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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />BIOLOGICAL OPINION <br /> <br />Status of the Species <br /> <br />in the lower Colorado and Gila River drainages (Baird and Girard 1853, Kirsch 1 889, Gilbert and <br />Scofield 1898, Miller 1961). <br /> <br />With their streamlined bodies, bony tail chub appear to be adapted to the Colorado River and large <br />tributary streams. Even with these adaptations, this species does not select areas of high velocity <br />currents and use of pools and eddies by the fish is significant (Vanicek 1967, Vanicek and Kramer <br />1969). Grinnell in 1914 captured bony tail chubs in a backwater along the LCR. There is limited <br />information on migrations or other movements. <br /> <br />Spawning takes place in the late spring to early summer (Jonez and Sumner 1954, Wagner 1955) <br />in water temperatures about 180 C (Vanicek and Kramer 1969). Riverine spawning of the bonytai I <br />chub has not been documented; however in reservoirs, gravel bars or shelves are used (Jonez and <br />Sumner 1954). Bonytail chub may be flexible in their spawning habitat needs as evidenced from <br />successful spawning in hatchery ponds at Dexter National Fish Hatchery and raceways at Willow <br />Beach National Fish Hatchery. <br /> <br />Habitat needs of larval and juvenile bony tail chubs are not well known. Few larvae have been <br />identified in the Lower Basin; in the Upper Basin, there is confusion between larvae of the bonytai I <br />chub and other chubs, so interpreting data is difficult. It is known that young fish prey on aquatic <br />invertebrates, especially chironomid larvae and mayfly nymphs (Vanicek and Kramer 1969). It <br />is likely that quiet water habitats are preferred habitats for young fish, given the succe ss of raising <br />them in man-made ponds. <br /> <br />Population Dynamics <br /> <br />The bony tail chub is adapted to the widely fluctuating physical environment of the historical <br />Colorado River. Adults can live 45-50 years, and apparently produce viable gametes even when <br />quite old. The ability to spawn in a variety of habitats is also a surviva I adaption. In the event of <br />several consecutive years with little or no recruitment (due to either too much or too little water), <br />the demographics of the population as a whole might shift, but future reproduction would not be <br />compromised. Fecundity measurements taken on adult females in the hatchery ranged from 1,015 <br />to 10,384 eggs per fish with a mean of 4,677 (USFWS 1990). With the fecundity of the species, <br />it would be possible to quickly repopulate after a catastrophic loss of adults. <br /> <br />Severe reductions in both population numbers and individual bonytail chub numbers can be traced <br />largely to impounding the LCR and introducing non-native fish into the modified environment. <br />Dams created reservoirs that favored survival and expansion of species adapted to lentic systems. <br />Deep water releases from large reservoirs created habitat immediately downstream of reservoirs <br />that was ideal for cold water species. Conversely the bony tail chub had adapted to a riverine <br />system tied to periodic flooding of the free flowing Colorado River. With physical modification <br />of the free flowing river and introduction of many non-native species, non-native species in the <br /> <br />39 <br />