Critical foraging locations and oceanographic relationships for Great Barrier Reef breeding seabirds

McDuie, Fiona (2016) Critical foraging locations and oceanographic relationships for Great Barrier Reef breeding seabirds. PhD thesis, James Cook University.

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Abstract

Pelagic seabirds are completely reliant upon the marine environment for all resources, but conservation strategies largely ignore critical at-sea food resources. Little is known of trophic mechanisms influencing prey availability for tropical seabirds. Until now, most studies of tropical regions focused on specific oceanographic characteristics in the marine environment, and often single species, with widely varying results. Consequently, is has been difficult to unequivocally determine the relative importance of mechanisms. Therefore, this study aimed to track a tropical Procellariiform through all life phases, to analyse multiple components simultaneously and to develop a comprehensive understanding of the linkages in tropical systems and model of tropical seabird foraging ecology.

Key foraging areas for wedge-tailed shearwaters breeding in the Australian Great Barrier Reef (GBR), and related oceanographic characteristics, were identified and mapped at three spatial scales. GBR breeding shearwaters are the first tropical Procellariiform to have been found to conduct a lengthy trans-equatorial migration, moving to a single core-use non-breeding foraging ground near Micronesia, ~6000km from the breeding colony. This area was characterised by low primary productivity ([Chl a]), low wind speeds and positive sea level anomalies (SLAs). A combination of characteristics suggest that foraging occurs at the peripheries of large to mesoscale eddies where fronts likely aggregate prey. The overlap of shearwater foraging activity with the Western Central Pacific Tuna Fishery (WCPTF), and documented links between tuna and frontal systems in this region suggest that sub-surface predator activity further enhances prey availability to these non-breeding seabirds.

During breeding, shearwaters undertook long, self-provisioning trips, to reach distant foraging sites between 300km and ~1000km from the colony. Foraging areas were characterised by deep water near areas of steep bathymetric change and commonly had strong associations with positive SLAs and medium current speeds. Again, these results are consistent with shearwaters foraging in convergence zones at the periphery of eddies. Contrary to expectations, foraging sites were not associated with elevated levels of primary productivity. Furthermore, identified foraging locations were not within currently managed areas of the GBR Marine Park. An overlap with tuna fishery suggests a similarly important association with sub-surface predators in this region.

Finally, short, chick-provisioning trips, were undertaken within 300km of the colony. This conclusively demonstrated the use of foraging grounds at two separate spatial scales during chick-rearing. Five separate topographically unique foraging zones were used during these short trips. All were in areas of steep bathymetric change where the activity of the 'Capricorn Eddy' generated frontal activity. A range of oceanographic parameters was shown to influence foraging activity, their relative importance differing among zones and seasons. More intense foraging was associated with rapidly changing sea-surface temperature (SST) gradients, as well as negative SST and [Chl a] anomalies. Combined, these results demonstrate strong links to the presence, movement and intensity of the Capricorn Eddy. An additional strong association with frontal formations where freshwater river plumes merge with the Eddy in a coastal, inshore foraging zone also highlights the previously unrealised importance of terrestrial inputs on shearwater reproductive success. This is, to my knowledge, a phenomenon not previously been seen to influence foraging pelagic seabirds in tropical regions. Therefore, resources used for chick provisioning are reliant upon two unique phenomena, the Capricorn Eddy and freshwater input.

The results in this study highlight a number of significant facts relevant to shearwater foraging ecology. First, wedge-tailed shearwaters forage at three spatial scales, implying that management and conservation priorities must be considered separately for each location. Second, mesoscale oceanographic phenomena, primarily eddies, their peripheries and convergence zones, are broadly important to shearwaters, influencing foraging activity at all spatial scales. Third, elevated [Chl a] is not an important influence in general, which directly contrasts with most previous seabird studies. Fourth, SSTs are generally important, as expected, and specific, narrow temperature ranges demonstrate links with foraging areas. Fifth, that sub-surface predator interactions may be very important to adult shearwaters when self-provisioning during both breeding and non-breeding. Finally, much of the environment shown to be important to shearwaters currently has little, if any, management and/or conservation strategies in place that consider the needs of pelagic seabirds.

These data form a general model of tropical seabird foraging ecology which demonstrates the importance of mesoscale oceanic phenomena, especially eddies, convergences, and freshwater input, to foraging pelagic seabirds.

Inter-annual variations suggest that climate-driven processes can heavily affect the oceanography and dynamics of foraging grounds, supporting previous links between climate-driven variation and variations in prey availability to wedge-tailed shearwaters. Therefore, these factors should be considered in future modelling and mapping of ideal or remaining habitat. Combined, these results reveal the complexity of oceanography and ocean dynamics that drive prey availability in tropical seabird foraging areas, and highlight the fact that some previously supposed factors of importance are not necessarily so. Therefore, determining optimal foraging habitat for tropical pelagic foraging seabirds is a complicated process and multiple oceanographic parameters must be assessed to best define the trophic mechanisms and processes that drive foraging activity and prey accessibility.

This comprehensive data set can be used to determine best practise strategies for the protection and conservation of wedge-tailed shearwaters. Identified foraging locations are priority target areas in which threats to seabirds can now be assessed. Threat assessment will help ensure optimal management and conservation efforts are implemented and carried out in critical foraging locations which will help to mitigate any impacts that may arise from climate-driven variation on foraging habitat. The results can also be applied in a global context to identify internationally recognised, candidate protected areas, such as Marine Important Bird Areas (MIBAs) in tropical systems. Additionally, my findings highlight potential new selection criteria, such as some quantification of sub-surface predator interactions that should perhaps be considered for use in identifying MIBAs in tropical regions.

Item ID: 46636
Item Type: Thesis (PhD)
Keywords: Ardenna pacifica; at-distance foraging; eddies; electronic tracking; foraging behaviour; Great Barrier Reef; habitat modelling; marine important bird areas (MIBAs); migration; oceanography; overwinter; PTT satellite tracking; sea-level anomalies; seamounts; self-provisioning; sub-surface predator; tropical Procellariiform; tropical seabirds; wedge-tailed shearwaters
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Additional Information:

Publications arising from this thesis are available from the Related URLs field. The publications are:

Chapter 2: McDuie, Fiona , and Congdon, Bradley C. (2016) Trans-equatorial migration and non-breeding habitat of tropical shearwaters: implications for modelling pelagic important bird areas. Marine Ecology Progress Series, 550. pp. 219-234.

Chapter 3: McDuie, Fiona, Weeks, Scarla J., Miller, Mark G.R., and Congdon, Bradley C. (2015) Breeding tropical shearwaters use distant foraging sites when self-provisioning. Marine Ornithology, 43. pp. 123-129.

Date Deposited: 12 Dec 2016 23:38
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060201 Behavioural Ecology @ 33%
06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 34%
05 ENVIRONMENTAL SCIENCES > 0501 Ecological Applications > 050102 Ecosystem Function @ 33%
SEO Codes: 97 EXPANDING KNOWLEDGE > 970106 Expanding Knowledge in the Biological Sciences @ 34%
96 ENVIRONMENT > 9605 Ecosystem Assessment and Management > 960507 Ecosystem Assessment and Management of Marine Environments @ 33%
96 ENVIRONMENT > 9603 Climate and Climate Change > 960305 Ecosystem Adaptation to Climate Change @ 33%
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