Factors Influencing Capture and Retention of Razorback Sucker in Light Traps

Abstract

Light trap captures of larval, endangered Razorback Sucker Xyrauchen texanus are used to guide flow release timing from reservoirs and measure Razorback Sucker annual reproductive success in the Green River, Utah. However, little is known regarding efficacy of light traps to capture or retain larvae under variable and field-relevant environmental conditions. We conducted laboratory experiments to investigate the effects of light trap set time, fish release distance from trap, light presence, turbidity, fish density, and cover on capture and retention probabilities of five early life stages of Razorback Sucker. Mean capture probability of protolarvae prior to swim bladder development (7-9 mm TL) was 0.40 (range = 0.28-0.55) over various treatments but rose to 0.76 (0.73-0.80) after swim bladder development in later protolarvae (9-10 mm TL). Mesolarvae (11-17 mm TL), the most common life stage captured in field sampling, had a similar mean capture probability (0.86; 0.82-0.90) as later protolarvae. Capture probabilities of metalarval (mean = 0.42, range = 0.21-0.63; 15-24 mm TL) and juvenile (mean = 0.24, range = 0.20-0.28; 22-37 mm TL) life stages were lower. Larvae retention probabilities were generally > 0.75 and increased to 0.97 for juveniles. Longer set times positively influenced capture probabilities, though capture probabilities were similar in 1-, 3-, and 5m release distances and relatively high (>0.75) for most the commonly sampled life stages in standard conditions. Light presence in traps greatly increased capture and retention of larvae compared with unlit traps. Our results confirm that light traps may be useful to detect first presence of Razorback Sucker larvae in Green River backwaters each spring, which triggers Flaming Gorge Reservoir releases to inundate floodplain wetlands and for annual monitoring of reproductive success. We also provide recommendations for gear set up, deployment, and techniques to increase capture and retention probabilities of Razorback Sucker larvae.

Light Trap Effectiveness to Capture Razorback Sucker Larvae in a Large Floodplain Wetland

“…Post hoc analyses also indicated that to achieve statistical power of 0.90 with the prevailing effect size, an unreasonable number of samples for each release distance ( n = 297) would need to be collected, assuming similar sample variances. Field study estimates of release distance effects corroborated laboratory tests conducted in troughs, which showed that recapture probabilities of Razorback Sucker larvae did not vary at release distances of 1, 3, or 5 m, even when trap set times were short (de Vlaming and Bestgen 2020). The recapture of larvae released 10 m from traps (10 m is about 400–500 fish body lengths) motivated us to examine the potential mechanisms that may be involved, given that low trap light levels emitted may not even be detectable by larvae in turbid environments 10 m distant.…”

“…We also conducted additional sampling in the Leota‐10 wetland to examine the retention of larvae in light traps (de Vlaming 2019), which was tested by placing larvae batches in lit traps and determining how many were retained. Retention test durations (night tests = 8 h, sunrise and daylight tests = 1.5 h) were identical to our laboratory experiments, where retention probabilities for mesolarvae were high and typically >0.70 (de Vlaming and Bestgen 2020). In contrast to laboratory studies, we found that field retention proportions were exceptionally low (≤0.08) and also contrasted with other field sampling in riverine backwaters, where abundant Razorback Sucker larvae (over 100) were often captured and retained (T. Jones, U.S.…”

“…The finding that release abundance did not affect the proportion of larvae recaptured in light traps (Figure 2) was corroborated in laboratory tests that found no effect of density on recapture probabilities when 25 or 50 fish were released into troughs (de Vlaming 2019; de Vlaming and Bestgen 2020). This indicates that, regardless of density, larvae would be captured in about the same proportions, information which can be used to estimate larvae abundance near a light trap.…”

“…Due to habitat complexity and heavy macrophyte growth in wetlands, typical larval sampling techniques like seining are difficult and ineffective. Light traps, a passive gear commonly used for sampling larval fish in low‐velocity habitats (Secor et al 1992; Mueller et al 1993; Snyder and Meismer 1997; Marchetti et al 2004; Hedrick et al 2009; Naus and Adams 2018), are presumed effective for monitoring population characteristics (Gregory and Powles 1985; Hickford and Scheil 1999; Gyekis et al 2006; Pierce et al 2007; Catalán et al 2014; de Vlaming 2019), and laboratory studies have demonstrated their effectiveness in capturing Razorback Sucker larvae (Snyder and Meismer 1997; de Vlaming and Bestgen 2020).…”

Laboratory and field evaluation of a new light‐emitting‐diode‐lit quatrefoil light trap for sampling fish early life stages

“…The effectiveness of CL light traps was demonstrated by de Vlaming and Bestgen (2020a, 2020b), where early life stages of endangered Razorback Sucker were captured in a variety of laboratory conditions, as well as in a large floodplain wetland. Additionally, light presence was the most important factor in both capture and retention of larvae in laboratory studies, such that when light was absent from the trap, capture and retention probabilities dropped by 90% and 75%, respectively (de Vlaming and Bestgen 2020a). Also, while probability of capture declined as larvae increased in age and size, the probability of trap retention of larvae was consistently high for all life stages tested (>75%).…”