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~~q~~~~Py`~~'~ i TxE Sovz~xw>•as~rExN NA~ruxAl.IST 39(4):371-374 ~~ DECEMSER 1994 PREDATION BY ODONATE NYMPHS ON LARVAL RAZORBACK SUCKERS (XYRAUCHEN TEXANUS) UNDER LABORATORY CONDITIONS MICHAEL J. HORN, PAUL C. MARSH, CORDON MUELLER, AND TOM BURKE Debartment of Zoology, Arizona State University, Tem~ie, AZ 85287 (MJH) Center for Environmental Studies, Arizona Slate University, Tem~be, AZ 85287 (PCM) National Biological Survey, Denver, CO 80225 (GMJ U.S. Bureau of Reclamation, Boulder City, NV 89006 (TB) Present address of MJH: U.S. Bureau of Reclamation, Boulder City, NV 89006 Aas'rxACZ'-High larval mortality has plagued efforts to raise razorback suckers (Xyrauchen texanus) in a Lake Mohave, Arizona-Nevada backwater. Observations indicate odonate nymph densities may be high enough to impact larval survival. In laboratory tests conducted in aquaria, damselfly (Coenagri- onidae: Enallagma sp.) and dragonfly (Libellulidae: Tramea sp.) nymphs consumed 81% and 76% respectively of 11.8 ± 0.7 mm total length larval razorbacks in 7 days compared to 12% mortality in controls. Larger razorback larvae (14 to 15 mm TL) were less susceptible than smaller fish, showing 53% mortality versus 18% in controls. Extensive growth of sago pondweed (Potamogeton pectinatus) may exacerbate predation effects in the backwater, by allowing odonates access to more of the water column. Predation on early life-stages is an important determinant of cohort strength in many fishes (Crowder, 1980; Hunter, 1981). Research, how- ever, has been directed primarily toward evalu- ating piscine predator impacts upon larval stages of fishes while relatively little attention has been directed toward other predator groups. Fresh- water insects, in particular, aquatic nymphs of dragonflies and damselflies are voracious pred- ators whose ability to impact invertebrate pop- ulations is well documented (Bennett, 1970; Benke and Benke, 1975; Clausen, 1972; Wissinger, 1988). Odonates often consume small fishes, and their population densities may become high enough to influence fish populations (Wilson, 1918; Wright, 1946; Pritchard, 1964; Tave et al., 1990). However, this potential impact on fish recruitment has often been ignored. The razorback sucker (Xyrauchen texanus) is an endangered Colorado River basin endemic, the largest remaining population of which occupies Lake Mohave, Arizona and Nevada. Although spawning is successful annually and larvae are produced in abundance, recruitment to juvenile size has been undetected over almost four decades (Minckley, 1983; Minckley et al., 1991). Unless augmented, the population, which is composed of old fish, is soon expected to disappear as aging adults perish (McCarthy and Minckley, 1987; Minckley et al., 1991). As a means of population augmentation, ra- zorback sucker adults were transferred from the body of Lake Mohave to an adjacent, fishless backwater where they spawned naturally. Larvae thus produced grew to sub-adult size in 1985, but subsequent propagation attempts were plagued with high mortality of fish <20 mm long (Marsh and Langhorst, 1988; Minckley et al., 1991). Odonate nymphs appeared to be abundant enough (unpubl. light trap collections) in 1992 to pose an apparent threat. This paper investigates whether damselfly (Coenagrionidae: Enallagma sp.) and dragonfly (Libellulidae: Tramea sp.) nymphs, the two most common species in the study area, will consume larval razorback suckers under laboratory con- ditions. It is part of an evaluation of constraints to on-site production in a management program to prevent extirpation of the Lake Mohave ra- zorback sucker population.