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Alert Diver is the dive industry’s leading publication. Featuring DAN’s core content of dive safety, research, education and medical information, each issue is a must-read reference, archived and shared by passionate scuba enthusiasts. In addition, Alert Diver showcases fascinating dive destinations and marine environmental topics through images from the world’s greatest underwater photographers and stories from the most experienced and eloquent dive journalists in the business.


LIFE AQUATIC PARROTFISH: GRAZERS OF THE REEF By Sarah Egner Parrotfish are more than just pretty faces — they may be vital for healthy coral reefs. These “beaked” and brightly colored fish delight divers, but there is much more to them than their visible characteristics. For example, the colors, patterns, shape and even sex of an individual parrotfish can vary as it moves through multiple life phases. Parrotfish are hermaphroditic, and many parrotfish species begin life as females with the ultimate goal of becoming “supermales.” Another interesting parrotfish fact is that at night some species cover themselves in a mucous cocoon that they secrete from an organ on their head. The fish remain motionless inside the cocoon, which is thought to protect them from predators and/or parasites. 1 Parrotfish teeth are fused into beaks that allow the fish to scrape food from the reef. Along with their food, the fish swallow limestone, which is ground up by the pharyngeal mill (teeth in the back of the throat) and excreted as fine sand. Most of the sand on the reef, and even on nearby beaches, is actually parrotfish poo. What these fish ingest is also important. Parrotfish spend 90 percent of their time feeding, 2 and many species of parrotfish are algal grazers. Herbivory is a key process on coral reefs that can assist reef-building corals, and more than 80 percent of herbivores on Caribbean reefs are parrotfish. 3 Herbivores remove the algae that is in constant competition with corals. The clear tropical waters that create the perfect environment for corals also provide the perfect conditions for algal growth. Macroalgae and corals are the dominant benthic groups in coral reefs and compete intensively for the available space. When corals face any kind of disturbance (e.g., bleaching, disease, hurricanes), macroalgae quickly colonize the newly available space. Today we see increasingly more reports that indicate a phase shift from coraldominated to algae-dominated reefs. STEPHEN FRINK WATERFRAME Parrotfish use their beaklike teeth to graze on algae that can enshroud a coral reef. Top: Locally harvested parrotfish are displayed for sale at a fish market in Pohnpei, Caroline Islands, Micronesia. Macroalgae and dense algal turfs can affect coral growth, settlement and survival. Algae can outshade, overgrow and abrade nearby corals. 4 Thick algal turfs can trap settlement, smothering corals. 3 Some algae can even compete allelopathically, causing coral mortality via the production of harmful chemicals. 4 Parrotfish grazing can facilitate coral recruitment by removing macroalgae. This creates space for growth 38 | SUMMER 2016

of coral or encrusting coralline algae, 3 which can promote coral larvae settlement and metamorphosis. 5 In July 2014 the Global Coral Reef Monitoring Network (GCRMN) of the International Union for the Conservation of Nature (IUCN) released a report with data collected by 78 principal investigators at 90 Caribbean reef locations in 35,000 reef surveys over 42 years, from 1970 to 2012. 6 Along with noting a 50 percent decline in living corals throughout the study period, the report also suggests that the loss of parrotfish and other grazers has had a greater negative impact on Caribbean reefs than climate change has. The study reported that the healthiest reef locations are where parrotfish are protected from overfishing (e.g., Flower Garden Banks National Marine Sanctuary, Bermuda and Bonaire). As for management, the study concluded that promoting parrotfish population growth would lead to greater resilience in coral reefs to disturbances such as warming temperatures and ocean acidification. We have already seen how a lack of herbivores can affect our reefs. Coral cover began declining throughout much of the Caribbean in the early 1980s after multiple stressors — hurricanes, coral disease and, finally, macroalgae overgrowth with the loss of the herbivorous urchin Diadema antillarum. 7 It is thought that the status of the reefs in the 1980s was made less resilient because many herbivorous fish had been heavily targeted by fishermen. The urchin was the last remaining key herbivore, resulting in the phase shift from coral to algal dominance. Overfishing continues to be a huge threat to parrotfish populations in the Caribbean. Herbivorous fish, especially parrotfish, have been heavily fished ALERTDIVER.COM | 39

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