Diet and habitat use of New Zealand fur seals at the western margin of their population expansion

New Zealand (NZ) fur seals (Arctocephalus forsteri) are now increasing their population size and expanding their number of breeding colonies and the extent of their range, as they recover from past exploitation from commercial sealing. This is particularly evident on the southwest coast of Western Australia (WA), which is the western margin of their distribution. While there has been significant effort directed towards researching the recovery of NZ fur seals, our understanding of how the expansion is unfolding in Western Australia is limited. The aim of this study was to help fill this gap by exploring the diet and habitat use of NZ fur seals in WA. Scats and regurgitates of NZ fur seals at Rottnest Island were examined to investigate the main prey species throughout the year, and to determine whether this marginal group displays differences in its dietary composition compared with other populations in the core of the range. The main species recovered from the scats and regurgitates were southern calamari (Sepioteuthis australis), beaked salmon (Gonorynchus greyi) and arrow squid (Nototodarus gouldi), suggesting a different diet composition from other populations. These differences are likely to be influenced by the location of Rottnest Island, on the margin of the species’ distribution. The population increase at the western edge does not seem to follow the same pattern as in the core of the Australian distribution to the east, and is unusual in that the breeding range is expanding northwards. Using data collected from three population surveys and GIS mapping tools, habitat use by fur seals was investigated to test the influence of physical characteristics of the ocean on the location of the colonies in WA.