| Summary: | Terns and noddies (subfamily Sterninae), comprising almost 15% of all seabird species, are largely piscivorous predators found from temperate to tropical regions in both coastal and pelagic systems. The largest species, the Caspian Tern (Hydroprogne caspia) has a cosmopolitan distribution, but despite this they are largely understudied, especially the finer-scale patterns of their distribution, abundance and foraging ecology throughout non-breeding periods. In south-western Australia, most H. caspia populations are restricted to two or three pairs, except for a large group of approximately 120 birds that breed annually on Penguin Island, Shoalwater Bay. This study investigated the foraging ecology of H. caspia over the first five months of their non-breeding season, from October 2018 to February 2019 on the Ramsar-listed Peel-Harvey Estuary. A single overnight roosting site for H. caspia was identified on the Estuary where a maximum of 147 birds were recorded in mid-February 2019. Tern foraging activity was concentrated at six main areas across the estuary, although tern counts and foraging activity varied between these areas, and foraging activity also differed significantly with time of day throughout the study period – it was greatest in the morning block. Overall, H. caspia were recorded taking 17 prey species (16 fish and one crustacean) on the Estuary, the most common being whitings (Sillaginidae), mullets (Mugilidae) and Eight-lined Trumpeter (Pelates octolineatus), comprising 35.0%, 33.9% and 14.4% respectively of their observed catch. The results of this study highlight the significance of the Peel-Harvey Estuary for the conservation of H. caspia in south-western Australia and the connectivity between the birds on the Estuary in the non-breeding period and those on Penguin Island during the breeding season. In addition, H. caspia appear to be strong candidates as biological indicators of Estuary health. The results of this study provide a basis for an ongoing monitoring plan to contribute towards the conservation and management of the birds and the environment at this Ramsar-listed site.
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