An Experimental Study of Vertical Habitat Use and Habitat Shifts in ...

elationship between mouth position and habitat use in fishes. Benthic-oriented fishes
typically have inferior of subterminal mouths and pelagic fishes have terminal or
supraterminal mouth positions (Mendelson 1975). Notropis telescopus and C. spiloptera
were more pelagically-oriented, each having mouths that are more terminally-positioned
than their congeneric counterparts. Mouth position is generally viewed as a phylogenetic
or historical constraint, with a strong proximate influence on habitat use. Increased
integration during syntopy as reported in this study provides evidence of a strong species
interaction influence over morphologically-correlated use of habitat in cyprinids.
In contrast to segregation by competitive displacement, studies have documented
integration through mixed-species shoals (Magurran and Pitcher 1983, 1987, similar to
mixed-species flocks of birds, Morse 1970). Previous interspecific contact may not be
required for two species, such as a native/non-native species pair, to closely-associate and
benefit symbiotically from interactions, such as by improved foraging success (Magurran
and Pitcher 1983) or by predator avoidance (i.e., safety in numbers, Allan and Pitcher
1986, Mathis and Chivers 2003). Based on general observations during my experiments,
the more pelagically-oriented species of each pair fed more aggressively during syntopic
trials, hence, there was no qualitative evidence for a mixed-shoal advantage of foraging
for both species. The number of individuals per tank in allotopic trials (n = 40) was equal
to that of syntopic trials, so the data do not support evidence for an influence of density
on syntopic integration, as one would expect from a “safety in numbers” explanation of
predator avoidance.
In conclusion, the laboratory-based experimental study demonstrates a difference
in vertical use of habitat between two native/non-native congeneric pairs. The
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