Ecology of rays on tropical coral reefs

Sherman, Carolyn Samantha (2019) Ecology of rays on tropical coral reefs. PhD thesis, James Cook University.

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Rays are a diverse group of elasmobranchs in both their morphology and ecology. They are among the most threatened elasmobranchs according to the International Union for the Conservation of Nature (IUCN), however, little is known about their life history, behaviour or population status. To be able to improve management of declining ray populations, their distribution and ecology must be better understood. Coastal species are most at risk from anthropogenic effects, however, the extent of impacts on rays have not been widely documented. Rays serve important ecosystems functions including stabilizing food webs and acting as ecosystem engineers through bioturbation. As mesopredators, rays prey on a variety of primary consumers, while also being prey for apex predators. Because of the many linkages in food webs among apex predators to producers, food web stabilization by mesopredators prevents trophic cascades. Some rays also serve their ecosystems through bioturbation, the biological reworking of sediments. During the act of feeding, some rays create feeding pits which oxygenate sediments, provides habitat for small teleosts and crustaceans, and facilitates meiofaunal movement. While rays serve important roles in their ecosystems, there is little species-specific information available. The lack of research is partially due to the cryptic nature of rays, making them difficult to study, and partially due to the charismatic nature of their relatives the sharks, which have received much more attention. Therefore, more research is needed to address the deficits in our knowledge of ray ecology and distribution.

Baited remote underwater video systems (BRUVS) are increasingly used to study fish communities, biomass, and animal behaviour. BRUVS entail deploying baited video cameras in the absence of human presence in order to survey fish and invertebrate populations. This methodology reduces human influence on the study species and encourages more natural behaviours than with human presence. Due to the popularity of BRUVS approaches, there are many analysis methods. MaxN, which refers to the maximum number of individuals observed of a species in a single frame of a video, is the most commonly used metric of relative abundance when analysing BRUVS data. Chapter 3 presents a novel metric for BRUVS analysis that involves identifying and counting distinct individuals (MaxIND) to quantify the accuracy of MaxN. Individual oriental bluespotted maskray (Neotrygon orientalis) and the bluespotted fantail ray (Taeniura lymma) were identified on BRUVS by spot patterns, tail characteristics, and sex at three sites in Malaysian Borneo. We demonstrated that MaxIND gave abundances that were 2.4 and 1.1 times higher than MaxN for N. orientalis and T. lymma, respectively. These differences between methods were consistent for each species between sites regardless of the presence of marine reserves. However, differences in abundance estimates from MaxN to MaxIND were apparent between species, indicating that correction factors need to be developed on a species basis to better estimate true abundance. While identifying individuals is time consuming, it provides improved accuracy and information about populations. We therefore recommend the use of MaxIND when rare and threatened species are present, in high density populations, and for behavioural analyses, where distinguishing features are present.

Ecological sampling must yield consistent results in order to reliably quantify predator populations. BRUVS are increasingly being used to evaluate and monitor predator communities in marine ecosystems. Many BRUVS studies compare multiple coral reef sites sampled at a single point in time. As coral reef monitoring using BRUVS grows in its capacity to provide data relevant to sustainable management, marine protected area efficacy, and overall reef health, understanding repeatability of sampling results is vital. Chapter 4 examined the repeatability of BRUVS results for the elasmobranch community both within and between seasons (dry and wet) and years, and explored environmental factors affecting abundances at two sites in Bau Bau, Indonesia. A total of 1139 elasmobranchs (69% rays, 31% sharks) were observed on 956 BRUVS across six sampling events. Consistent results were found both in species composition and abundances within a season and across multiple years using the same sampling protocol (number and location of BRUVS). However, abundances of all sharks and rays were significantly higher in the wet season in both years. The elasmobranch community was significantly different between the two sites sampled in a consistent manner. The results demonstrated that while BRUVS are a consistent, reliable and repeatable method for surveying elasmobranchs, care must be taken in timing of sampling various regions to ensure accuracy when comparing multiple locations as season was an important factor in the results.

Coral reef ecosystems are highly dynamic environments with complex trophic interactions and environmental drivers. Rays are important members of these systems, however, in areas like Southeast Asia they are often heavily fished. Their conservation is difficult, as many countries in which they are fished do not have the capacity for effective fisheries management. For chapter 5, BRUVS were deployed at 70 reefs in 11 countries across the Coral Triangle and Australasian regions to determine ray abundances and assemblage. In 3426 BRUVS deployments, 1069 ray individuals were observed. The three most abundance species / genera were maskrays (Neotrygon spp.), fantail rays (Taeniura spp.), and eagle rays (Generas: Aetobatus, Aetomylaeus, Myliobatus). Ray assemblage was relatively consistant across the study area, however, ray abundances varied greatly with only a single individual in Vietnam to a very high abundance of rays in Indonesia. The differences in abundances are likely a reflection of fishing pressure and fisheries management. In countries with low fishing pressure, communities were species rich in both rays and sharks. Countries with moderate fishing pressure began to lose species richness, especially of sharks, although abundances of rays remained similar. With high fishing pressure, only small productive species of rays were present, and these were abundant due to the lack of top predators (i.e. sharks). Finally, in countries with extremely high fishing pressure, even productive species were absent. In order to conserve rays and their ecosystem services, fisheries management must be addressed. In some cases this requires fisheries management implementation and in some cases may involve increased management efficacy. Additionally, habitat quality and characteristics also affect the ray community at finer scales than fishing pressure. Benthic relief was most important to all rays with some species preferring low relief areas and some preferring high relief (coral dominated) areas. Thus, in addition to fisheries management, habitat quality and conservation is also important for ray species.

Sharks are decreasing in abundance in many coral reef habitats, but the ecosystem effects of this loss are poorly understood. Rays are a prevalent mesopredator in tropical coral reef ecosystems experiencing low fishing pressure that are preyed upon by top predators like sharks. Across Southeast Asia and the Western Pacific there are varying abundances of coral reef predators that consume rays. Studies have suggested reduced predator abundances leads to increases in mesopredator abundance (mesopredator release) and potentially trophic cascades. In this study, we examined the relationship between top predator abundance (sharks) and the abundance and behaviour of two genera of small benthic rays using BRUVS at 19 sites across six countries. Where predators were more abundant, the bluespotted maskray complex (Neotrygon spp.) and two species of fantail rays (Taeniura lymma and T. lessoni) were sighted less often, possibly because of lower abundances. However, small ray behaviour was significantly affected by predator abundance. Individuals of focal ray species visited BRUVS significantly fewer times at sites with higher predator abundances. Where predators were less abundant, rays spent significantly more time in the video frame, visited BRUVS more often, and were more likely to feed from bait bags. In addition to predator abundance, small ray presence was significantly influenced by relief and depth. Neotrygon spp. were more abundant on deeper, lower relief habitats, while Taeniura spp. were more prevalent in reef-associated shallow, high relief habitats. Overall, this chapter found that predator abundance had a significant effect on small benthic ray abundance and behaviour in the presence of BRUVS. The results demonstrate that changes in behaviour associated with the loss of predators may make the interpretation of phenomena like mesopredator release more challenging unless behavioural effects are taken into account.

This thesis demonstrates the many uses of BRUVS as a tool for surveying ray abundances, behaviours, and assemblage. A variety of analysis techniques were used for BRUVS data, with results proving the effectiveness of this survey method. Using the newly described metric MaxIND, more accurate abundance estimates and behavioural analyses are able to be performed in a natural setting. As there is limited data about rays on coral reefs globally, this thesis provides basic information about ray assemblages and abundances across the Coral Triangle and Australasian regions. Countries within these regions have extremely variable fishing pressure and management capacity leading towards changes in ray populations. In order to conserve ray species, improved fisheries management and habitat preservation are needed.

Item ID: 64608
Item Type: Thesis (PhD)
Keywords: rays, ecology, coral reefs, marine reserves, conservation
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Copyright Information: Copyright © 2019 Carolyn Samantha Sherman.
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One publication arising from this thesis is stored in ResearchOnline@JCU, at the time of processing. Please see the Related URLs field.

Chapter 3: Sherman, C. Samantha, Chin, Andrew, Heupel, Michelle R., and Simpfendorfer, Colin A. (2018) Are we underestimating elasmobranch abundances on baited remote underwater video systems (BRUVS) using traditional metrics? Journal of Experimental Marine Biology and Ecology, 503. pp. 80-85.

Date Deposited: 20 Oct 2020 04:09
FoR Codes: 05 ENVIRONMENTAL SCIENCES > 0501 Ecological Applications > 050102 Ecosystem Function @ 50%
06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 50%
SEO Codes: 96 ENVIRONMENT > 9605 Ecosystem Assessment and Management > 960507 Ecosystem Assessment and Management of Marine Environments @ 50%
96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 50%
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