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The ecology and biology of stingays (Dasyatidae) at Ningaloo Reef, Western Australia

O'Shea, Owen (2012) The ecology and biology of stingays (Dasyatidae) at Ningaloo Reef, Western Australia. PhD thesis, Murdoch University.

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Abstract

Batoids make up a significant portion of the biomass in coastal and nearshore ecosystems, yet few data are available on the functional role and life history characteristics of rays in these environments. Given their conservative life history traits and vulnerability to extrinsic pressures, urgent information is required to further understand this little known group of fishes. The objectives of this research were to assess the biological and ecological characteristics of tropical stingrays at Ningaloo Reef, Western Australia. More specifically, I wanted to quantify the physical and biological impacts associated with predation by stingrays, prey specificity and trophic resource partitioning and age and growth of five sympatric species (Himantura uarnak, Neotrygon kuhlii, Pastinachus atrus, Taeniura lymma and Urogymnus asperrimus). A technical assessment for safe and ethical lethal sampling protocols for large dasyatid rays is discussed as a foundation to the research that was conducted for this degree. Strict codes of practice for the welfare of animals in scientific research demand up to date methodologies for ethical consideration, especially where death is an endpoint. Safe and humane techniques were developed as part of this study in order to sample the rays required using lethal methods. These techniques proved successful with both considerations met and it is hoped, will provide a framework for safe practices for any future work where lethal sampling of large, potentially hazardous demersal elasmobranchs is required. Age and growth parameter estimates were evaluated for these five species by sectioning and counting calcium band-pair deposition in vertebral samples. Due to less than ideal sample sizes on account of logistical constraints, a multi-analytical approach was adopted to optimise parameter estimates and generate realistic results. This included using a Bayesian framework to approximate the posterior distribution of the growth parameters. Growth rates of smaller-bodied species were faster than for larger-bodied species, but longevity was shorter. The oldest recorded age from these samples was 27 years and although validation was not possible, annual deposition is assumed based on previous accounts of similar species. This is the first time that growth parameter estimates in dasyatid rays have been assessed using this approach, yet the application is highly relevant for other rare, vulnerable or endangered species where optimal sample sizes may not be possible. The characterisation of ray diets was assessed through stomach content analysis from 170 individuals of five species. Five broad taxonomic prey categories were common to all species of ray; however, H. uarnak is shown to be a crustacean specialist while the remaining four species showed high levels of overlap within their diets. Assessment of the physical impacts related to stingray foraging within an intertidal embayment, previously identified as an area of intense feeding by rays, demonstrated high levels of sediment excavation. As a direct result of bioturbation by stingrays over 21 days, 760 kg of sediment was excavated from an experimental area of 1,500 m2. Predation effects by rays were examined by experimentally manipulating densities in fixed areas to prevent feeding. Results indicated that some, but not all prey-taxa differed significantly in abundance between treatment and controls. Sampling also allowed a quantitative assessment of infaunal taxa common within the Marine Park, and the potential importance as a prey source for rays, as well as other epibenthic predators. Throughout the course of this study, a new species locality record and parasite-host relationships was described for the parasitic leech Pterobdella amara and Himantura leoparda and Urogymnus asperrimus. This is the first time this leech has been encountered in Western Australia and in combination with a significant gnathiid isopod larvae infestation; the effects on an individual stingray are documented. A methods paper is also included in this thesis detailing a cost-effective method of tag attachment and retrieval for short-term tracking in reef associated elasmobranchs. Field-testing of galvanic timed releases and the practical application in tagging two individual sharks (Carcharhinus melanopterus) and three large stingrays (Pastinachus atrus n = 2 and Urogymnus asperrimus n = 1) are discussed. Preliminary results of these short-term tracks demonstrated that these methods are a rapid and effective means of tagging elasmobranchs with limited impact on the animal’s welfare. This research is the first of its kind at Ningaloo Reef and details critical functional processes and highlights the ecological significance of rays within coral reef environments. It also details current methodologies and techniques trialled for the first time within the context of ecological studies on tropical elasmobranchs. Data presented here can be used to develop or contribute to, conservation and management strategies for this overlooked, yet vulnerable group of fishes.

Publication Type: Thesis (PhD)
Murdoch Affiliation: School of Biological Sciences and Biotechnology
Supervisor: van Keulen, Mike, Meekan, M. and Thums, M.
URI: http://researchrepository.murdoch.edu.au/id/eprint/13760
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