Refining the ecological role of stingrays in coral reef ecosystems

Barbosa Martins, Ana Paula (2019) Refining the ecological role of stingrays in coral reef ecosystems. PhD thesis, James Cook University.

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Coral reefs are some of the most diverse and complex ecosystems in the world. They are known for their ability to support a high diversity of marine life. Unfortunately, coral reefs and their associated marine life are under intense anthropogenic pressures; climate change, habitat loss and overfishing are some of the main threats. These pressures also have negative effects on stingrays inhabiting coral reef areas. Management and conservation of stingrays is currently hindered by a lack of knowledge on essential aspects of their ecology. Therefore, this thesis aimed to refine the roles juvenile stingrays play in coral reef habitats and determine the importance of coral reef habitat to stingray populations in a nursery area in the Great Barrier Reef. To do so, this thesis addressed five specific aims: (1) review the current knowledge on batoid nursery areas; (2) identify movement patterns and determine habitat use of juvenile cowtail stingrays Pastinachus ater; (3) evaluate the accuracy of towed-float GPS tags to assess movement patterns and habitat use of stingrays; (4) identify diel movement patterns and habitat use of juvenile mangrove whiprays Urogymnus granulatus; and (5) investigate the relative trophic relationships of four juvenile elasmobranch species within a communal nursery area, with an emphasis on stingrays.

Nursery areas are crucial for many elasmobranch species, providing advantages such as increased access to prey and reduced mortality. To date, batoid nurseries have been poorly studied in comparison with shark nurseries. The current decline of batoid populations worldwide, resulting in serious extinction threats, highlights the importance of better understanding these critical habitats. This dissertation presented a synthesis of the available knowledge on batoid nurseries and suggests the use of a combination of well-established criteria to standardize batoid nursery definition.

Two telemetry approaches were used to examine movement patterns and habitat use of two stingray species commonly found in coral reef habitats: cowtail stingrays Pastinachus ater and mangrove whiprays Urogymnus granulatus. First, active acoustic telemetry was used to investigate cowtail stingray movements. Active acoustic telemetry provided fine-scale results, but had some limitations (e.g. potential human disturbance and difficulties in performing night tracks). Therefore, a new method was developed – towed-float GPS telemetry –- to investigate mangrove whipray activity patterns and habitat use. Lastly, stable isotope analysis was used to define the contribution of potential carbon sources in the food web and the trophic position and relationship of four juvenile elasmobranch species (mangrove whipray, cowtail stingray, blacktip reef shark Carcharhinus melanopterus and giant shovelnose ray Glaucostegus typus).

Active acoustic telemetry results generated a total of 14 active tracks of cowtail stingrays ranging from 4.91 to 9 hours. Cowtail stingrays moved at an average speed of 2.44 m.min⁻¹ + 0.87 SE, with minimum distances travelled ranging from 546 to 1446 meters. Tracking data showed that juvenile cowtail stingrays move in response to tidal cycles, moving faster and with straighter pathways during incoming and outgoing tides, compared to low and high tides. Juvenile cowtail stingrays also showed a strong affinity to sand flat areas and mangrove edge areas, but were infrequently detected in mangrove root habitats. These areas provide food resources and potential refuges for juvenile rays to avoid potential predators. Reef crest habitats were identified as secondary refuge for juveniles during the lowest tides.

Towed-float GPS tags were tested on juvenile stingrays with active tracking performed simultaneously for comparison. Individuals travelled 1332.15 ± 269.58 m SE across Pioneer Bay at an average speed of 6.87 m.min⁻¹ and average tracking time of 3.7 hours. Stationary tests demonstrated that the quality of the data obtained by towed-float GPS tags could not be matched by active, acoustic or ARGOS telemetry – on average reaching 99% of successful location recording and <15 meters accuracy. Location Error varied significantly based on the number of satellites detected, with error decreasing as satellite number increased.

Towed-float GPS telemetry showed juvenile mangrove whiprays travelled distances from 394 to 2189 meters during tracks, moving at a mean rate of movement of 4.51 m.min⁻¹ ± 3.1 SE with track durations ranging from 1.5 to 9.0 hours. Juvenile mangrove whipray movements were strongly influenced by tidal cycles and rate of movement was significantly different between day and night. Individuals moved faster and chose more direct paths during the outgoing and incoming tide, and were significantly faster during the day than at night. Juvenile mangrove whiprays showed preference for mangrove root habitats during high tides. These areas are thought to reduce juvenile stingray predation risk.

Stable isotope analysis showed all juvenile elasmobranchs in Pioneer Bay are at a trophic level of ~4. Isotopic niche size of blacktip reef sharks was smaller than both stingrays, and cowtail stingrays showed the largest niche size. Results showed strong evidence of niche partitioning between mangrove whiprays and cowtail stingrays with differences in feeding strategy. Nearshore pelagic and benthic prey items (e.g. crabs, annelid worms and small baitfishes) contributed most to juvenile elasmobranch diets, while mangrove or offshore prey and carbon sources appear to not have significant input.

Results of telemetry and stable isotope analysis revealed juvenile stingrays were fully dependent on the Pioneer Bay system. This dissertation confirms juvenile stingrays play important roles as mesopredators and energetic links within the Pioneer Bay nursery area. By looking at fine scale movements and trophic relationships, this PhD provides important information to better understanding juvenile stingray's ecology, but also to support management and conservation policies.

Item ID: 63975
Item Type: Thesis (PhD)
Keywords: coral reef ecology, stingrays, ecological roles, habitat use, batoids, nurseries, elasmobranch, biotelemetry
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Copyright Information: Copyright © 2019 Ana Paula Barbosa Martins.
Additional Information:

Four publications arising from this thesis are stored in ResearchOnline@JCU, at the time of processing. Please see the Related URLs. The publications are:

Martins, A.P.B, Heupel, M. R., Chin, A., and Simpfendorfer, C. A. (2018) Batoid nurseries: definition, use and importance. Marine Ecology - Progress Series, 595. pp. 253-267.

Martins, A. P. B., Heupel, M. R., Oakley-Cogan, A., Chin, A., and Simpfendorfer, C. (2020) Towed-float GPS telemetry: a tool to assess movement patterns and habitat use of juvenile stingrays. Marine and Freshwater Research, 71 (1). pp. 89-98.

Heupel, Michelle R., Kanno, Shiori, Martins, Ana P.B., and Simpfendorfer, Colin A. (2019) Advances in understanding the roles and benefits of nursery areas for elasmobranch populations. Marine and Freshwater Research, 70 (7). pp. 897-907.

George, Lachlan W., Martins, Ana P.B., Heupel, Michelle R., and Simpfendorfer, Colin A. (2019) Fine-scale movements of juvenile blacktip reef sharks Carcharhinus melanopterus in a shallow nearshore nursery. Marine Ecology Progress Series, 623. pp. 85-97.

Date Deposited: 05 Aug 2020 00:24
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 50%
06 BIOLOGICAL SCIENCES > 0602 Ecology > 060201 Behavioural Ecology @ 50%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 100%
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