A comparison of stereo-videos and visual census methods for assessing subtropical rocky reef fish assemblage

Visual census conducted by divers has remained the most common method for obtaining quantitative data on reef fish assemblages since it was first applied in the 1950s. More recently, stereo-video systems are increasingly becoming popular as a sampling method to complement underwater visual counts. Understanding which method is more suitable to estimate fish assemblage metrics for particular objectives and hypotheses is important to design effective studies. Here, we compared rocky reef fish assemblage characteristics sampled by diver-based stationary point count and transect visual census, as well as back and forth diver-operated stereo-video systems (stereo-DOVs) and baited remote underwater stereo-video systems (stereo-BRUVs) in a coastal island in Southwestern Atlantic. Diver-based methods sampled a higher richness and abundance of fishery non-target species closely associated with the reef, as well as cryptobenthic small-bodied species. Conversely, stereo-BRUVs' samples focused more on larger and mobile carnivorous fishery target species of the families Carangidae and Lutjanidae. The stereo-DOV recorded higher species richness compared to other methods and required less sample effort in the field. Limitations faced by the four methods were discussed; however, the higher sampling efficiency of the adapted back and forth stereo-DOVs, combined with the diverse advantages of video techniques, such as permanent record of data and decreased interobserver variability for species identification and estimates of body size, indicates stereo-DOVs as a highly efficient method to assess the entire fish assemblage, across feeding guilds and commercially important species. In addition, when combined with a remote technique, such as stereo-BRUVs, both of these methods efficiently capture estimates of diversity metrics as well as body size information especially for fishery target species in rocky reefs.