Shark surveys at Scott and Mermaid Reefs

A total of 103 sharks from 11 species were seen on video footage collected using four deployment methods in shallow and deep habitats around Mermaid and Scott Reefs. BRUVS were set on the seabed in lagoon waters (14-48 m) and on the sides of deep drop-offs (58-72 m) at both reefs. Single BRUVS or a stereo-BRUVS were hung at 15m range from the surface over shallow and very deep (&gt;300m) waters at Mermaid Reef. On average, the rate of sighting of sharks on video tapes was much higher on the surface hangs and much higher at Mermaid Reef. There were many fewer sharks seen at Scott Reef and these occurred much later in tape sets (generally over half-way through the tape), indicating that they were less abundant there. In particular, the sharks more valuable for the trade in fins (Silvertip Whalers Carcharhinus albimarginatus, Scalloped Hammerheads Sphyrna lewini) were not sighted at Scott Reef, but were relatively abundant at Mermaid Reef. One of the few Grey reef sharks seen at Scott Reef had a wound in its left jaw that may have been caused by fishing gear. There were clear differences amongst habitats in terms of the species composition sighted on tapes. Silvertip whalers (C. albimarginatus) were seen only in deeper waters, and White-tip Reef sharks (Triaenodon obesus) were only seen on the seabed mostly in the lagoons and reef edges. The Grey Reef Shark (C. amblyrhynchos) was seen in all habitats sampled, at the surface and on the bottom, but more commonly outside the lagoons. The largest (~3m) Tiger (Galeocerdo cuvieri) and Great Hammerhead (Sphyrna mokarran) sharks were sighted on one BRUVS in Mermaid Reef Lagoon. The northern top section of Mermaid Reef had an abundant and diverse fauna of Grey Reef, Silvertip and Scalloped Hammerhead sharks that were visibly associated with schools of Rainbow Runners (Elegatis bipinnulatus), pelagic surgeonfish (Naso hexacanthus/lopezi) and long-toms (Strongylura spp). Single individuals of the poorly-known Thresher Shark (Alopias pelagicus), Sicklefin Hound Shark (Hemitriakis spA) and Fossil Shark (Hemipristis elongata) were sighted on tapes, with the remaining species comprising Tawny (Nebrius ferrugineus) and Leopard (Stegastoma fasciatum) sharks. Few sharks fed on the bait canisters. Coarse comparisons of shark images with scale grids on the bait arms indicated the possibility that Grey Reef and Silvertip Whalers seen on the surface baited hangs in very deep water were smaller than those seen on bottom-set BRUVS in 40-70m, but this requires further image analysis. Few shark targets were seen on the stereo-video sets, and the accurate measurements of those sharks with Vision Metrology Software was not completed at the time of writing this draft report. It is recommended that further studies of these reefs, and other localities within and outside the MOU Box, are undertaken to ascertain the importance of habitat and effects of fishing in determining shark abundance. It is possible that multiple sightings of up to 6 S.lewini at one time are evidence for aggregations of these Scalloped Hammerheads occurring at Mermaid Reef, similar to the large aggregations known elsewhere in the Indian Ocean.

Tags: Baited Remote Underwater Video Stations (BRUVS), Fishing Effects, Human Dimensions | Environmental Impacts, Marine Features (Australia) | Mermaid Reef, WA, Marine Features (Australia) | Scott Reef, WA, Oceans | Marine Biology | Fish, Sharks
Organization: Australian Ocean Data Network