Relative influences of biotic and abiotic factors on coral reef fish biomass in the Anthropocene

Background/Question/Methods

Coral reefs are the most diverse marine ecosystems, yet intensifying disturbances from overfishing to global climate change threaten the future of these ecosystems and the livelihoods of the people who rely upon them. Several recent studies have revealed a negative relationship between humans (specifically, human population density, a proxy for fishing pressure) and reef fish biomass. Yet, these studies typically have been limited to examining data only from single island groups, and thus have not been able to disentangle anthropogenic factors from potentially confounding biophysical factors, such as oceanographic productivity and sea surface temperature. Here, we investigate the relative influences of biotic and abiotic factors on overall reef fish biomass and reef shark biomass using an ocean-basin scale standardized monitoring data set from the National Oceanic and Atmospheric Administration’s (NOAA) Coral Reef Ecosystem Division. This data set is based upon >1600 large (>1ha each) towed-diver surveys conducted at 46 reefs including the Hawaiian Archipelago, Johnston and Wake Atolls, U.S. Line and Phoenix Islands, American Samoa, and Mariana Archipelago, spanning >4,750 km latitude and >6,600 km longitude. The surveyed region ranges from extremely remote uninhabited atolls to islands with very high human population densities. It also is characterized by substantial variation in sea surface temperature, primary productivity, and habitat complexity. 

Results/Conclusions

Estimates of shark densities from towed-diver surveys were substantially lower (<10%) than published estimates from surveys along small transects (<0.02 ha), casting doubt over the inverted biomass pyramids (predator biomass greater than prey biomass) reported for pristine reefs in other studies. Models of these towed-diver surveys show that densities of gray reef sharks (Carcharhinus amblyrhynchos), whitetip reef sharks (Triaenodon obesus), and a group of “all reef sharks” increased substantially as human population decreased and as primary productivity and minimum sea surface temperature (or reef area, which was highly correlated with temperature) increased. Simulated baseline densities of reef sharks in the absence of humans were 1.1-2.4/ha for the main Hawaiian Islands, 1.2-2.4/ha for inhabited islands of American Samoa, and 0.9-2.1/ha for inhabited islands in the Mariana Archipelago, suggesting that density of reef sharks has declined to 3-10% of baseline levels in these areas.