Optimal deployment durations for baited underwater video systems sampling temperate, subtropical and tropical reef fish assemblages

Baited remote underwater stereo-video systems (BRUVs) are being adopted globally as a sampling method for surveying fish assemblages. The literature demonstrates that researchers are using different deployment times, rarely with justification for the time chosen. We aimed to determine the optimal deployment time of BRUVs to sample reef fish assemblages in temperate, sub-tropical and tropical locations along the west coast of Australia. Observations of fish species richness and abundance were compared between deployment periods of 0–15, 15–30, 30–45 and 45–60 min duration. Within the subtropical and tropical locations there was no significant difference in the composition of the fish assemblage beyond 15–30 and 30–45 min respectively, suggesting 30 and 45 min would be suitable. Whereas within the warm temperate location, fish assemblages differed across all deployment periods with at least 60 min required to best characterise the fish assemblage. Most species (60–71%) and individuals (50–75%) were observed within 15 min at all locations and trends in the arrival times of individual species proved to be variable and species-specific. Critically, some larger-bodied target species continued to arrive in high proportions up to 45–60 min at all locations. Despite differences in optimal deployment durations between locations, we recommend that 60 min of imagery be collected as the cost of collecting, storing and annotating the extra imagery is minimal in comparison to the full costs of a program or recollecting imagery. Researchers may choose to analyse the first 30 or 45 min of imagery if sampling subtropical or tropical fish assemblages, yet the full 60 min is available should there be a need for standardised soak times to address questions involving spatial and temporal data collected by different users.