Abstract
AbstractWe evaluated how the use of bait as a fish attractant influenced the species and size composition of demersal fishes viewed with a stereo video lander at 160 sampling sites at Stonewall Bank, a deepwater rocky reef complex off the Oregon coast. We also studied the effectiveness of stereo video for generating estimates of fish length and distance from the cameras. Bait altered the species composition of fish encountered, increasing the mean counts of demersal fishes by 47%, with increases of 135–250% for Rosethorn Rockfish Sebastes helvomaculatus, Northern Ronquil Ronquilis jordani, and Spotted Ratfish Hydrolagus colliei. Increases in the mean counts of 35–150% for unidentified sculpins (Cottidae), Yelloweye Rockfish S. ruberrimus, and Quillback Rockfish S. maliger were nonsignificant. The calibrated stereo video lander provided acceptably precise estimates of fish length and camera‐to‐fish distance (range of three replicate measurements less than 3 cm for length and 20 cm for distance) for 34.3% of the demersal fishes that were counted. The precision of length and distance estimates declined with increased distance; acceptable estimates of distance were typical when fish were within 200 cm and were infrequent when fish were beyond 500 cm. Bait reduced the mean distance at which acceptable estimates of length and distance were obtained from 264 cm to 200 cm, but had no effect on mean fish length for the three most frequently encountered demersal species. The combined effect of bait on demersal fish counts and mean distance more than doubled the efficiency of the stereo video lander for generating fish length and fish–camera distance estimates.Received January 22, 2014; accepted April 9, 2014
Highlights
The implementation of numerous no-take marine reserves on the U.S west coast has led to increased interest in further development of visual survey methods, for demersal fishes inhabiting rocky reefs that are too deep for diver surveys
Our second objective was to evaluate the effectiveness of a stereo video lander for generating precise fish length and camera-to-fish distance estimates
Bait Effects on Fish Counts and Time to MaxN The presence of bait altered the species composition of fish counted with the stereo video lander (Table 1; ANOSIM: P < 0.05, R = 0.017), increasing the mean counts for demersal fishes, as a group, by 47% (P < 0.01)
Summary
The implementation of numerous no-take marine reserves on the U.S west coast has led to increased interest in further development of visual survey methods, for demersal fishes inhabiting rocky reefs that are too deep for diver surveys. Landers are low-cost sampling devices designed to be used in very rugged rocky habitat and can be deployed from vessels equipped with only a hydraulic block (Hannah and Blume 2012) They have been shown to be useful for distinguishing differences in fish abundance and species assemblages and for identifying species–habitat relationships (Hannah and Blume 2012; Easton 2013). Calibrated stereo video systems have been shown to be effective for estimating these parameters in other areas and applications (Harvey et al 2001; Harvey et al 2004; Williams et al 2010; Langlois et al 2012) but have not been evaluated on a video lander for sampling temperate deepwater rocky reef fishes. Stereo video capability facilitated objectives of evaluating the effect of bait on the size composition of demersal fishes viewed and the effect of bait on the distance at which fish were being counted and measured
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