Cameras or Divers? How Baited Remote Underwater Videos and 'Long Swims' Underwater Visual Census Complement Each...

Conventional methods for surveying coral reefs, such as underwater visual census (UVC), often underestimate large-bodied, mobile, or cryptic fish species due to limitations in diver mobility and visual detection. To address these limitations, rapid timed 'Long Swim' (LS) UVC protocols have been developed to better capture large predator assemblages, yet their performance relative to baited remote underwater videos (BRUV) remains poorly evaluated in topographically complex oceanic reef environments. This study provides the first site-specific comparison of BRUVS and LS-UVC protocols for predatory reef fish from nine selected families on the reefs of Pemba Island, Tanzania. To ensure robustness, we implemented post hoc spatial filtering, a multivariate sensitivity analysis excluding bait-attracted Carangidae, and site-level depth- and effort-matched bootstrap resampling. Community composition differed significantly between methods (PERMANOVA p = 0.001), a pattern that persisted even when bait-attracted carangids were excluded from the analysis. After standardising for effort and depth, BRUVs detected higher species richness and a greater number of unique species, particularly mobile and deep-dwelling taxa. In contrast, LS-UVC excelled at detecting residential reef-associated taxa but recorded fewer unique species. Jaccard similarity between methods remained consistently low (< 0.2), indicating that each method samples distinct ecological components of the predator community. These findings highlight that the primary value of integrating BRUVs and LS-UVC lies in capturing complementary components of the predator assemblage rather than strictly increasing overall richness estimates. Their combined use provides a more comprehensive and ecologically meaningful assessment, essential for managing predatory reef fish communities in complex habitats and depths.