Abstract
Indo-Pacific bottlenose dolphins (Tursiops aduncus) inhabit the coastal and estuarine waters of Perth, Western Australia, a rapidly-expanding city of 1.8 million. To assess dolphin distribution and abundance, we conducted boat-based photo-identification surveys between June 2011 and February 2013 using systematic transect routes in estuarine (Swan Canning Riverpark, SCR) and adjacent coastal areas and balanced sampling effort across seasons defined according to the Australasian calendar. Seasonal abundances were estimated using mark-recapture methods following the Robust Design. The highest overall abundance was in winter 2011 (n = 252 ± 43.6, 95% CI 180-353) and the lowest abundance in summer 2012-3 (n = 109 ± 12.7, 95% CI 86-136). Estimates of dolphin abundance within the SCR were constant (n =18 ± 1.04 individuals) over the study period. The number of groups encountered in coastal areas was the lowest during summer (n =11 ± 2.7 in 2011-2 and n = 12 ± 2.5 in 2012-3), with smaller group size in summer 2012-3 (n = 3 ± 0.6 individuals per group) than summer 2011-2 (n = 6 ± 0.9 individuals per group). Most of the individuals sighted (n = 11 of 20, 55%) in each season were observed in the SCR. Many individuals (n = 89 of 198, 45%) were only observed in two seasons, with most of these sighted in winter and spring 2011. The apparent peak in abundance in 2011 may reflect the La Niña conditions present that year, perhaps through greater prey availability (e.g. baitfish assemblages). While SCR appears to maintain a small resident community, populations in coastal areas appear less defined. Continued study will further examine seasonal and spatial trends and assess possible explanatory factors, including oceanographic conditions, dolphin ranging patterns, and limitations of the sampling design.