Use of the Robust Design to Estimate Seasonal Abundance and Demographic Parameters of a Coastal Bottlenose Dolphin (Tursiops aduncus) Population

Abstract

As delphinid populations become increasingly exposed to human activities we rely on our capacity to produce accurate abundance estimates upon which to base management decisions. This study applied mark–recapture methods following the Robust Design to estimate abundance, demographic parameters, and temporary emigration rates of an Indo-Pacific bottlenose dolphin (Tursiops aduncus) population off Bunbury, Western Australia. Boat-based photo-identification surveys were conducted year-round over three consecutive years along pre-determined transect lines to create a consistent sampling effort throughout the study period and area. The best fitting capture–recapture model showed a population with a seasonal Markovian temporary emigration with time varying survival and capture probabilities. Abundance estimates were seasonally dependent with consistently lower numbers obtained during winter and higher during summer and autumn across the three-year study period. Specifically, abundance estimates for all adults and juveniles (combined) varied from a low of 63 (95% CI 59 to 73) in winter of 2007 to a high of 139 (95% CI 134 to148) in autumn of 2009. Temporary emigration rates (γ') for animals absent in the previous period ranged from 0.34 to 0.97 (mean = 0.54; ±SE 0.11) with a peak during spring. Temporary emigration rates for animals present during the previous period (γ'') were lower, ranging from 0.00 to 0.29, with a mean of 0.16 (± SE 0.04). This model yielded a mean apparent survival estimate for juveniles and adults (combined) of 0.95 (± SE 0.02) and a capture probability from 0.07 to 0.51 with a mean of 0.30 (± SE 0.04). This study demonstrates the importance of incorporating temporary emigration to accurately estimate abundance of coastal delphinids. Temporary emigration rates were high in this study, despite the large area surveyed, indicating the challenges of sampling highly mobile animals which range over large spatial areas.

Overlooking a potential hotspot at Great Barrier Island for the nationally endangered bottlenose dolphin of New Zealand

“…This implies that some individuals leave the study area for multiple seasons but subsequently return. Temporary emigration rates estimated in Bunbury (γ' = 0.34−0.97) (Smith et al 2013) were similar to those estimated in this study (γ' = 0.03−0.75) and also varied seasonally. The apparent survival estimated here for adults and juveniles combined (0.918, SE = 0.058) is marginally lower than the estimate for adults only (0.950, SE = 0.081).…”

“…Specifically, abundance estimates varied from a low of 60 dolphins (95% CI = 53−67) during winter 2012 to a high of 131 (95% CI = 114−151) during autumn 2012 (excluding summer estimates). This is comparable to a recent study conducted on a coastal population of bottlenose dolphins in Bunbury, Western Australia, that also implemented the Robust Design (Smith et al 2013); that population also exhibited a seasonal fluctuation in abundance with peak numbers detected in summer and autumn (139 dolphins in autumn 2009) and lower numbers during winter months (63 dolphins in winter 2007). In BOI, a seasonal fluctuation was also apparent with a lower number of dolphins during winter months (TezanosPinto et al 2013).…”

“…Additionally, a recently reported mortality event at GBI involved a fatal boat strike to a juvenile bottlenose dolphin (Dwyer et al 2014). Adult survival estimates for GBI are slightly higher than those reported for BOI (0.928, SE = 0.008) for − 2006(Tezanos-Pinto et al 2013) but similar to Bunbury (0.95, SE = 0.02) for 2007−2009 (Smith et al 2013).…”

“…In Bunbury, seasonal fluctuations in abundance were explained by an influx of adult males into the study area during the breeding season (summer/ autumn) and their subsequent departure during the non-breeding months (Smith et al 2013). Breeding is also seasonal in BOI (Constantine 2002, TezanosPinto 2009) so it is possible that a similar situation occurs in GBI waters, where the sex of most dolphins is currently largely unknown, especially males and non-reproductive females.…”

Estimates of abundance and apparent survival of coastal dolphins in Port Essington harbour, Northern Territory, Australia

“…This is an advance on standard open-population models in which all immigration and emigration are assumed permanent. Examples of robust-design studies on coastal dolphins include Balmer et al (2008), Rosel et al (2011) and Smith et al (2013).…”