Effectiveness of light traps for detecting Razorback Sucker larvae
Date
2019
Authors
de Vlaming, Catherine M., author
Bestgen, Kevin, advisor
Bailey, Larissa, committee member
Wohl, Ellen, committee member
Journal Title
Journal ISSN
Volume Title
Abstract
Current management strategies for improving the status of wild and endangered Razorback Sucker Xyrauchen texanus rely on adequate larval sampling of wetland and riverine backwater habitats to evaluate post-reproductive survival, spatial and temporal patterns of distribution and abundance, and entrainment into wetlands. One strategy uses the detection of Razorback Sucker larvae to prompt flow releases to inundate Green River floodplain wetlands, habitat which may increase survival of those early life stages. Light traps, a passive sampling gear which exploits the innate attraction of fish early life stages to light, are thought an effective gear type for sampling, but little is known of their efficacy to capture or retain larvae. Therefore, we assessed usefulness of light traps for sampling or retaining Razorback Sucker larvae under a variety of environmental conditions using laboratory and field experiments. In the laboratory, we investigated effects of light trap set time, release distance from trap, light presence, turbidity, light source, cover, and trap aperture on capture and retention probabilities of five early life stages of Razorback Sucker. Mean capture probability of protolarvae prior to the development of a swim bladder (7-9 mm total length [TL]) was 40% (28-55%) over the various treatments, but rose to 76% (73-80%) after protolarvae formed a swim bladder (9-10 mm TL). Mesolarvae (11-17 mm TL), the most commonly captured life stage in field sampling, had similar mean capture probabilities as later protolarvae at 86% (82-90%). Capture probability of metalarval (mean = 42%, range 21-63%; 15-24 mm TL) and juvenile (mean = 24%, range 20-28%; 22-37 mm TL) life stages were lower. Retention probabilities of larvae placed in traps were generally >75% and increased to 97% for juveniles, but some fish nearly always escaped. The relationship between set time and release distances of 1, 3, and 5 m on capture indicated longer set times positively influenced capture probabilities while distance had little effect. Light presence in traps greatly increased capture and retention of larvae compared to unlit traps, and indicated traps lit with light-emitting diodes (LED) increased capture of Razorback Sucker larvae due to increased light intensity when compared to chemical-light-stick-lit traps. Light trap aperture widths of 4 or 6 mm did not influence capture or retention. Overall, laboratory experiments provided valuable information on how specific variables affect capture and retention of Razorback Sucker larvae in light traps and provide a framework for interpreting and designing field studies, which we were able to subsequently carry out. Field experiments consisted of experimental releases of unmarked, single, and double-marked (immersion in oxytetracycline hydrochloride [OTC]) Razorback Sucker larvae over three nights in a managed wetland of the Green River, Utah at the Ouray National Wildlife Refuge. Batches of released larvae were paired with 1 of 12 light traps each night in various densities (10, 50, 250, 1,000 fish per trap), 3 and 10-m release distances from light traps, LED and chemical-light-stick light sources, and two release times to evaluate effects on larvae capture probabilities. In addition, batches of 25 single-marked larvae were placed in light traps and set on a fourth night in various environmental light conditions (night, sunrise, and sunlight) to evaluate effects on larvae retention. Light traps recaptured larvae each night, even with low density releases in the 53.5 ha wetland, and recapture probabilities ranged from 0 to 0.68. The LED trap capture probabilities were up to 2.5X greater than for chemical light stick traps, but capture probabilities were not influenced by release distance or larvae density. Inexplicably, retention was very low, a result inconsistent with the previous laboratory tests. Both laboratory and field experiments indicated light traps are a useful gear to monitor abundance of larvae, evaluate reproductive success of adults, and detect even low densities of larvae in large and open habitats. Additionally, light traps are suitable to detect presence of Razorback Sucker larvae in riverine backwaters each spring, the timing of which is used to begin high flow releases from Flaming Gorge Dam to inundate Green River, Utah, floodplain wetlands. Expanded ecological understanding of early life stages of Razorback Sucker will contribute to their conservation in the Colorado River basin.