HERBIVORE BEHAVIOR ON CORAL REEFS

Bonaire, Antigua, and Barbuda

 

Herbivores play a critical role in coral reef ecosystems. They facilitate coral growth and recruitment by grazing down algae that can otherwise smother and poison corals. Overfishing of herbivorous fish and a disease-induced mass mortality of Diadema sea urchins have greatly herbivory levels on Caribbean reefs, leading to a widespread shift from coral-dominated to algal dominated reefscapes.

Maximizing herbivory on coral reefs has become a primary goal for managers, researchers, and conservationists alike. Recent studies have helped reveal potential thresholds in the relationship between herbivore populations and coral health, as well as the relative role that different herbivores play in suppressing diverse algal communities. However, the mechanisms that determine the potential impact of a given herbivore population are still poorly understood.

Recent studies in both marine and terrestrial systems have highlighted the role of risk effects in driving herbivore behavior, and the potential implications these behavioral shifts can have. One of the clearest examples comes from the reintroduction of wolves in Yellowstone National Park, where the mere presence of wolves caused deer to reduce grazing in order to increase vigilance. This grazing suppression led to a spike in populations of aspen - typically a preferred snack for local ungulates.

As our global population continues to grow, the spatial overlap between human activities and wild animal populations is greater than ever. Humans today play a role in arguably every ecosystem, and coral reefs are no exception. Countless studies have documented the direct effects of humans on reef populations through fishing, habitat modification, etc. However, little is known about the potential for human activities to alter risk environments on reefs, and specifically to modify the critical grazing behaviors of herbivorous fish.

My research aims to address the following two hypotheses:

  1.  Herbivorous fish exhibit reduced grazing in areas with high risk of spearfishing
  2.  Herbivorous fish exhibit reduced grazing in areas with low densities of herbivorous fish, due to the social nature of feeding behaviors

To address these questions, I am conducting surveys of parrotfish (Sparisoma viride, Scarus vetula, and Sparisoma aurofrenatum) feeding behavior across varying sites in the Caribbean. As of now, my field sites have included the islands of Bonaire, where spearfishing has been banned since the 1970s; Barbuda, where the fishing of parrotfish was banned in 2016, and Antigua, where spearfishing is still commonplace.

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To quantify grazing behavior in these various sites, my field assistants and I conduct focal follows where we track and observe (from as great a distance as visibility allows) an individual fish for two minutes and record the amount of time it spends grazing vs. other activities. While grazing we also record the number of bites taken, which can be used to calculate both bite rates during feeding forays and total bite rates over the course of the two minute follow. To test for the potential effects of other environmental drivers as well as the role of herbivore densities, we also assess fish and benthic communities, reef structure, water quality, temperature, and visibility at each site. The levels of spearfishing and recreational dive activities at each site are scored based on local reports from fishermen, managers, dive operators, and personal observation.

To assess overall grazing levels at each site we also deploy mounted GoPro cameras for two-hour periods. We can then analyze the video to quantify the amount of grazing on a given substrate area, as well as the species, phase, and approximate size of fish responsible.