9 months ago

AD 2016 Q3

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 in some locations of the Caribbean for thousands of years. 8,9 The largest species in the Caribbean, including the rainbow parrotfish (Scarus guacamaia) and midnight parrotfish (Scarus coelestinus), are rare or absent on most Caribbean reefs. 9 Furthermore, these species are most abundant in places with little or no fishing pressure. The decline in parrotfish populations, particularly in the Caribbean, is a loss not only for the ecosystem but also for the economy. Caribbean coral reefs generate more than $3 billion annually from tourism and fisheries, 6 benefitting 38 different countries. Many people who visit these countries want to swim with large, colorful fish. Although restoring healthy parrotfish populations will make Caribbean reefs more colorful, it won’t solve every problem they face. The good news, as far as parrotfish are concerned, is that overfishing is not a universal problem like ocean acidification or global climate change. Parrotfish populations can be managed locally. A variety of herbivores is needed to help control algal growth. Herbivorous reef fishes are diverse with specific dietary preferences and assorted feeding techniques, which have different impacts on the reef. For example, parrotfish in genus Sparisoma prefer macroalgae, whereas those in genus Scarus target turf algae. 9 Parrotfish tend to be “scrapers,” while surgeonfish are “grazers.” Scrapers, which remove portions of the underlying limestone as they feed, are important in clearing new space for colonization by coral or crustose coralline algae, but scrapers also bioerode the reef. There are many important invertebrate grazers as well, such as the famed Diadema urchin. Maintaining a diverse herbivore population is key for reef resilience. Parrotfish predation may be a source of coral mortality. 10 More research is necessary to determine if the positive effects of herbivory outweigh the negative effects of corallivory. A 2012 study revealed that parrotfish coral predation intensity could increase as coral density declines. 11 Another study found that parrotfish eat polyps with the highest number of gonads and concluded “chronic grazing by parrotfishes has negative fitness consequences for reef-building corals.” 12 A contradicting study from the same year, however, stated, “Corallivory may constitute a source of acute mortality in coral recruits, but the available evidence implies that any negative impacts are outweighed by positive effects in removing algal competitors.” 3 Even the evidence that herbivorous fish can promote coral recovery on Caribbean reefs has been inconsistent. While the long-term GCRMN study found a strong correlation between healthy reefs and healthy parrotfish populations, not all studies have come to the same conclusion. Paulina Guarderas and colleagues at Oregon State University studied herbivorous fish within a marine protected area and found that protection of herbivorous fish was not associated with increases in coral cover when compared with a fished location. 13 The “herbivore/algae/healthy coral” paradigm has many layers and is extremely context dependent. Reefs are unique in structure, species and stressors, and each factor influences a reef’s resilience. Coral reefs are complex ecosystems, and the diversity that makes them such exciting places to spend a few hours are also important in keeping them healthy. Each organism has a niche — a role it plays on the reef. Many of these niches, like that of herbivorous parrotfish, are not yet completely understood, and understanding them better is perhaps more important than ever as we place increasingly more stress on reef habitats. Save a parrotfish, save a reef? Maybe, maybe not, but these colorful fish certainly play a significant role in the coral reef ecosystem. AD References 1. Grutter AS, Rumney JG, Sinclair-Taylor T, Waldie P, Franklin CE. Fish mucous cocoons: the ‘mosquito nets’ of the sea. Biol Lett. 2011; 7(2):292-4. 2. Bruggemann JH, Kuyper MWM, Breeman AM. Comparative analysis of foraging and habitat use by the sympatric Caribbean parrotfish Scarus vetula and Sparisoma viride (Scaridae). Mar Ecol Prog Ser. 1994; 112:51-66. 3. Mumby PJ. Herbivory versus corallivory: are parrotfish good or bad for Caribbean coral reefs? Coral Reefs 2009; 28(3):683-90. 4. Rasher DB, Hay ME. Chemically rich seaweeds poison corals when not controlled by herbivores. Proc Natl Acad Sci. 2010; 107(21):9683-88. 5. Tebben J, Motti CA, Siboni N, et al. Chemical mediation of coral larval settlement by crustose coralline algae. Sci Rep. 2015 June 4; 5:10803. 6. Jackson JBC, Donovan MK, Cramer KL, Lam W, eds. Status and trends of Caribbean coral reefs: 1970-2012. Gland, Switzerland: Global Coral Reef Monitoring Network, IUCN; 2014. Available at: downloads/caribbean_coral_reefs___status_report_1970_2012.pdf. 7. Hughes TP. Catastrophes, phase shifts, and large-scale degradation of a Caribbean coral reef. Science 1994; 265:1547-51. 8. Fitzpatrick SM, Keegan WF. Human impacts and adaptations in the Caribbean islands: a historical ecology approach. Earth Environ Sci Trans R Soc Edinb. 2007; 98(1):29-45. 9. Adam TC, Burkepile DE, Ruttenberg BI, Paddack MJ. Herbivory and the resilience of Caribbean coral reefs: knowledge gaps and implications for management. Mar Ecol Prog Ser. 2015; 520:1-20. 10. Rotjan RD, Lewis SM. (2005) Selective predation by parrotfishes on the reef coral Porites astreoides. Mar Ecol Prog Ser. 2005; 305:193-201. 11. Burkepile DE.(2012) Context-dependent corallivory by parrotfishes in a Caribbean reef ecosystem. Coral Reefs 2012; 31(1):111-20. 12. Rotjan RD, Lewis SM. Predators selectively graze reproductive structures in a clonal marine organism. Mar Biol. 2009; 156(4): 569-77. 13. Guarderas AP, Hacker SD, Lubchenco J. Ecological effects of marine reserves in Latin America and the Caribbean. Mar Ecol Prog Ser. 2011; 429:219-25. 40 | SUMMER 2016

RESEARCH EDUCATION MEDICINE A manatee hovers just beneath the placid surface of Three Sisters Springs in Florida’s Crystal River National Wildlife Refuge. 42 DAN WAS THERE FOR ME / 44 ADVANCED DIVING / 48 EXPERT OPINIONS / 54 SAFETY 101 56 FROM THE MEDICAL LINE / 60 SKILLS IN ACTION / 62 INCIDENT INSIGHT JAMES CARNEHAN/WATERHOUSE MARINE IMAGES ALERTDIVER.COM | 41

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