A multi-method assessment of elasmobranch diversity and abundance in the southern Red Sea

Abstract

Elasmobranchs (sharks and rays) are vital to marine ecosystems as predators exerting top-down control on lower trophic levels. Unfortunately, they are also among the most threatened vertebrate taxa, with vulnerability compounded for many species by limited actionable data. Localised species distributions and community compositions are poorly known, and baseline assessments may be biassed by the limitations of individual methodologies. Here we adopt a multi-method approach to assess elasmobranchs diversity estimates from baited remote underwater video systems (BRUVS), unoccupied aerial vehicle surveys (UAVS), and environmental DNA (eDNA), from a two-week survey of five islands in the southern Red Sea. Species richness and relative abundance (i.e., cumulative maximum number of species (MaxN) for the video surveys; relative read counts for the eDNA) were compared across methodologies. A total of 11 species of elasmobranchs were detected, with BRUVS identifying the most species (7) but lower relative abundances (27 cumulative MaxN) than UAVS (6 species, 31 cumulative MaxN). A total of five elasmobranch species were detected from 10,385 amplicon sequence variants in eDNA samples. Differences among methods reflected biases and limitations that could be mitigated adopting an integrative approach. Time- and cost-efficiency analyses showed that eDNA surveys required minimal field effort compared to visual surveys but involved greater post-field processing time. Yet, this method yields potential for larger datasets. Overall, these findings provide a foundation for facilitating top-predator assessments suggesting that multi-method surveys may offer a more holistic approach while not substantially extending the field time required to complete the survey.