BRUV analysis

As part of the UNDP/GEF Protected Area project, Baited remote underwater video (BRUV’s) were taken from 22 sites around Denis Island with the help of MCSS in April 2014. Sites were chosen at two of the main dive sites and then around from the North point of the island around the East coast and around the South point.  It was originally planned to have at least 4 surveys in each area but because of sea conditions and other unforeseen circumstances, in the end only 20 sites were done.

The videos were viewed and the monitoring started between 3 and 12 minutes after deployment in order avoid disturbance from boats and people.  The area surveyed covered the area between camera and bait canister and approximately 1m behind the bait canister.  All predatory fish in this area were recorded and the highest number of each species in one shot at any one time during each video was recorded.  All other sightings of megafauna were also recorded throughout the video whether they were in the survey area or not.  This included sharks, rays, turtles, Bumphead Parrotfish and other rare/unusual sightings.  The time and video of these sightings were recorded in a separate sheet.  Each shark will also be analysed to try and determine number of individuals and estimate abundance of each species. Abundance from each site was accumulated into the designated areas and the total abundance was calculated.

 

As expected the largest abundance and diversity was found at the dive sites with more being found at Batfish compared to Aquarium.  This was also true of Megafauna sightings with all sharks in this area being seen at Batfish.  Turtles and Bumphead Parrotfish were seen at both sites.  Here the size of the predators was also much larger than around the Island as expected.

The shark sightings were initially only recorded for the first 60 minutes of monitoring, but most videos were longer than that and it was found that many sharks were only attracted to the BRUV after this time and so monitoring was extended to allow monitoring of all sharks in the entire video time.  4 different species of shark were observed using the BRUV surveys; Whitetip Reef Shark (Triaenodon obesus), Grey Reef Shark (Carcharhinus amblyrhynchos), Tawny Nurse Shark, (Nebrius ferrugineus) and Sicklefin Lemon Shark (Negaprion acutidens). An additional sighting of a Black Tipped Reef Shark was also captured on video in front of the dive centre at the time of the surveys.  Sharks were analysed using sharks per hour as all the videos were different lengths.  Using sharks per hour as opposed to abundance counteracts this discrepancy.

These surveys show that Denis Island boasts a healthy population of predators, especially sharks.  The dive sites are used by the larger predators such as the adult Groupers, Twinspot Red Snapper, and larger sharks shown by the presence of Grey Reef Sharks only at the dive sites and not on the reef flats.  The shallows are used by the smaller juvenile fish such as small emperors, snapper and grouper.  Looking specifically at the Paddletail Snapper (Seen in the image below with the Black tail), which was seen both at the dive sites and the shallow sites, the size difference shows that the reef flats are used as a nursery for these species.  The shallow areas also seem to be a nursery for some shark species such as Nurse Sharks which were much smaller in the shallows compared to on the dive sites.

This data also shows Denis Island as a very important area for sharks as not only were 5 different species seen, but also very different sizes from very small juveniles, to adults show that Denis is a host to different species of sharks all through their life cycle. 

Turtles were also seen using the BRUV’s both on the dive sites and in the shallows, showing that turtles also use this area frequently.  Combining this with the shallow water surveys that were done at the same time, and the turtle nesting data, it can be seen that many turtles, both Green and Hawksbill, use this area for everything from grazing and feeding to nesting. 

These results are promising, but are far from complete. Monitoring over different seasons is essential in determining the role of Denis Islands’ reef flats as a nursery and possibly a pupping area for the different species seen in this exercise. GIF and Denis Island hope to continue gathering this important data together with MCSS to determine how best to protect these areas.