|
<br />tions. Chironomids, baetid mayflies, amphipods, planar-
<br />ians, oligochaetes, and snails tend to be predominant in
<br />tailwaters below deep-release dams. Common benthic
<br />invertebrates in potamon reaches in the upper basin and
<br />lower river are oligochaetes, chironomids, gastropods,
<br />leeches, turbellarians, sphaeriid clams, odonates, beetles,
<br />simuliids, net-spinning caddisflies, and baetid mayflies.
<br />Introduced freshwater shrimp and crayfishes are locally
<br />abundant in the lower river, and the introduced Asiatic
<br />clam, Corbicula fluminea, occurs as far upstream as Lake
<br />Mead. The Colorado River System appears unique in that
<br />unionacean clams are virtually absent from its waters and
<br />isopods are usually absent from Iotic sites.
<br />The fish fauna of the Colorado River Basin bears little
<br />resemblance to its original state. Approximately
<br />100 species are now present; some 67 non-native fish spe-
<br />cies in 16 families have been introduced since the turn of
<br />the century and are now predominant in most fish commu-
<br />nities (Miller and Lowe 1964; Minckley 1973, 1979; Moyle
<br />1976; Carothers and Minckley 1981; Tyus et al. 1982). In
<br />terms of numbers of species, cyprinids, centrarchids,
<br />salmonids, catostomids, and ictalurids head the list of
<br />introduced fishes. Of the 54 natives listed in Table 2, 17 are
<br />either threatened, endangered, or extinct, and most have
<br />experienced drastic abundance and range reductions (Miller
<br />1972; Minckley 1973, 1979; Joseph et al. 1977; Behnke
<br />and Benson 1983). Two species (Pahranagat spinedace and
<br />Las Vegas dace) are extinct, and the woundfin is almost
<br />gone. The cutthroat trout is threatened in the upper basin
<br />(Behnke 1979), and most stream- and spring-inhabiting
<br />fishes of the middle and lowermost Colorado River drainage
<br />are legally protected or of special concern (Johnson 1987).
<br />All of the "big-river" fishes are in jeopardy (Minckley
<br />1973, 1983; Carothers and Minckley 1981; Tyus et al.
<br />1982; Behnke and Benson 1983; Hickman 1983). Wild
<br />Colorado squawfish are gone from the lower basin, and the
<br />flannelmouth sucker is extirpated from the Gila River drain-
<br />age (Williams et al. 1985). Tyus (1987) considered razor-
<br />back sucker one of the rarest fishes in the Colorado River
<br />Basin. The humpback chub persists tenuously in the Little
<br />Colorado River and Grand Canyon (Kaeding and Zimmer-
<br />man 1983) and occupies a few scattered canyon areas in the
<br />upper basin (Behnke and Benson 1983). The bony tail, origi-
<br />nally widespread and abundant in the basin, is functionally
<br />extinct; a few scattered individuals exist in the Green and
<br />Upper Mainstem Colorado rivers and in Lake Mohave in the
<br />lower basin (Behnke and Benson 1983). Behnke and Benson
<br />(1983), said of the bony tail's demise that "If it were not for
<br />the stark example provided by the passenger pigeon, such
<br />rapid disappearance of a species once so abundant would be
<br />almost beyond belief". Several authors (Minckley 1979;
<br />Behnke 1980; Hubbard 1980; Molles 1980; Behnke and
<br />Benson 1983; Williams et al. 1985; Stanford and Ward
<br />1986c) have attributed decline of native fishes to (1) modifi-
<br />cation and loss of habitat and (2) introduction of non-native
<br />specIes.
<br />Construction and regulation of dams have had severe
<br />impacts on the fish fauna of the Colorado River, and little
<br />unaltered habitat remains (Tyus 1984). Coats (1984)
<br />described general lack of regard for minimum flow needs
<br />of fishes in operating Colorado River dams. Extreme fluctu-
<br />ations and alteration of seasonal flow regimes have been
<br />implicated in alleged loss of 1983 and 1984 year classes of
<br />
<br />232
<br />
<br />the Colorado squawfish in its most productive remaining
<br />nursery habitat (Jones and Tyus 1985).
<br />Williams et al. (1985) discussed adverse impacts of
<br />introduction of non-native species on native fishes in most
<br />of the 15 aquatic ecosystems in North American deserts that
<br />they considered. Schoenherr (1981) described behavioral
<br />interactions between introduced redbelly tilapia, Tilapia zil-
<br />iii, and sailfin molly, Poecilia latipinna, leading to replace-
<br />ment of desert pupfish. Schoenherr (1981), MinckIey et al.
<br />(1977); Meffe et al. (1983), and Meffe (1984, 1985) studied
<br />predation by mosquitofish, Gambusia affinis, resulting in
<br />endangerment of Sonoran topminnow. Behnke and Benson
<br />(1983) discussed possible redside shiner, Richardsonius
<br />balteatus, competition with Colorado squawfish in the
<br />upper basin. Colorado squawfish interactions with other
<br />non-natives, e.g., "choking" on channel catfish, Ictalurus
<br />punctatus, (McAda 1983; Pimentel et aI. 1985) and compe-
<br />tition with northern pike, Esox lucius, (Wick et al. 1985)
<br />need further research.
<br />Surveys by Moffett (1942, 1943), Dill (1944), and Wallis
<br />(1951), stimulated stocking of game fishes, and threadfin
<br />shad (Dorosoma petenense), and various invertebrates were
<br />stocked as forage. Trouts (Salmo gairdneri, S. clarki, and
<br />Salvelinus fontinalis) and Pacific salmon (Oncorhynchus
<br />kisutch and O. nerka) were stocked in reservoirs, and col-
<br />dwater fisheries developed. Striped bass, Morone saxatilis,
<br />were introduced in Lake Mead in 1969, and a successful
<br />fishery developed in the 1970's. Rainbow trout and thread-
<br />fin shad populations declined as a result of predation by
<br />striped bass (Baker and Paulson 1983).
<br />A number of attempts were made to remove "coarse"
<br />fishes to make room for introduced species. In 1962,
<br />700-800 km of the Green River and its tributaries were
<br />treated with rotenone to allow Flaming Gorge Reservoir and
<br />the streams to realize their full potentials as trout fisheries
<br />(Miller 1963; Dexter 1965; Pearson et al. 1968). Down-
<br />stream detoxification failed, and rare endemic fishes were
<br />killed in Dinosaur National Monument. Binns (1967)
<br />reported that Colorado squawfish, razorback sucker, and
<br />rare mayflies had not reestablished populations in the treated
<br />area after 2 yr. Rotenone was also applied in the San Juan
<br />River prior to closure of Navaho Reservoir and on the Gila
<br />River upstream from San Carlos Reservoir. Impacts of these
<br />incidents have never been fully assessed.
<br />Non-native trout fisheries downstream from Colorado
<br />River dams (Mullan et al. 1976) have become valuable
<br />assets. Flaming Gorge Dam has been modified to improve
<br />such a fishery through increase in tail water temperatures
<br />(Holden 1979).
<br />Other fisheries in the Colorado River Basin are dependent
<br />on non-native centrarchids and striped bass, but fishing for
<br />channel and flathead catfish (Pylodictis olivaris), walleye
<br />(Stizostedion vitreum), and northern pike is popular locally
<br />(Behnke et aI. 1982; Stanford and Ward 1986c). Threadfin
<br />shad are important as food for piscivores in Lake Powell and
<br />lower-basin reservoirs (Johnson 1970, 1971; Stanford and
<br />Ward 1986c). Martin et al. (1982) estimated the annual eco-
<br />nomic value of Lake Mead fisheries at $69 million. Mullan
<br />et al. (1976), Carothers and Dolan (1982), Persons and
<br />Bulkley (1982), Morgenson (1983), and Baker and Paulson
<br />(1983) considered fishery management and cited similar
<br />studies on the Colorado system.
<br />Studies of fish production in the system are rare. Scarnec-
<br />
|