Exotic Aquatic Organisms - International Development Research ...

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bottom substrates by European carp during feeding. However, Fletcher et al. (198,5) concluded
that populations of carp have not caused increased turbidity levels in the Lower Goulburn River
Basin, Victoria.
Moyle et al. (1986) highlighted the direct and indirect influences of carp on shallow lakes in
North America. When carp are removed, turbidity decreases and native invertebrate-feeding
fishes that locate their prey by sight increase (Taylor et al. 1984). The impact of carp on turbidity
in Australian aquatic ecosystems seems to be obscured by the naturally high md variable
turbidity of many inland waters (Kirk 1977) and the interaction of hydrological factors with soil
type and land degradation.
Fletcher et al. (1985) also concluded that Potamogeton pectinatus, a shallow rooted, soft
leaved species, may be the only aquatic plant reduced by carp in Australia. Most of the aquatic
plants affected by carp in the United States and France are species of Potamogeto'z and Chara
(Crivelli 1983).
Xiphophorus helleri, the swordtail, is predominantly herbivorous, taking only a few aquatic
invertebrates, but there is no clear evidence that its feeding has contributed to changes in aquatic
macrophyte or algal communities in areas of introduction. Rather, the loss of endemic aquatic
macrophytes from Brisbane's urban creeks has resulted from dredging, channel modifications,
siltation, dumping of domestic and garden rubbish, bank erosion and the invasion of introduced
plants (Arthington et al. 1983). The control of water hyacinth (Eichhornia crassipes) and other
introduced plants with herbicides may also have contributed to the decline of so:ne endemic
macrophytes.
Numerous studies elsewhere have attempted to separate the effects of introduced species per
se from the influence of other disturbances and natural environmental phenomena, with only
limited success (Moyle et al. 1986). Australian researchers have experienced the same
difficulties, as Arthington et al. (1983) and Fletcher et al. (1985) show in work on poeciliids and
carp. Nevertheless, this is an area requiring much more study than it has so far received.
Impacts of Competition on Endemic Fishes
Resource competition for food, perhaps mediated by interference and aggressiDn, seems a
likely form of impact of many introduced species, particularly the mosquitofish, G. affinis.
Taylor et al. (1984) and Arthington and Mitchell (1986) point out that successfu] introduced
fishes typically exhibit generalist feeding habits and trophic opportunism. There is consequently
considerable overlap in the diets of introduced and endemic fishes in many systems.
G. affinis and Australian melanotaeniids, atherinids, galaxiids and a retropinnil have very
similar diets (Arthington unpublished; Lloyd 1987). Moreover, the diet of the rainbowfish,
Melanotaenja duboulayi (Castelnau) changes where large numbers of mosquitofish and small
numbers of rainbowfish coexist. Also M. duboulayi tends to select larger prey items in the
presence of G. affinis, which actively selects very small prey from the invertebrates in the stream
environment. These shifts reduce dietary overlap between the two species, and may contribute to
an effective partitioning of limited food resources.
Environmental disturbances causing loss of aquatic macrophytes required for rainbowfish
spawning habitat may also have been involved in the decline of this species (Arthington et al.
1983; Milton and Arthington 1984).
There is other evidence that Australian fishes display niche shifts in coexistence with S.
affinis but Lloyd (1987) recorded an expansion of niche width rather than a riduction as