We tested the efficacy of a stereo camera (SC) system adapted for use with a remotely operated vehicle (ROV) to estimate fish length distributions at reef sites in the northern Gulf of Mexico. A pool experiment was conducted to test the effect of distance (1, 2, 3 or 5 m), angle of incidence (AOI; 0° to 40° at 5° increments), and SC baseline distance (BD; BD1 = 406, BD2 = 610, and BD3 = 762 mm camera separation) on the accuracy and precision of fish model length (288, 552, or 890 mm fork length) estimates compared to a red laser scaler (RLS). A field experiment was then conducted at 20 reef sites with SCs positioned at BD1 to compare fish length distribution estimates between the SC and RLS systems under in situ conditions. In the pool experiment, mean percent errors were consistently within the a priori selected threshold of ±5% at AOIs ≤10° at all distances with all four systems. However, SCs produced accurate estimates at AOIs up to 30° at all distances tested; 2–3 m was optimal. During reef site surveys, SCs collected 10.4 times as many length estimates from 4.3 times as many species compared to the RLS. Study results demonstrate that, compared to laser scalers, ROV-based SC systems can substantially increase the number of available fish length estimates by producing accurate length estimates at a wider range of target orientations while also enabling measurements from a greater portion of the cameras’ field of view.
Marbled Grouper Dermatolepis inermis were categorized as “near threatened” by the International Union for Conservation of Nature in 1996, and the status of the species was set for review in 2018. Analysis used to support its global threatened status included basic parameters, such as numbers seen in a year and spatial maps of positive observations, but included no statistical approaches to interpret abundance trends. In an effort to improve the understanding of Marbled Grouper status and trends, we used the Southeast Area Monitoring and Assessment Program reef fish video survey time series in the northern Gulf of Mexico, USA, to estimate relative abundance, proportion of positive stations, and the impact of habitat variables on fish abundance. Marbled Grouper were consistently observed in low numbers on the Louisiana shelf‐edge banks throughout the survey. Generalized linear mixed models using a negative binomial data distribution showed significant effects for the presence of reef and depth, while the percent coverage of rock was marginally significant. These results indicate that detection and abundance of Marbled Grouper are strongly associated with high rugosity and deep (60–100 m) reef tracts. Interpretation of annual trends was difficult due to high interannual variability, but the data appear to show no detectable trends. Given that the previous classification of near threatened was based primarily on the consistent but rare observation of the species in its preferred habitat on a global basis, and given that the annual indices produced for the northern Gulf of Mexico region were highly variable, we recommend that the current International Union for Conservation of Nature regional status be maintained. This study highlights the utility of optical surveys for collecting data on species that are rare or that are not observed using traditional fisheries sampling gears and also indicates the importance of synchronous collection of habitat information.
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