cages, and all three populations became extinct between
<br />days 13 and 24 (Fig. 2). Surviving adults were physically damaged in the
<br />first two weeks, and all were eaten by the end of the experiment. Allopatric
<br />(age populations all experienced successful recruitment and were extant at
<br />the end of the 24 day experiment (Fig. 2).
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