Oceanographic drivers of near-colony seabird foraging site use in tropical marine systems

Abstract

Wedge-tailed shearwaters of Australia’s southern Great Barrier Reef (GBR), obtain food for their chicks on short trips adjacent to the breeding colony. GPS tracking (from February to April in 2013 and 2014) indicated that most trips were of 1 d duration (70 to 85%) and that all were within 300 km of the colony. Oceanographic characteristics of foraging and non-foraging areas were compared to identify mechanisms driving prey availability. Foraging generally occurred adjacent to the Capricorn Shelf, where the largest oceanographic feature in the region, the Capricorn Eddy, creates increased frontal activity and upwellings. Shearwaters consistently revisited 4 bathymetrically and topographically distinct foraging zones influenced by this mesoscale eddy. In 2013, strong sea surface temperature (SST) fronts associated with relatively intense eddy activity influenced foraging activity in all foraging zones. In 2014 the dominant oceanographic factors influencing foraging were SST and chlorophyll a concentration ([chl a]) or their anomalies. However, these influences varied among zones, suggesting a weakened effect of the eddy and elevated importance of fine-scale phenomena such as localised upwellings. Foraging in the coastal foraging zone was also significantly influenced by terrestrial inputs: specifically, birds foraged in association with freshwater flood plumes with higher [chl a]. The oceanographic mechanisms underlying prey availability to shearwaters in this system are tightly linked to variations in climatic conditions. Consequently, predicted changes associated with climate change, such as increased frequency or severity of El Nino events, are likely to seriously diminish the profitability of identified foraging locations and the reproductive output of impacted colonies. Currently, most identified foraging areas are without specific management or protection status.