7726

• UCREFRP Catalog Number: 7726
• Author: Stanford, J. A. and J. V. Ward.
• Title: Fish of the Colorado System.
• USFW Year: 1986.
• USFW - Doc Type: 385-403
• Copyright Material: YES

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have replaced the Gila spinedace (Medafulgida), and exotic mosquito fish (Garn-
busia affinis) have replaced the Gila topminnow (Poeeiliopsis oeeidentalis oeci-
dentalis) in the Gila drainage. Mosquitofish also have replaced desert pupfish in
the Salton Sea, and exotic largemouth bass (Micropterus salmoides) and sailfin
mollies (Poecilia latipinna) have replaced Moapa dace (Moapa eoriaeea) and
White River springfish (Creniehthys baileyl) (Minckley & Deacon 1968; Minckley
1973; Deacon 1979). Minckley (1979) believes that the greatest impact is in
predation on juvenile natives, especially by juvenile exotics. In some cases,
hybridisation between native and exotic species has "swamped" the native gene
pool (e,g, rainbow X cut-throat trout: Behnke & Benson 1980). Further, range
restrictions and habitat change~, including the effects of exotic species, may be
enhanced by hybridisation between native species (e.g. razorback X flannel-
mouth sucker: Hubbs & Miller 1953; Tyus et al. 1982).
Exotics have been especially successful in reservoirs (Stanford & Ward 1986b)
and closely regulated segments (Stanford & Ward 1986a: Fig, 1). The major
fisheries in Lower Basin reservoirs involve the planktivorous threadfin shad,
which provides food for largemouth (Micropterus salmoides) and striped
bass (Morone saxatilis) (Dill 1944; Kimsey 1957; Minckley 1973). In Upper
Basin reservoirs a host of introduced suckers, minnows and trout are pre-
sent. Throughout the system, introduced fish have become established in the
remaining river segments through stocking or migration from the reservoirs
(Table 2).
The Serial Discontinuity Concept of Ward & Stanford (1983) - the idea that
regulated rivers tend to ecologically reset or mimic upstream or downstream
lotic conditions - predicts that native species should be re-established some
distance downstream from a dam, However, in the Colorado system there may
no longer be sufficient river lengths for this to occur, especially with the presence
of the exotic species, Big-river endemic species survive in the lower Green River
and the mains tern above Lake Powell because conditions approach pre-regulation
conditions, but even these segments are likely to be affected by dams proposed
or under construction on the lower reaches of the Yampa, White, Dolores and
Gunnison rivers,
I
; Productive exotic sport fisheries have been created below several dams:
growth rates of trout reportedly are more than 38--40 cm annually (Wiley &
Duffer 1980). Some fisheries have developed immediately below dams (e,g, Lee
Ferry fishery below Glen Canyon Dam), and others (e,g. lower Gunnison Rivcrl
are most productive some distance downstream from the dam, as cold tern.
peratures in the tailwaters are ameliorated by warm air temperatures and sid~'
flows (Stanford & Ward 1983, 1984).
A productive fishery developed in the Green River below Flaming Gorgt'
. Dam as the reservoir was filling (1962-68), but declined dramatically in respoTl'C
to cold (40C) releases after the reservoir filled (maximum depth 134m), In an
effort to salvage the fishery in 1978 the dam was fitted with multi-level relea'c

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gates to allow a constant summer tailwater temperature of e. 130 C. Trout
growth increased 3--4 fold in the warmer water (Larson et al. 1980).
The Lee Ferry fishery below Lake Powell involves a simple food chain, and
produces fast-growing rainbow and other introduced trout (Plate Ig). The clear,
nutrient-laden water from the hypolimnion of Lake Powell stimulates growths of
the green alga Cladophora glomerata on the river bottom, sustaining high produc-
tion of introduced amphipods (Gammarus laeustris; see Ward et al. 1986), in
turn fed upon by trout. This fishery extends well into the Grand Canyon and is
a very different trophic system to the one that existed before impoundment.

Conclusion

The indigenous fish of the Colorado system have few marine affinities (Miller
1958) and are 85% endemic (66% at the species level), suggesting long isolation.
For some forms, however, survival may depend on artificial propagation
(Greger & Deacon 1982; Hammon & Inslee 1982), The system now is so altered
by regulation that questions about ecology of fish in the virgin river are largely
academic. Non-native species abound throughout, and the prospect for further
introductions is strong. Populations will likely fluctuate widely as interspecific
competition and the ecological consequences of regulation differentiate "winners
and losers".
Many research opportunities remain. Relationships between processes in
reservoirs and tail waters and successful fish populations are little understood.
The life histories and needs of forage organisms and factors controlling primary
productivity must be related to the fish communities, especially if sport species
are to be preferred, The regulated Colorado system provides experimental
macrocosms in which organism-level (e.g. gene expression) or ecosystem-level
(e.g. reservoir influences) theories may be tested (cf. Ward & Stanford 1984),
Answers to these questions may help to couple the benefits of regulation with
the natural attributes of the system.

Acknowledgements

We thank Dr R. Behnke and G. Wilde for reviewing the manuscript and R.
Valdez, J. Rinne, W. Dreyer, J. Johnson, C. Carlson, R. Harris and G. Bryant
for unpublished material.

References

Behnke, R. r 1981. Systematic and zoogeographical interpretation of Great Basin trouts. In R. J.
NallJ1an & D. L. Soltz (eds). FIshes in North AmerIcan Deserts. John Wiley & Sons, N,Y.:
95-124. '

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