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1~9 The Southwestern Naturalist
<br />vol. 30, no. 1
<br />~ length) to the nearest 0.1 mm and assigned to a developmental phase according to Snyder (1976).
<br />~ During the lan•a1 period (59 days), daily growth ranged from 0.0 to 0.7 mm. Slean daily gror+~th
<br />was 0.3 mm (ranging from a mean of 0.2 for mesolarvae to 0.4 mm for protolarvae), compared to
<br />0.5 to 0.6 mm for hatchery-reared larval humpback chub and bonytail, respectively, cultured at
<br />temperatures ranging from 12.9 to 25.5°C (Hamman, 1982a; 1982b). Total length o[ larvae at
<br />selected developmental events (Fig. l) correlated closely with data presented by Snyder (United
<br />States Dep. Inter. Bur. Land Mgt. Bio. Sci. Ser. No. 3, Denver, Colorado, 1981) for field-collected
<br />roundtail chub mesolarvae and metalarvae.
<br />One of the major problems confronting aquatic biologists working in the Colorado River Basin
<br />is identifying young-of-the-year roundtail chub, humpback chub and bonytail (Valdez and
<br />Clemmer, pp, 109-1 l9 in Fishes of the Upper Colorado River System: present and future [W. H.
<br />Miller, H. M. Tyus, and C. A. Carlson, eds.], Am. Fish. Soc., Bethesda, Maryland, 1982). Reliable
<br />discriminating characters must be determined before collections of larval and early juvenile Gila
<br />can be used to monitor these fish populations and answer many questions regarding life history
<br />and habitat requirements. We anticipate that the taxonomic research being conducted by the Colo-
<br />. rado Division of Wildlife and Colorado State University will contribute significantly to solving
<br />this problem. The roundtail chub developmental series produced from the present study is a vital
<br />component in this research.
<br />This research was Funded under Federal-Aid matching grant SE-3-5 administered through the
<br />Nongame Research Group, Colorado Division of Wildlife. G. Dean, M.'Grode, R. Hufziger, :~?.
<br />!'loth. S. Piatania, G. Skiba, D. Snyder, R. Van Buren and E. Nick assisted in the field or
<br />laboratory.-RoeERr T. Mures, Larval Fish Lab., Dept. of Fishery and Wildlife Biol., Colorado
<br />State Univ., CHARLES M. HAYNES, Nongame Research Group, Colorado Div. of Wildlife, Fort Co1-
<br />lirzs, CO 80526 and CLARE\CE A. CARI$ON, Dept. of Fishesy and Wildlife Biol., Colorado Slate
<br />= Univ., Fort Collins, CO 80523.
<br />CHOKING OF COLORADO SQUAWFISH, PTYCHOCHEILUS LUCIL'S (CYPRINIDAE),
<br />ON CHANNEL CATFISH, ICTALURUS PUNCTATUS (ICTALURIDAE), AS A CAUSE OF
<br />MORTALITY.-The Colorado squawFish (Ptychocheilus Lucius), the largest cyprinid native to
<br />• North America, is endemic to the Colorado River basin and historically was widely distributed in
<br />the Colorado River and its major tributaries. Following major water developments on the Colo-
<br />rado River system, this species declined and is now listed by the U.S. Fish and Wildlife Service as
<br />endangered (U.S. Fish Wildlife Service, Federal Register 39:1175, 1974).
<br />Historically the Colorado squawfish has preyed on soft-rayed fishes. Early in [his century the
<br />channel catfish (Ictalurus purzctatus) was established in the Colorado River system as a sport fish.
<br />Since that time, there were undocumented reports of Colorado squawFish Found dead with channel
<br />catfish lodged in their esophagus (Seethaler, ~f.S. Thesis, Utah State University, Logan, 1978;
<br />Vanicek and Kramer, Trans. Amer. Fish. Soc., 98:193-208, 1969). Until now, the dnly documented
<br />evidence of this phenomenon was reported by ;VlcAda (Southwestern Nat. 29:119-]20, 1983) who
<br />collected an adult Colorado squawfish (550 mm TL) with a channel catfish (120 mm TL) lodged
<br />head first in its esophagus, posterior to the gill arches.
<br />Some concern exists that the lodging of juvenile channel catfish in throats of Colorado squaw-
<br />fish could be one of the causes of the decline in numbers of this endangered species. Hence, a
<br />laboratory experiment was designed to study the phenomenon in more detail. Objectives of the
<br />study were to 1) determine if adult Colorado squawfish would prey on channel catfish without
<br />prior conditioning, 2) determine if channel catfish could be swallowed or tend to lodge in [he
<br />esophagus, and 3) determine if Colorado squawfish are injured in the process of trying to eat
<br />channel catfish. Since the completion of this study, three more cases of Colorado squawfish chok-
<br />ing on channel catfish were reported from the Green River in L'tah. This previously unpublished
<br />information is included here and contrasted with the laboratory study.
<br />Colorado squawfish used in the laboratory experiment were hatched in 1974 and reared at the
<br />Willow Beach National Fish Hatchery. Sixty individuals ranging in length from 360 to 512 mm
<br />TL (z = 949 mm, SD=29 mm) were used in these tests. Fish were fed pelleted commercial trout
<br />feed or rainbow trout (Sa1mo gairdneri) on alternating days. Channel catfish were hatched in June
<br />1982 and reared at a commercial hatchery in southern Idaho. Leng[hs ranged from 51 to 125 mm
<br />TL (x = 77, SD = 10.5, n=600) at the time o[ testing. Both species were held at 15 t 1 C and a
<br />photoperiod o[ 19L:lOD in Logan City dechlorinated water.
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