Incorporating environmental factors is critical for determining conservation baselines for shark relative abundance of coral reefs: Insights from the world’s largest shark sanctuary.

Background/Question/Methods

: Populations of sharks and other large marine predators, including those inhabiting coral reef ecosystems, have been undergoing dramatic global declines. While the relative importance of shark declines to the overall health and resilience of coral reefs is still unclear, the magnitude of global declines and their importance in other ecosystem types suggests that rebuilding populations should be a priority in many locations. Such rebuilding efforts, however, require estimates for what shark relative abundances might be expected in the absence of intense fishing. Therefore, it is important to elucidate the factors – both environmental and anthropogenic - that drive the relative abundance of sharks as a guild, and of key species, in relatively intact reef systems to set restoration baselines. French Polynesia, the world’s largest shark sanctuary, provides an ideal model system for such efforts. We used baited remote underwater video surveys (BRUVS) to assess the relative abundance of sharks on 35 reefs, located on 17 different sites representing a diversity of island geomorphologies across the large geographic range of French Polynesia. Boosted regression trees were used to model results.

Results/Conclusions

: Island geomorphology was the most influential factor for predicting relative abundance of sharks. Overall, abundances at atolls (mean maxN = 3.88; SD = 2.44), were nearly three times greater than for high islands (mean maxN = 1.31; SD = 1.25), highlighting that conservation targets that do not take underlying environmental factors into account could lead to suboptimal restoration. Island group, temperature and net primary productivity had smaller effects, while human pressures as measured by market gravity were not a significant factor. Although models examining species specific patterns varied, our findings suggest that environmental factors, and island geomorphology in particular, should be taken into account when setting conservation targets for shark abundances. Further studies, exploring the role of interspecific interactions in driving relative abundances in the absence of large anthropogenic effects would further enhance our understanding of the structure and abundance of natural reef shark communities and optimize setting conservation targets.