Holzwarth, S. R., DeMartini, E. E., Schroeder, R. E., Zgliczynski, B. J., Laughlin, J. L.

Sharks and jacks in the Northwestern Hawaiian Islands from towed diver surveys 2000-2003

2006

Atoll Research Bulletin

543

257-279

"Sharks (Carcharhinidae) and jacks (Carangidae) were surveyed using towed divers at the atolls and banks of the Northwestern Hawaiian Islands (NWHI) during annual surveys from 2000 to 2003. We compared numerical and biomass densities of these predators among reefs, among habitats within atolls (forereef, backreef, channel, and lagoon) and banks (insular and exposed), and mapped the spatial distribution of predators at the reefs where they were most abundant. Shark and jack densities were both very high at two of the three pinnacles in the chain, Necker and Gardner Pinnacle. Otherwise, shark densities were highest at Maro Reef and Midway Atoll, and jack densities were highest at Pearl and Hermes Atoll and Lisianksi-Neva Shoals. Galapagos sharks (Carcharhinus galapagensis) and gray reef sharks (C. amblyrhynchos) were observed most frequently in forereef habitats within atolls, and on exposed reefs within banks. Whitetip reef sharks (Triaenodon obesus) showed no significant habitat preferences on either atolls or banks. Giant trevally (Caranx ignobilis), bluefin trevally (C. melampygus), and amberjack (Seriola dumerili) were most frequently observed in forereef habitats within atolls, although the difference was significant only for amberjack. Jack densities were similar on exposed and insular reefs within banks. Maps of the spatial distribution of Galapagos sharks at Maro Reef and Midway Atoll and giant and bluefin trevally at Pearl and Hermes and Lisianski Island-Neva Shoals showed localized hotspots (areas of high density) within these habitats. We conclude that towed-diver surveys provide an effective method to assess shark and jack populations at the remote, expansive atolls and banks of the NWHI. Continued tow surveys will enable us to monitor the status of these important apex predators in an ecosystem relatively undisturbed by humans."

FY2005 CRCP Project ID 1221; Project Title: Pacific Reef Assessment and Monitoring Program (Pacific RAMP): Biennial Monitoring for the U.S. Pacific Islands and Atolls; Principal Investigator: Rusty Brainard