The communal nursery area paradigm revisited: niche overlap versus niche separation among juvenile shark species of Cleveland Bay

Kinney, Michael J. (2011) The communal nursery area paradigm revisited: niche overlap versus niche separation among juvenile shark species of Cleveland Bay. PhD thesis, James Cook University.

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Abstract

The ecology of multiple sympatric juvenile shark species inhabiting a single common, or 'communal', nursery environment during early life was the focus of this thesis. Previous work on these communal shark nurseries hypothesized a number of potential advantages gained by young sharks via their use, including greater protection from predation compared to single species shark nurseries, and an abundance of dietary resources which allowed sympatric species to feed and grow to maturity with limited competition despite high levels of dietary overlap. Almost twenty years have passed since the establishment of these original hypotheses, and yet little subsequent work has been done on communal shark nurseries to test them, resulting in the persistence of these original hypotheses in numerous current scientific works. This thesis endeavors to reassess the ecology of the originally defined communal shark nursery of Cleveland Bay, in northern Queensland, Australia, with primary aims including: an evaluation of spatial, temporal, and dietary niche partitioning between sympatric species; an investigation into the potential protective advantages gained through communal nursery areas use; and finally an assessment into the potential place that communal nursery area conservation could play in the larger picture of shark management.

Sampling of shark populations occurred both on commercial fishing boats (fisheries-dependent) and on research vessels (fisheries-independent). Fisheries-depended efforts focused principally on the determination of dietary resource use through the collection of stomach contents from euthanized sharks, while fisheries-independent efforts centered on the collection of catch data to determine spatial and temporal variation in abundance. Additionally, tissue samples for stable isotope analysis were collected from sharks and teleosts captured during fisheries-independent activities. Tissue samples were used in combination with stomach content data to explore issues of dietary resource partitioning among various shark and teleost species within the nursery environment.

Of the thirteen juvenile shark species captured in Cleveland Bay, six of them were found to use the area as a nursery: Carcharhinus fitzroyensis, Rhizoprionodon acutus, Carcharhinus amboinensis, Carcharhinus sorrah, and Carcharhinus limbatus/tilstoni. Two additional species, Sphyrna mokarran, and Sphyrna lewini, were also potentially using the area as a nursery, although data for these species were limited. Off the six species found to use the bay as a nursery area, niche partitioning was apparent between each of them to varying degrees. Dietary resource partitioning among sharks revealed three general groups: 1) R. acutus and R. taylori, 2) C. fitzroyensis, and C. sorrah, and 3) C. amboinensis, and C. limbatus/tilstoni. Across all three groups of sharks the diets of each species contained a large proportion of small teleost prey, but the families of the most prevalent prey varied between groups, and species. Stable isotope values indicated a wide range of primary carbon sources which were partitioned between individual shark species adding further evidence to the hypothesis of partitioned dietary resources. Spatial and temporal separation was also apparent between the juvenile sharks in Cleveland Bay with species occupying different areas of the bay, under diverse environmental conditions, often indicating unique seasonal patterns. For example, species such as C. sorrah, occurred most frequently in the deeper, exposed waters of the bay, while the majority of other species inhabited varying areas among the shallows. Seasonal patterns suggested that species such as C. amboinensis occurred predominantly during the summer, while other species, such as R. acutus and C. limbatus/tilstoni, occurred more often during the winter and spring.

Despite the general pattern of spatial and temporal separation displayed among the sympatric juvenile sharks of Cleveland Bay, discrete species rich areas were also identified in both the shallow, 0-5 meter, and slightly deeper, 5-10 meter, waters of the bay. These species-rich areas indicate that juvenile sharks of several species, while segregated over much of their range within the nursery, can sometimes be found inhabiting the same specific areas, potentially sheltering from predators, or adverse environmental conditions, or exploiting dietary resources in more exposed sections of the bay. Therefore, these species-rich aggregations may provide juvenile sharks within communal nursery areas additional protective benefits not available to sharks in single species nurseries where such aggregations do not occur, or occur at a reduced scale.

The findings of this thesis have improved our understanding of communal shark nursery areas by revealing them to be highly diverse environments, both in terms of species composition, and patterns of individual species' habitat and dietary resource use. Contrary to original assessments, it is likely that a delicate balance of resource partitioning strategies, rather than an overabundance of resources, is what allows several juvenile shark species to utilize the same nursery environment. The understanding, and maintenance, of this balance is imperative to the management of these important early life environments. With concerns over the proliferation of identified shark nursery environments potentially leading to unmanageably vast conservation areas, communal shark nurseries could provide a useful alternative where juvenile sharks of several species could be protected via the management of a single discrete area.

Item ID: 33056
Item Type: Thesis (PhD)
Keywords: Cleveland Bay; ecology; fisheries; juveniles; management; North Queensland; nursery areas; shark conservation; sharks
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Publications arising from this thesis are available from the Related URLs field. The publications are:

Chapter 2: Kinney, Michael John, and Simpfendorfer, Colin Ashley (2009) Reassessing the value of nursery areas to shark conservation and management. Conservation Letters, 2 (2). pp. 53-60.

Chapter 5: Kinney, Michael J., Hussey, Nigel E., Fisk, Aaron T., Tobin, Andrew J., and Simpfendorfer, Colin A. (2011) Communal or competitive? Stable isotope analysis provides evidence of resource partitioning within a communal shark nursery. Marine Ecology Progress Series, 439. pp. 263-276.

Date Deposited: 08 May 2014 01:35
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060202 Community Ecology (excl Invasive Species Ecology) @ 50%
06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 50%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 100%
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