7169

• UCREFRP Catalog Number: 7169
• Author: Meffe, G. K.
• Title: Predation and Species Replacement in American Southwestern Fishes
• USFW Year: 1985
• USFW - Doc Type: A Case Study
• Copyright Material: YES

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FIG. 2.-Results of field coexistence study. Plotted is individual population sizes of I-
occidentalis versus link for populations allopatric to, and sympatric with, G. affinis. t.im,
connect population means,
Fin Damage Analysis.-In the native population at Bylas Springs, 122 of
178 (68.5%a) topminnows examined from October 1980 to March 1982 had
slight to heavy caudal fin damage. Forty-two topminnows collected in 1968.
before mosquitofish colonization, had no damage. Over 100 topminnotss
examined between March and July, 1981 from an adjacent spring lackin4
mosquitofish exhibited <1% fin damage.
Similar damage appeared on laboratory topminnows coexisting wide
mosquitofish. After 52 days in aquaria, 73 of 130 (56.2%) topminnows held
with G, affinis had damaged fins, while none of 144 held in allopatry was
damaged. There was also a slight, but significant decrease in mean survival
of populations of 12 topminnows adults when in sympatry (allopatric
= 11.92, SD = 0.29; sympatric: X = 10.92, SD = 0.90; t = 3.664, P =
.0029). When the species were placed together, mosquitofish femaics
immediately became aggressive, and chased and bit topminnows. Within F)
minutes some fish were damaged; most were attacked within an hour.
Mosquitofish did not generally act aggressively toward one another, and no
fin damage was observed on that species.
Stress Analysis.-Topminnows sympatric with mosquitofish in the first
laboratory stress experiment were significantly lighter in somatic weight.
and produced fewer embryos and mature ova, than did allopauit
individuals (Fig. 3, Table 1). In the second trial, significant difference.
appeared in ovary weight and fertility (Fig. 4, Table 1). There was nn
detectable effect of sympatry versus allopatry on total weight or ovar}
weight in the field (Fig. 5, Table 1) and trends (regression slopes) were in
the opposite direction of laboratory results. There was a significant
difference in fertility between allopatric and sympatric field populations.
but again in the opposite direction of that seen in the laboratory.
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STANDARD LENGTH (m m)
FIG. 3.-Results of laboratory stress analysis 1. A) Plotted is wet body weight (upper regressions)
,lid wet ovary weight (lower regressions) versus standard length [or allopatric
I). - -0) and sympaui( (• 0) Sonoran topminnow p(pulations. 13) Plotted is fertility versus
,undard length for allopatric (0- - -0) and sympatric (• -) Sonoran topminnow populations.
DiscuSSION.-Available data suggest that replacement of Y. occidentalis by
G. affinis is primarily due to predation on juveniles, with stress responses
to adults potentially playing a minor role. Other hypotheses, such as
resource competition, hybridization, parasitism, or disease, are rejected for
lack of supportive evidence.
Predation on topminnows by mosquitofish was repeatedly observed in the
himratory and the field. In both experimental settings, the native species
did not recruit successfully when in sympatry with mosquitofish. A possible
stress response, expressed as lighter, less-fecund female topminnows, was
inconsistently observed in laboratory experiments. Since similar results
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