7982 - Laserfiche WebLink

Home Browse Search

398 Recovery of Long-lived Species .1 [ J ; I q recovery initiatives. The reverse of stocking- elimination or removal of non-natives-is worth considering but has not yet been mean- ingfully addressed. Introductions of non- native fishes in the Colorado basin have been documented by many, and data on their pres- ent status was provided by Minckley (1973, 1982) and Tyus et al. (1982a). Competition of non-native fishes with Colorado squawfish has been proposed by various workers (Hol- den 1977a, 1979; Joseph et al. 1977; USFWS 1978b, 1989a; Holden and Wick 1982; Behnke and Benson 1980; Osmundson 1987). The impacts of such introductions on native species remain problematic because they dif- fer in their effects on various life stages, both temporally and spatially. For example, larval squawfish are eaten by other fishes, and prob- ably suffer competition from the young of other species as well. Studies of predation are difficult because the fragile larvae are rapidly digested by predators (Marsh and Langhorst 1988). Postlarval Colorado squawfish attain 40- 50 mm TL in their first year of life, and they may begin to eat other fishes at that time. It is possible that larger or more aggressive indi- viduals experience better growth in backwa- ters where small non-native fishes are used as food. Some studies indicate that squawfish postlarvae consume appreciable volumes of unidentified fishes Uacobi and Jacobi 1982); however, our stomach analysis of hundreds of small squawfish collected in the Green River in 1986-1988 (USFWS unpub. data) indicated that age-o fish subsisted mainly on inverte- brates, and seldom other fishes, in agreement with findings ofVanicek and Kramer (1969). On the other hand, aggressive behavior to- ward squawfish larvae by some introduced spe- cies has been observed in aquaria (Karp and Tyus 1990) and ostensibly occurs in nature as well. Postlarvae may also serve as prey for some larger fishes, particularly centrarchids, which commonly enter backwaters. Osmund- son (1987) recorded predation on Colorado squawfish by largemouth bass (Micropterus salmoides), green sunfish (Lepomis cyanellus), black crappie (Pomoxis nigromaculatus) and black bullhead (Ameiurus melas). Fish in the juvenile life stage are large enough to navigate freely along most river channels, and presumably do so. There are possible advantages and disadvantages to them from introduced species, the squawfish eating smaller species and individuals (Van- icek and Kramer 1969) and being eaten by larger ones until adulthood is attained. How- ever, stomachs of 123 northern pike (Esox lucius), 61 walleye (Stizostedion vitreum), and 755 channel catfish ([ctalurus punctatus) taken from the Green and Yampa rivers from 1984 to 1988 (Tyus and Beard 1990; Tyus and Nikirk 1990) provided no evidence that these predators ate squawfish (some fish re- mains were not identified). The fish remains most commonly identified were other intro- duced forms. Growth of adult Colorado squawfish may be more rapid than formerly if their prey base has increased due to introduction of non- native fishes, but lack of precise data on prey abundance makes such comparisons difficult. Some introduced prey may be lethal; large squawfish have been known to choke on chan- nel catfish (Vanicek 1967; McAda 1983; Pimentel et al. 1985). Larger introduced fishes such as northern pike and walleye are poten- tial competitors for food and space. Large squawfish readily take fishing baits or artificial lures and can frequently be ob- served and fished at the water surface. There- fore sport fishing must be considered a factor limiting future recovery. Sport fishing mortal- ity of adults (intentional or not) is a serious problem in areas of concentration, particu- larly in the Yampa River, a popular fishing spot (Saile 1986; Tyus and Karp 1989). In the