Influence of marine-based industries on coral health and disease

The recent global emergence of coral disease outbreaks and subsequent coral mortality is commonly linked with human activities, however almost nothing is known about the influence of marine-based industries on coral disease. Given the growing demand for coastal development and natural resource extraction in locations that overlap with coral reefs, and growth of industries that rely on coral reefs, particularly tourism and fishing, resource managers need tools to combat coral disease epizootics and prevent future outbreaks. Research presented in this thesis identifies factors associated with industries that influence coral disease and evaluates existing and potential management tools for mitigating their impacts. Concentrating tourism activities can be an effective way to closely manage high-use parks and minimise the effects of visitors on plants and animals, however, the effects of reef-based tourist facilities on coral health have not been assessed. In partnership with reef managers and the tourism industry, in Chapter 2, I test the effectiveness of concentrating tourism activities on reefs with and without permanent tourist platforms as a strategy for managing tourism on coral reefs in the Great Barrier Reef Marine Park. Coral diseases were 15 times more prevalent at reefs with offshore tourism platforms than at nearby reefs without platforms. The maximum prevalence and maximum number of cases of each disease type occurred at reefs with permanently moored tourism platforms. Diseases affected 10 coral genera from 7 families at reefs with platforms, but only 4 coral genera from 3 families at reefs without platforms. The greatest number of disease cases occurred within the spatially dominant acroporid corals, which exhibited 18-fold greater disease prevalence at reefs with versus without platforms. Neither the percent cover of acroporids nor overall coral cover differed significantly between reefs with and without platforms, which suggests that neither factor was responsible for the elevated levels of disease. Identifying how tourism activities facilitate coral disease will help ensure ongoing conservation of coral assemblages and tourism. Recreational scuba diving on coral reefs is one of the fastest growing tourism sectors globally. Although physical injury and sedimentation associated with intensive dive tourism has been documented extensively, other impacts on coral health are unknown. In Chapter 3, I compare the prevalence of 4 coral diseases and 8 other indicators of compromised health at five of the highest and lowest used dive sites around the small community-managed island of Koh Tao, Thailand. The mean prevalence of healthy corals at low-use sites (79%) was twice that at high-use sites (45%). I found a 3-fold increase in coral disease prevalence at high-use sites, and significant increases in sponge overgrowth, physical injury, tissue necrosis from sediment, and non-normally pigmented coral tissues. Sediment necrosis was strongly associated with white syndrome prevalence across all sites. Injured corals were more susceptible to skeletal eroding band disease only at high-use sites, suggesting that additional stressors associated with use intensity facilitate disease development. Unexpectedly, I observed 113 corals entangled in derelict fishing line, of which 87% had ciliates associated with skeletal eroding band disease initiating from lesion boundaries, increasing disease susceptibility 5-fold compared to non-entangled corals, an unreported mechanism of coral mortality associated with fishing gear. Use of numerous indicators of coral health increases understanding of impacts associated with rapid tourism growth. Identifying practical management strategies, such as spatially separating multiple reef-based activities, is necessary to balance the expansion of tourism and maintenance of coral health. The rapid pace of coastal development near sensitive coral reef ecosystems necessitates a comprehensive understanding of the impacts that development activities have on all aspects of coral health. While elevated sedimentation and turbidity are often cited as drivers of reef decline, their influence on coral disease prevalence has never been investigated in situ. In Chapter 4, coral health surveys were conducted along a dredging-associated sediment plume gradient to assess the relationship between sedimentation, turbidity and coral health near Montebello and Barrow Islands, Western Australia. Reefs exposed to the highest number of days under the sediment plume (296 to 347 days) had 2-fold higher levels of disease, largely driven by increases in white syndromes, and a 6-fold increase in other signs of compromised coral health, relative to reefs with little or no plume exposure (0 to 9 days). Multivariate modeling and ordination incorporating sediment exposure level, coral community composition and cover, predation and multiple thermal stress indices provided further confirmation that the level of sediment plume exposure was the main driver of elevated disease and other indicators of compromised coral health. This study provides the first empirical evidence linking sedimentation and turbidity with elevated coral disease prevalence in situ. Minimising sedimentation and turbidity associated with coastal development will provide an important management tool for controlling coral disease epizootics. A limited number of options are available for directly managing diseases in marine environments. In Chapter 5, the utility of marine reserves for mitigating coral disease was assessed for the first time in the Great Barrier Reef Marine Park. Comparisons of coral disease assemblages and the prevalence of six individual diseases among sites with protection versus sites with fishing revealed that no-take reserves resulted in a 3- fold reduction in pooled coral disease prevalence. Of the 31 explanatory factors tested, including habitat and environmental characteristics, fish assemblages, and differences in fishing gear restrictions, a multivariate regression demonstrated that protection from fishing was the primary factor explaining variability in coral disease assemblages. Further, significant partial correlations with coral damage and the abundance of derelict fishing line indicate that direct damage associated with line fishing is the primary driver of differences between protection levels. Gear restrictions within fished zones did not improve coral health, instead I found significantly greater levels of skeletal eroding band disease, white syndromes, coral damage, and derelict fishing line when gear was restricted, compared to unrestricted. Moreover, within fished zones, the prevalence of skeletal eroding band disease, coral damage, and fishing line increased with increasing proximity to the nearest reserve boundary, signifying that fishers target areas just outside of reserve boundaries due to ease of accessibility from boat moorings located within reserves or perceptions that fish stocks are less depleted near reserve boundaries. This study concludes that both protection from fishing and spatially managing use-intensity within fished areas are important strategies to improve coral health. This thesis consistently demonstrates that reducing stressors associated with marine-based industries can ameliorate coral health and alleviate the impacts of disease. Identifying and implementing effective management strategies to improve coral health represent practical tools for increasing the resilience of vulnerable reef ecosystems in a changing climate and developing world.