NOAA Survey Data Used in Statistical Model to Predict Baseline Density of Reef Sharks in U.S. Pacific Islands

Researchers in the PIFSC Coral Reef Ecosystem Division (CRED) recently teamed with several partners to analyze variations in the density of reef sharks on U.S. Pacific coral reefs in relation to several explanatory factors.

Gray reef sharks (Carcharhinus amblyrhynchos) at Kure Atoll in the Northwestern Hawaiian Islands. NOAA photo by Paula 
               Ayotte.
Gray reef sharks (Carcharhinus amblyrhynchos) at Kure Atoll in the Northwestern Hawaiian Islands. NOAA photo by Paula Ayotte.

Using statistical models, the researchers examined the relationship between the density of reef sharks observed in towed-diver surveys (conducted by CRED as part of the Pacific Reef Assessment and Monitoring Program) and human population size (number of humans living within 200 km of the reef), concurrently accounting for the influence of oceanic habitat factors including primary productivity, sea-surface temperature, reef area, and reef physical complexity. The fitted models were used to predict the density of sharks in the absence of humans. Densities of gray reef shark (Carcharhinus amblyrhynchos), whitetip reef shark (Triaenodon obesus), and the group "all reef sharks" were predicted to be much higher on reefs where the human population size was low and where primary productivity and minimum sea-surface temperature (or reef area, which was highly correlated with temperature) were higher. Model predictions of reef shark density in the absence of humans (or absent any impact of humans) were 1.1-2.4 sharks/ha for the main Hawaiian Islands, 1.2-2.4 sharks/ha for inhabited islands of American Samoa, and 0.9-2.1 sharks/ha for inhabited islands in the Mariana Archipelago. Given these results, the authors infer that the density of reef sharks has declined to 3%-10% of "baseline levels" (i.e., absent human influence) in these areas.

CRED scientists and coauthors from the University of Victoria, Scripps Institution of Oceanography (University of California San Diego), the PIFSC Fisheries Biology and Stock Assessment Branch, and the NOAA Pacific Islands Regional Office recently published the study results as "Re-creating Missing Population Baselines for Pacific Reef Sharks", in the peer-reviewed journal Conservation Biology.