Foraging niche specialisation and resource use in tropical seabirds: implications for management

Seabirds are a group of wide-ranging species with complex life histories, and an entirely marine foraging niche. Intra-species niche specialisation in seabirds appears widespread with different populations, sexes and age groups specialising in different marine resources. Seabirds in the tropics are understudied relative to those at higher latitudes. Knowledge gaps in tropical seabird foraging ecology lead to uncertainty about the prevalence of niche specialisation in tropical seabirds. Filling these gaps not only advances our understanding of how tropical seabirds use marine resources but also has important conservation management implications. This study uses regional eastern Australia to investigate three aspects of tropical seabird foraging ecology thought to drive intra-species foraging niche specialisation. The applied ecological findings of each investigation are then translated into direct management actions aimed at safeguarding the regional marine resources of the study populations and tropical seabirds in general. Firstly, multiple years of data on foraging patterns, habitat and resource use in Wedge-tailed Shearwaters Ardenna pacifica breeding on a tropical and sub-tropical colony were used to investigate local adaptation of foraging strategy. Consistent dual-foraging was observed over three years in the tropical shearwater population, whereas the sub-tropical population switched from unimodal to dual-foraging and then back again. Chicks from the tropical population were consistently provisioned with local resources from southern Great Barrier Reef waters whereas adults self-provisioned on resources obtained in the central Coral Sea. The sub-tropical population used local resources in the northern Tasman Sea to sustain chicks. This local resource was shared by adults under unimodal foraging, but under dual-foraging self-provisioning adults shifted to exploit the same Coral Sea resources as tropical conspecifics. As such, chicks and adults have differently specialised foraging niches at the tropical colony but frequently shared the same foraging niche at the sub-tropical colony. Both populations showed a degree of flexibility in their foraging strategy that is related to the long-term reliability of local resources, and used persistent and reliable at-distance resources in the central Coral Sea to buffer local resource depletion. Secondly, Wedge-tailed shearwaters from the same tropical and sub-tropical populations were used to investigate the importance facilitated foraging with tuna. Shearwater foraging behaviour was assessed relative to oceanographic covariates and predicted distributions for multiple tropical tuna species and age-classes, simulated by an existing ecosystem model (SEAPODYM). Shearwaters from both colonies undertook long-trips to deep, pelagic waters close to seamounts and foraged most often at fronts and eddies. At broad-scales, shearwaters consistently foraged in areas with higher predicted adult skipjack and micronektonic tuna densities and avoided adult Bigeye Tuna. At finer-scales, dynamic ocean features aggregated tuna of all sizes. Enhanced tuna density at these locations increased the likelihood of shearwater foraging activity. Long-trips in the tropics targeted oligotrophic waters with higher tuna densities. Long-trips in the subtropics targeted enhanced primary productivity, but in some years shifted to target the same oligotrophic, tuna-dense waters used by tropical conspecifics. As such, the foraging niche of the tropical population is consistently specialised on facilitated foraging, whereas the foraging niche of the subtropical population becomes specialised on facilitated foraging in years of low marine productivity. Finally, GPS tracking, dietary and nutritional analysis was used to investigate sex-specific foraging in a tropical Brown Booby Sula leucogaster population. Sex-specific segregation was observed in: a) foraging location: females undertook longer trips, foraging at more distant locations than males; b) foraging time: male activity and foraging occurred throughout the day, while female activity and foraging increased from midday to an afternoon peak; and c) prey type: females mostly consumed flying fish, whereas males consumed equal proportions of flying fish and squid. Brown Booby diets contained five tropical prey species that significantly differed in their nutritional composition, but despite this variation no differences were found in the overall nutritional content of prey caught by each sex. The observed sex-specific niche specialisation by prey type, location and time of capture are likely driven by a combination of a division of labour, risk partitioning and competition. However, Brown Boobies may flexibly partition foraging niche by sex in response to varying competitive and environmental pressures. Combined, these studies found intra-species niche specialisation to be prevalent in tropical seabirds from eastern Australia, and identified multiple drivers operating over several levels of resolution. Inter-population niche specialisation in Wedge-tailed Shearwaters was driven by colony-specific access to different foraging resources. Patterns of resource availability at each colony were in-turn related to differences in near-colony primary productivity and differential interactions with sub-surface predators such as tuna. Divergence in the foraging resources used by chick and adult Wedge-tailed Shearwaters was mediated by local adaptation and flexibility in adult foraging strategies, also linked to colony-specific patterns of foraging resource availability. Finally, sex-specific niche specialisation in Brown Boobies was likely driven by a combination of previously identified factors, but not convincingly by sex-specific nutritional demands. These drivers reveal the complexity and colony-specific nature of tropical seabird foraging resources. Local resource availability varies in relation to oceanography and competition, meaning tropical seabirds have to display plasticity in foraging niche. However, this work also found that some oligotrophic, pelagic tropical waters form an important and apparently reliable resource base for tropical seabirds. This study implies that the traditional view of poor quality, unpredictable resources in tropical habitats can be offset by facilitated foraging opportunities provided by high tuna biomass. Combining results on the persistence of foraging niche specialist groups and the location of required foraging resources, this study was able to assess current management of regional tropical seabird resources and suggest improvements. Most regional Brown Booby populations receive adequate protection of resources that are contained within the Great Barrier Reef Marine Park (GBRMP). Similarly, Wedge-tailed Shearwaters breeding within the GBRMP have resources that sustain their chicks relatively well protected by GBRMP zoning. However, the self-feeding resources used by these Wedge-tailed Shearwater populations and conspecifics in sub-tropical waters fall under the management of the Australian Marine Park Network, which is currently a 'paper park' network. The successive reduction of high protection areas in revisions of this network and the priority given to industry have led to important seabird resources being zoned within areas primarily managed by fisheries. By providing comprehensive data on spatial use, scientific rationale for resource requirements, and ecological insight to reduce conflicts with other stakeholders, this study provides a road map for inclusion of tropical seabirds in regional zoning activities