Macro-zooplankton availability for foraging seabirds and the influence of meso-scale oceanographic features

The Houtman Abrolhos Islands (28°-29°S, 114°E) are the most significant seabird breeding sites in the south-east Indian Ocean for large populations of lesser noddy (Anous tenuirostris), brown noddy (A. stolidus), sooty tern... [ view full abstract ]

The Houtman Abrolhos Islands (28°-29°S, 114°E) are the most significant seabird breeding sites in the south-east Indian Ocean for large populations of lesser noddy (Anous tenuirostris), brown noddy (A. stolidus), sooty tern (Onychoprion fuscata) and wedge-tailed shearwater (Ardenna pacifica) during the spring/summer period. The availability of neustonic fishes and squid for prey is pivotal in seabird breeding success and juvenile survival. A large set of surface macro-zooplankton samples collected in August/September 2011 across three transects radiating out 300 km from the Abrolhos, as well as strategically targeted stations in meso-scale oceanographic features was available for study. The potential prey field was examined by distance from the island colonies and oceanographic features (e.g, Leeuwin Current, warm-core and cold-core eddies) which were present throughout the duration of sampling. CTD profiles and IMOS sea-surface temperature imagery indicated strong Leeuwin Current (>20°C) activity near the shelf-edge and cooler, more saline, Sub-Tropical Surface Water (<17.5°C) in the south of the study area in which a complex eddy field was present. Settled volumes of macro-zooplankton obtained using a surface net with 1 mm mesh ranged from 40 – 377 ml/1000 m3. In total, 248 phyllosoma larvae of the Western Rock Lobster, 36 squid larvae and 1,175 larval and post larval fishes were collected. The Myctophid Hygophum hygomii was dominant in the samples and contributed approximately 9% of all fishes caught. Most of the phyllosoma larvae were stages 7 and 8, and the greatest numbers were found in warm-core eddies. This study indicated that areas of higher prey concentrations were within the foraging range of the seabirds. However, warm-core eddies could be targeted by the seabirds as they appear to have more available surface prey than the surrounding ocean.