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400 The Southwestern Naturalist vol. 38, no. 4 STYROFOAM FLOATS 5-mm ENTRANCE L T 4- 11 ??\\ RL SCALP; ] 0 - z tx. FIG. 1-Schematic of a larval light trap using chem- ical light sticks (taken from Killgore, 1991). cies are spawning and rearing razorback suckers in isolated backwaters on Lake Mohave. Their goal is to raise suckers to a length of 30 cm, a size believed large enough to reduce predation from game fish when suckers are re-introduced into the reservoir. This paper describes the de- sign, use, and results of a light trap developed to monitor razorback sucker spawning success. It provides details of the first concerted attempt to collect larval catostomids with traps, and rec- ommends modifications that may improve light trap effectiveness for other species. Razorback sucker larvae exhibit a positive pho- totactic response and have been collected by using hand-held spotlights and dipnets (Marsh and Langhorst, 1988; Minckley et al., 1991; Bozek et al., 1991). Light traps have been previously used, but were ineffective for capturing larval razorback sucker (W. L. Minckley and M. A. Bozek, pers. comm.). Low capture rates were attributed to light intensity and problems asso- ciated with short battery life. Light traps of various designs have successfully captured other phototactic fish species; however, catostomids have not been targeted or collected in appreciable numbers (Kurien et al., 1952; Paul- son and Espinosa, 1975; Faber, 1982; Muth and Haynes, 1984; Gregory and Powles, 1985; Kill- gore, 1991; Dewey and Jennings, 1992). Our traps were patterned from descriptions reported by Killgore (1991) and constructed of PVC material 6 mm thick (Fig. 1). Four 20-cm lengths of pipe 10 cm in diameter were spaced 1 cm apart in a shamrock pattern and fused to two flat disks 25 cm in diameter. Killgore's design was modified by painting the tops and bottoms black to restrict illumination to the trap's sides and entrances. Floatation was removed to allow the traps to be placed directly on the substrate, and trap entrances were enlarged to 10 mm. Four potential attractants were tested inside the traps: dry dog food, cyalume chemical light sticks (11 lux), and 12-watt (2,100 lux) and 55- watt (4,500 lux) halogen bulbs. Light intensities were measured 20 cm from the light source through tap water (NTU's = 1.7) using a Sperry Model SLM-110 light meter. Previous problems with short battery life were solved by extending electrical cable from the traps to 12-volt, DC, gel- cell batteries located on shore. Field tests were conducted twice a week from 19 February to 30 April 1992, in Yuma Cove (isolated) and Arizona Bay, two razorback sucker spawning sites in Lake Mohave, Arizona-Ne- vada. Water transparency was excellent and nor- mally exceeded Secchi depths of 4-5 m at both sites. Presence of razorback sucker larvae was confirmed through collections made by hand-held spotlight and dipnet. Traps were set along shore and allowed to fish overnight. Traps were re- trieved by carefully lifting the trap off the bottom and surrounding it with a dipnet constructed of 505-µm mesh to prevent incidental escape of lar- vae. Traps were flushed with filtered water and samples were examined at the sampling site by using a portable light table. Larval fish were counted and released unharmed except for ref- erence collections. During the first two weeks of sampling only cyalume light sticks and dog food were used at depths of 1, 2, and 3 m. Although larval suckers were captured with spotlights and dipnets, the cyalume light and bait failed to attract any sucker larvae during 60 overnight trap sets. Twelve-watt and 55-watt white lights were added to the test protocol on 4 March. Sampling effort was also reduced from overnight, to sets of 1 to 2 h duration (2000-2400 h) and limited to 1-m depths. The final protocol consisted of two sets of 4 traps each, testing dog food, cyalume sticks, and 12- and 55- watt light bulbs. One set of traps was used in Yuma Cove, the other in Arizona Bay. A total of 226 trap sets were completed, rep- resenting 1,327 trap hours of effort, including 758 trap hours (n = 135) in Yuma Cove and 569 hours (n = 91) in Arizona Bay. The initial trap- ping effort using the cyalume sticks and dog food are included in this tabulation (Table 1).