A comparison of density and length of Pacific groundfishes observed from 2 survey vehicles: a manned submersible and a remotely operated vehicle

Abstract

Abstract-Visual surveys of seafloor communities in deep water are becoming more common and provide fishery-independent abundance estimates that could improve stock assessments for some groundfish species. However, limitations of the survey vehicle must be considered when developing methods. To that end, we estimated densities of demersal fishes from 28 paired strip-transect surveys, using a manned submersible (a human-occupied vehicle, HOV) and a remotely operated vehicle (ROV) in 3 types of habitats (high-relief rock, low-relief mixed rock, and soft sediments) at water depths of 75-315 m off central California. Differences in fish detection, identification, and measurements were observed between vehicles (e.g., densities of unidentified fishes, unidentified rockfishes, and unidentified species of Sebastomus were significantly higher in ROV surveys). Species most closely associated with the seafloor were observed at higher densities in HOV surveys than in ROV surveys-a result possibly due to the greater reactions of fish to the ROV. The percentage of fish for which we could not estimate size was greater from video images collected with the ROV than from in situ observations made from the HOV. Results of our study will be useful for evaluation of the limitations and biases of these survey vehicles in assessments of demersal fishes.

Modern technologies and integrated observing systems are “instrumental” to fisheries oceanography: A brief history of ocean data collection

“…The use of AUVs has evolved from anecdotal “that's cool” curiosity—basically “RUVs” (recreational underwater vehicles)—to nondestructive quantitative visual survey tools (Sward et al, 2019). From in situ crewed submarine surveys and later adding video systems to uncrewed ROVs, fishery scientists and marine geologists have collaborated to observe the relationship between demersal species and physical and geological features and ultimately map fish density and biomass throughout the benthic marine habitat (Laidig & Yoklavich, 2016). Starting with coarse spatial scales, submersible surveys evolved to identifying resident fish on rock outcrops with 1 m accuracy, documenting strong fidelity of individual fish to an outcrop.…”

A comparison of two remotely operated vehicle (ROV) survey methods used to estimate fish assemblages and densities around a California oil platform

“…The authors speculated that these differences could be due to the presence of a tether on the ROV (absent from the HOV), differences between the ROV and HOV in the positioning of lighting (forward versus starboard, respectively), difference in sounds emitted by the vehicles, and the difference in the size of the vehicles. Following up on this work [ 23 ], these researchers compared fish diversities, densities, and sizes obtained during this same study. They found that 1) density estimates of taxa associated with the seafloor were higher in the HOV surveys, 2) a greater percentage of HOV-observed fishes could be sized, and 3) a higher percentage of fishes in the HOV surveys could be identified to species.…”

A Systematic Review of Remotely Operated Vehicle Surveys for Visually Assessing Fish Assemblages

“…Few studies have compared ROV with Autonomous underwater vehicles (AUVs) and manned submersibles, despite employing similar methodology. In a study comparing the abundances and lengths of fish collected by ROV and manned submersible surveys across different habitats and depths in California, manned submersibles were able to record a greater number of species, body length estimates, and abundances of species that were found closer to the seafloor (Laidig and Yoklavich, 2016). While the number of studies comparing these methods are limited, ROVs are a more practical and inexpensive tool for monitoring (Koenig et al, 2005), and possess greater maneuverability than AUVs and manned submersibles.…”