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

Abstract

Abstract, and grey shading in the table). The errors were due to transcription error and in no way affect the outcome and conclusions of the study. AbstractKey results. Abundance estimates for the three species varied across the sampling periods, with the estimated number of Australian snubfin dolphins ranging from 136 (s.e. 62) to 222 (s.e. 48); from 48 (s.e. 17) to 207 (s.e. 65) for the Australian humpback dolphin; and 34 (s.e. 16) to 75 (s.e. 26) for the bottlenose dolphin. AbstractContext. Three dolphin species occur in coastal waters of monsoonal northern Australia: the Australian snubfin (Orcaella heinsohni), humpback (Sousa sp.) and the bottlenose (Tursiops sp.). Their overall population size and trends are poorly known, and their conservation status has been difficult to resolve, but can be expected to deteriorate with likely increased development pressures.Aims. We sought to provide an estimate of abundance, and apparent survival, of the three dolphin species at the largely undeveloped harbour of Port Essington (325 km 2 ), Northern Territory, with repeated sampling over a 2.9-year period. Given increasing obligations to undertake population assessments for impact studies at proposed development sites, we assess the strengths and limitations of a systematic sampling program.Methods. We used photo-identification data collected during systematic boat-based transect surveys undertaken from 2008 to 2010 and Pollock's robust capture-recapture design model. Key results. Total abundance estimates for the three species were variable across different sampling periods. The estimated number of individuals in the sampled area varied per sampling episode from 136 (s.e. 62) to 222 (s.e. 48) for snubfin, from 48 (s.e. 7) to 207 (s.e. 14) for humpbacks and from 34 (s.e. 6) to 75 (s.e. 9) for bottlenose dolphins. Apparent survival was estimated for snubfin at 0.81 (s.e. 0.11), humpbacks at 0.59 (s.e. 0.12) and bottlenose at 0.51 (s.e. 0.17) per annum.Key conclusions.(1) The values derived here provide some of the only estimates of local population size for these species across monsoonal northern Australia; (2) population-size estimates varied considerably among seasons or sampling episodes; (3) the low apparent survival probabilities indicated that many individuals may move at scales larger than the study area; (4) density of snubfin and humpback dolphins in the present study area exceeded the few other estimates available for these species elsewhere in Australia.Implications. The present study provided the first baseline estimates of abundance and apparent survival for three coastal dolphin species in monsoonal northern Australia. Such information is becoming increasingly important as development pressures intensify in coastal areas. Sampling protocols for future monitoring and impact assessment need an enhanced consideration of seasonality and scale issues.

Site-Specific Assessments of the Abundance of Three Inshore Dolphin Species to Inform Conservation and Management

“…The abundance estimates of bottlenose dolphins at Cygnet Bay were similar to those reported for larger sites within the Northern Territory (Palmer et al, 2014;Brooks and Pollock, 2015). However, the abundance of 157 (CI 137-186) bottlenose dolphins at Beagle Bay is comparably large; this represents a density approximately comparable to those reported in highproductivity, sub-tropical embayments (e.g., Lukoschek and Chilvers, 2008;Nicholson et al, 2012), and suggests particularly favorable habitat to this species.…”

“…However, apparent survival estimates of 0.62 for humpback and 0.85 for bottlenose dolphins are indicative of permanent emigration of individuals during the course of the study, suggesting both resident and more transient components within the sampled populations (cf. Silva et al, 2009;Palmer et al, 2014). The documented movement of two bottlenose dolphin individuals between Cygnet and Beagle Bays supports this conclusion.…”

“…This method was favored over those based upon a random selection of images (e.g., Parra et al, 2006a;Palmer et al, 2014) due to potential issues of photographer bias toward more distinctive (see Read et al, 2003) or approachable individuals within a group.…”

“…Transects designed to give even coverage of the study site 3.2 As some degree of variability in the extent of overlap between individuals' home ranges and our study sites is expected, it is inevitable that some individual heterogeneity of capture probabilities was present. We did not attempt to model for such effects; incorporating covariates into capture-recapture models to address potential heterogeneity adds complexity, and may not be feasible where sample sizes are small (e.g., Palmer et al, 2014). However, any resulting downward bias in abundance estimates is expected to be minimal, as abundance estimates within a sampling period were broadly consistent with the total number of individuals identified at a study site, which in almost all cases reached a plateau (Figure 2) 4.…”

“…Globally, their distribution is restricted to shallow coastal and estuarine waters of northern Australia and southern New Guinea (Parra et al, 2002(Parra et al, , 2004Beasley et al, 2005Beasley et al, , 2016Jefferson and Rosenbaum, 2014). Available data suggest that both species are discontinuously distributed as small populations of 50-200 (e.g., Parra et al, 2006a;Palmer et al, 2014; Table 1), which exhibit site fidelity (Parra et al, 2006a;Cagnazzi et al, 2011Cagnazzi et al, , 2013a, limited gene flow between populations (Cagnazzi, 2011;Brown et al, 2014), and are reliant upon near-shore habitats (Parra et al, 2006b;Parra and Jedensjö, 2014). While snubfin and humpback dolphins are data deficient, both species have been assigned precautionary "near threatened" 1 statuses by the International Union for Conservation of Nature (IUCN) in light 1 The IUCN Red List status assessment of "near threatened" for humpback dolphins in Australia was prior to the description of S. sahulensis (Jefferson and Rosenbaum, 2014) and, therefore, considered both S.sahulensis and S.chinensis as a single species (Reeves et al, 2008a).…”