Human influence on Pacific coral reef ecosystems revealed

A recent study set out to understand what coral reefs would be like in the absence of human influence, through studying extremely remote and uninhabited reefs in the Pacific.

A number of islands and atolls in the Pacific Ocean remain virtually untouched by human activity, situated hundreds of kilometers from the nearest human populations. Due to this great distance from human influence, biological communities at those isolated reef ecosystems have been used as baselines against which to assess the extent of human impacts to reef ecosystems around human-populated islands.

The study was conducted by scientists at the University of Hawai’i at Mānoa (UHM) School of Ocean and Earth Science and Technology (SOEST), the National Oceanic and Atmospheric Administration (NOAA), and the University of Victoria, and was published in the journal PLOS ONE. Researchers drew on data from nearly 40 of these islands and atolls across the central and western Pacific, including 25 unpopulated islands, to investigate the relative influence of environmental variation and human presence on reef fish assemblages.  An extensive dataset of fish abundance across the Pacific was used in this study, gathered by divers conducting visual surveys of reefs during more than 2,000 hours of underwater observation at reef sites spanning 39 U.S. Pacific islands. The full dataset includes surveys of reef fish, coral habitat and satellite-derived measurements of oceanographic conditions at each reef location including sea surface temperature, wave energy, and oceanic productivity.

The remote islands and atolls of the Pacific

The remote islands and atolls of the Pacific

Over the last decade, several studies have highlighted dramatic differences in reef fish assemblages between human-populated and remote coral reef areas, with reef fish biomass at remote reefs typically being several times that of  at human-populated areas; and with greatest differences for heavily-targeted, larger-bodied and upper trophic level fishes 1,2.

The estimates of depletion at human-populated islands from this study are broadly consistent with those previous studies. After accounting for environmental variation among the reefs, the team of scientists estimated that human presence is associated with large reductions in reef fish biomass compared to projections for an uninhabited state—20 percent to 78 percent depletion at reefs in the main Hawaiian islands, up to 69 percent depletion in the Mariana Archipelago and up to 56 percent depletion in American Samoa. At all but the most lightly populated islands, it was estimated that reef fish biomass was ~20–40% of what it would be in the absence of humans, and also that human effects were much stronger on piscivores (carnivorous animal which eats primarily fish) than on other consumer groups.

The study also found however, that the absence of humans from remote, uninhabited reef areas in the Pacific was not always associated with spectacular fish abundance. Reefs in the Northwest Hawaiian Islands, though extremely remote, had many fewer fish than uninhabited U.S. Line Islands situated close to the equator where regional and local upwelling bring nutrient-rich waters to the surface, enhancing phytoplankton production.

Based on their findings, the team of researchers caution against any assumption that the spectacular high biomass fish assemblages seen at some remote reefs represent a natural level that all reefs would attain in the absence of humans.

“The association between oceanic productivity and fish biomass that we document for Pacific reefs is an important reminder that not all coral reefs have the same capacity to sustain high fish biomass. There is natural variability among reefs that is unrelated to their history of human influence,” said Kate Hanson, a postdoctoral fellow with UH at Mānoa and a co-author on the study.

“Natural variability in fish communities amongst reefs implies that there is no single target for what a healthy reef should look like,” notes Julia Baum, Assistant Professor at the University of Victoria, Canada and a co-author on the study. “However, the consistent declines in fish abundance with even low levels of human presence suggest that fully protected no-take zones will be necessary to maintain coral reef fish communities in their natural state.

Article Reference:

  1. Ivor D. Williams, Julia K. Baum, Adel Heenan, Katharine M. Hanson, Marc O. Nadon, Russell E. Brainard. Human, Oceanographic and Habitat Drivers of Central and Western Pacific Coral Reef Fish Assemblages. PLOS ONE, 2015; 10 (4): e0120516 DOI: 10.1371/journal.pone.0120516

Photo Header Credit: 

Fleetham Dave via Getty Images

Additional References:

  1. Friedlander AM, DeMartini EE. Contrasts in density, size, and biomass of reef fishes between the northwestern and the main Hawaiian islands: the effects of fishing down apex predators. Mar Ecol Prog Ser. 2002;230: 253–264. Available: <Go to ISI>://000175588600022. doi: 10.3354/meps230253
  2.  Stallings CD. Fishery-Independent Data Reveal Negative Effect of Human Population Density on Caribbean Predatory Fish Communities. PLoS One. 2009;4. doi: 10.1371/journal.pone.0005333

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