Diversity in shark nursery area function in the Great Barrier Reef

Yates, Peter Matthew (2014) Diversity in shark nursery area function in the Great Barrier Reef. PhD thesis, James Cook University.

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Abstract

Understanding spatio-temporal patterns in habitat use is critical for assessing the ecological role of a species, its vulnerability to human impacts and environmental change, and the efficacy of conservation and management strategies. Investigating these patterns for young sharks (neonates, young-of-the-year [YOY], and juveniles) can also improve understanding of the functioning of shark nurseries, and the potential benefits of marine protected areas (MPAs). The overall objectives of this thesis were to (1) characterise spatio-temporal patterns in coastal shark distributions, (2) explore the ecological drivers of these patterns, and (3) evaluate the potential implications of heterogeneous space use for population sustainability and the efficacy of coastal MPAs.

Portfolio theory predicts that contributions from a diverse range of young shark habitats may reduce variability in the overall production of adults, and maintain population resilience. This thesis examined case studies of portfolio effects in teleost fish and evaluated the relevance and potential implications of these processes for shark populations. Environmental heterogeneity in young shark habitats can result in locally adapted habitat-use patterns and life-history traits. Therefore, young shark habitats may be differentially impacted by anthropogenic disturbance or environmental change, with different habitats being productive at different times. In addition, increased stability in productivity may occur when the effects of localised disturbance in one area are buffered by production in others. Understanding intra-population variations in abundance and habitat use, and the extrinsic drivers of these, is fundamental to understanding the potential for portfolio effects in shark populations.

In the context of portfolio effects, multi-year fishery-independent surveys of shark communities were used to investigate diversity in shark nursery area function along a 400 km stretch of coastline in north Queensland, Australia. Multivariate analyses were used to explore spatio-temporal variations in immature shark community structure. Generalised linear models were used to identify spatial, seasonal and inter-annual variations in the occurrence or abundance of young sharks, and the ecological drivers of these variations. Potential benefits of MPAs for tropical coastal sharks were explored on the basis of variations in shark abundance, length-frequency distributions and species composition. In addition, tag-recapture data from fishery-dependent and fishery-independent sources were used to investigate the movements of individuals across MPA boundaries.

A total of 1987 sharks from six families and 22 species were captured in fishery-independent surveys, with 19 species of Carcharhiniformes dominating (99.2%) the total catch. Australian sharpnose Rhizoprionodon taylori (52%) and blacktip Carcharhinus tilstoni/Carcharhinus limbatus (12%) sharks were numerically dominant. Spot-tail Carcharhinus sorrah (8%), pigeye Carcharhinus amboinensis (6%), scalloped hammerhead Sphyrna lewini (5%), milk Rhizoprionodon acutus (5%) and whitecheek Carcharhinus coatesi (5%) sharks were moderately abundant. In total, 642 sharks were classified as immature, including 383 YOY individuals. Immature sharks from 18 species were present; however, interspecific variation in life-history-stage composition was apparent. Catch data also indicated community-wide spatial structuring of sharks on the basis of body size rather than life-history stage. Multivariate analyses identified significant spatial heterogeneity in immature shark communities among bays. In addition to building on traditional shark nursery paradigms, these results demonstrated that data on nursery function from restricted areas may not accurately portray patterns occurring over broader geographic scales.

General and species-specific patterns in shark abundance were characterised by a range of biotic and abiotic variables. Relationships with turbidity and salinity were similar across multiple species, highlighting the importance of these variables in the functioning of communal shark nurseries (i.e. those used by multiple species). In particular, turbid environments were important for all species at typical oceanic salinities. Mangrove proximity, depth and water temperature were also important, however their influence varied between species. These extrinsic factors, along with intrinsic life-history-associated factors, were identified as potential drivers of interspecific variations in the occurrence of young sharks. Seasonal variations characterised the occurrence of YOY pigeye, YOY spot-tail and neonate blacktip sharks, whereby relative occurrences peaked during all or part of the summer wet season. In contrast, spatial variations were more pronounced for YOY blacktip and scalloped hammerhead sharks, with higher respective occurrences in Repulse and Rockingham Bays compared to other locations. Such varied utilisation of coastal ecosystems has important implications for the management of coastal habitats and the conservation of shark populations.

Species composition varied significantly between management zones, and overall shark abundance was higher inside MPAs. In addition, length-frequency distributions of blacktip and pigeye sharks inside MPAs comprised a greater proportion of larger-bodied individuals compared to those in open zones. Tag-recapture locations indicated repeated or prolonged occupancy of MPAs by some species. These results suggested that sub-bay-sized coastal MPAs may increase the survival of young sharks to maturity or shelter parts of breeding stocks. Therefore, MPAs do not necessarily need to be large to benefit multiple sympatric species comprising diverse life histories and habitat use patterns.

Spatial heterogeneity in shark occurrence and abundance suggested that proximate bays are likely to vary in terms of the services they provide to young sharks, and thus their level of contribution to adult populations. Further, the occurrence of young sharks appeared to be more complex than would be predicted by the timing of parturition alone. The ecological drivers of habitat use identified in this thesis may promote spatial diversity in habitat use along environmentally heterogeneous coastlines, and may therefore have important implications for population resilience. By being among the first studies to investigate the implications of heterogeneous space use by young sharks, this thesis may serve as a model to facilitate future research on portfolio effects and the associated benefits for shark populations.

Item ID: 40806
Item Type: Thesis (PhD)
Keywords: Abbot bay; Bowling Green Bay; Cleveland Bay; coastal ecology; coastal sharks; Edgecumbe Bay; fisheries; gill netting; Great Barrier Reef; habitat conservation; habitat function; Halifax Bay; Hinchinbrook Channel; Hinchinbrook Island; immature sharks; infant sharks; juvenile sharks; marine ecology; marine protected areas; population dynamics; Repulse Bay; Rockingham Bay; sampling; shark habitat; shark nurseries; shark size; sharks; tropical coast; Upstart Bay; Woodwark Bay; young sharks
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Publications arising from this thesis are available from the Related URLs field. The publications are:

Chapter 2: Yates, Peter M., Heupel, Michelle R., Tobin, Andrew J., and Simpfendorfer, Colin A. (2012) Diversity in young shark habitats provides the potential for portfolio effects. Marine Ecology Progress Series, 458. pp. 269-281.

Chapter 4: Yates, Peter M., Heupel, Michelle R., Tobin, Andrew J., Moore, Stephen K., and Simpfendorfer, Colin A. (2015) Diversity in immature-shark communities along a tropical coastline. Marine and Freshwater Research, 66 (5). pp. 399-410.

Chapter 5: Yates, Peter M., Heupel, Michelle R., Tobin, Andrew J., and Simpfendorfer, Colin A. (2015) Ecological drivers of shark distributions along a tropical coastline. PLoS ONE, 10 (4). pp. 1-18.

Chapter 6: Yates, Peter M., Heupel, Michelle R., Tobin, Andrew J., and Simpfendorfer, Colin A. (2015) Spatio-temporal occurrence patterns of young sharks in tropical coastal waters. Estuaries and Coasts, 38 (6). pp. 2019-2030.

Date Deposited: 02 Nov 2015 04:04
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 100%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 100%
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