Are we underestimating elasmobranch abundances on baited remote underwater video systems (BRUVS) using traditional metrics?

Baited remote underwater video systems (BRUVS) are increasingly used to study fish communities, biomass, and animal behaviour. Due to the abundance of BRUVS data, there are many analysis methods. The most commonly used method for analysis of BRUV data is MaxN which refers to the maximum number of individuals observed of a species in a single frame of a video. Here, we present a novel method for BRUVS analysis that involves identifying and counting distinct individuals (MaxIND) to quantify the accuracy of MaxN. Individual oriental bluespotted maskray (Neotrygon orientalis) and the bluespotted fantail ray (Taeniura lymma) were identified on BRUVS by spot patterns, tail characteristics, and sex at three sites in Malaysian Borneo. We demonstrated that MaxIND gave abundances that were 2.4 and 1.1 times higher than MaxN for N. orientalis and T. lymma, respectively. These differences between methods were consistent for each species between sites regardless of the presence of marine reserves. However, differences in abundance estimates from MaxN to MaxIND were apparent between species, indicating that correction factors need to be developed on a species basis to better estimate true abundance. While identifying individuals is time consuming, it provides improved accuracy and information about populations. We therefore recommend the use of MaxIND when rare and endangered species are present, in high density populations, and for behavioural analyses.