Shark and fin-fish surveys of Indian Ocean atoll reefs using baited remote underwater video stations (BRUVS (TM))

Two field surveys were undertaken for this study. In June 2003, Mermaid Reef (Rowley Shoals, unfished MPA) and Scott Reef (MOU74 Box, currently fished) were surveyed. In October 2004, Ashmore and Cartier Reefs (MOU74 Box, historically fished) and Clerke and Imperieuse Reefs (Rowley Shoals, unfished MPAs) were surveyed.

In both field surveys, BRUVS (TM) were deployed in 2 modes. Firstly, at all reefs they were deployed along depth contour lines with each unit separated by approximately 400m in the shallow (5-30m) reef crest and deeper reef slope (40-70m) on the outside of reefs, and on the lagoon floor (20-30m) where these could be accessed by surface vessels. Secondly, at Scott, Ashmore and Mermaid Reefs a zodiac was allowed to drift in deep (50-300m+) water 500 m off the edge of drop offs and BRUVS (TM) hung from the side at 15 m depth.

The total numbers of benthic BRUVS (TM) deployments at each locality were: Ashmore Reef, 58; Cartier Reef, 6; Scott Reef, 24; Mermaid Reef, 30; Clerke Reef, 24; and Imperieuse Reef, 41. The majority of reef deployments occurred in deep (40-70m) water and all deployments (including zodiac deployments) were spread throughout daylight hours from 07.00 - 16.00 hrs.

In the 2003 pilot surveys, at Mermaid and Scott Reefs, interrogation of each tape provided the time the BRUVS (TM) settled on the seabed and for each shark: its species; the time of first sighting; time of first feeding at the bait; a coarse initial estimate of length; and a list of fish species. This enabled the identification of different sharks on each tape for cumulative summaries of shark visits to be developed for each BRUVS (TM) set.

In the 2004 surveys at Ashmore, Cartier, Clerke and Imperieuse Reefs, interrogation of each tape provided the time the BRUVS (TM) settled on the seabed and, for each species of shark and fish, the time of first sighting, time of first feeding at the bait, the maximum number seen together in any one time on the whole tape (MaxN), the time at which MaxN occurred, and the intraspecific and interspecific behaviour. No attempts at measurement were made in these 2004 surveys, but fish and sharks were classified as 'adult' or 'juvenile', based on their size. A 'reference collection' of images of each species was made from the BRUVS (TM) tapes, and identifications based on these images were verified by taxonomists and other fish biologists. The benthos visible in each BRUVS (TM) set was classified and an image was stored for later reference.