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,4~ December 1994 Horn et al.-Odonate predation on razorback sucker In both experiments, numerous odonate nymphs disappeared through mortality to unknown sources, such as cannibalism, or in most cases emergence. Shed exuviae and winged adults pro- vided evidence of the latter. It is unknown when losses occurred. DISCUSSION-Odonate nymphs clearly are ca- pable of destroying substantial numbers of larval razorback sucker under experimental conditions. Small larvae exposed to dragonfly nymph pre- dation experienced only 24% survival and those exposed to damselflies had 19% survival com- pared to 88% survival in control aquaria. Larger larvae had higher survival, but were still impacted heavily (47% survival with predators present compared to 82% survival of controls). Larger larvae tended to remain in the water column, where nymphs were only occasionally observed to be swimming. In contrast, newly hatched ra- zorback sucker larvae, which do not have a func- tionally buoyant gas bladder (Minckley and Gus- tafson, 1982), were incapable of sustained swimming, and spent most of their time on the aquarium bottom. Larvae in or on the substrate would presumably be more vulnerable to pred- atory odonates. Further, larger larvae in our ex- periment appeared to react quicker to odonate encounters than small larvae and often escaped. Observations of odonate attacks on larvae sug- gest that odonates randomly encounter larvae sit- ting on the bottom, or as larvae swim within reach. In the laboratory experiment, nymphs, al- thoughoccasionally observed to swim in the water column, were confined to the bottoms and sides of aquaria. Many odonates climb among aquatic vegetation and adopt an ambush strategy to cap- ture prey (Corbet, 1963). Tramea, the large li- belluide used in this experiment was the most common nymph observed in backwaters of Lake Mohave, and is commonly associated with veg- etation. Extensive growth of sago pondweed (Pot- ¢mogeton ~iectinatus) and spiny naiad (N¢jas ma- rina) in Lake Mohave backwaters may exacerbate predation by nymphs on young razorback suckers by allowing nymphs perches from which to am- bush passing fish. Historically, backwater habitats would have provided ideal shelter for young larvae. The num- ber, and species, of predatory fishes was low prior to establishment of the tremendous variety of in- troduced centrarchids, catfishes and minnows now occupying the system. Much of the decline in 373 native fish populations has been attributed to these introduced fishes. Most aquatic insect taxa, on the other hand, are native or ubiquitous in dis- tribution, posing no new or novel threat to ra- zorback sucker larvae. Within the backwaters themselves we have not, however, quantified what percentage of predation is from odonates versus other predaceous insect taxa. Under present conditions, isolated backwaters of the lake favor thick growths of vegetation and may permit insect populations such as odonates to become extremely large. In addition, many of these backwaters retain water throughout the year, allowing predators with longer generation times to enter the system, and allowing continued pres- ence of short-lived predators. Prior to construc- tion of the dams, river levels varied widely on a seasonal basis, drying up or flushing out many backwaters, thus limiting both growth of vege- tation and densities of potential predatory insects. Further, isolated backwaters do not have any fish- es that might feed on odonates. Within Lake Mo- have itself, odonate nymphs are uncommon. This is probably a result of the lack of vegetation and the presence of insectivorous fishes. Based upon our observations of the potential impacts of odonate nymphs and other insect pred- ators, several new approaches are currently under investigation to increase survivorship of larval suckers in backwaters. New backwaters have been selected that dry completely on a seasonal basis because of fluctuating lake levels. Drying kills back all macrophytes. Although macrophytes do reappear every year, young fish get a 6 to 8 week period of time when macrophytes are not present. After this time fish are too large (50 mm) to be captured by odonates or other predaceous insects that utilize macrophytes for perches. In addition, no nymphs, or other species, are able to over- winter because of the dry substrate. Populations of nymphs appear every spring following breed- ing by adult odonates moving into the area. Small odonates appear about the same time the fish are stocked, but by the time these nymphs are large enough to take fish, razorback sucker have out- grown their stage of vulnerability. Finally, larval razorback sucker are now raised to 20 mm TL before being released to backwaters in an effort to further reduce the potential for predation. In 1993, implementation of such measures resulted in a survival rate of 25% for 20 mm TL razorback sucker larvae versus 1% or less for eggs and newly hatched larvae (M. Horn, pers. obser.). Fish