Feeding ecology of little terns Sterna albifrons sinensis in south-eastern Australia and the effects of pilchard mass mortality on breeding success and population size

Abstract

Little terns Sterna albifrons sinensis breeding on Rigby Island, Gippsland Lakes in south-east Australia fed their chicks entirely on juvenile fish of the families Clupeidae, Engraulidae, Pomatomidae and Carangidae, including pilchard Sardinops neopilchardus, southern anchovy Engraulis australis and blue sprat Spratelloides robustus. The entrance channel to the Lakes was an important feeding site. Numbers feeding there increased on the flood tide and decreased on the ebb tide. Their dive rates followed the same pattern, suggesting they depended on shoals of juvenile fish entering the estuary during high tide. The number feeding varied from day to day, and dive rates were positively correlated with numbers, suggesting that the abundance of juvenile fish entering the channel also varied from day to day. There was no evidence that breeding success or number of breeding pairs were adversely affected by the 1995 mass mortalities of pilchard in the area. However, breeding success was reduced significantly in 1999 and 2000 following the 1998/1999 pilchard mortality. The 1995 mortality affected mainly larger size classes of pilchard, whereas the 1998/1999 mortality also affected younger age classes. This difference may explain why little terns seemed only to be adversely affected by the second mortality event.