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

Abstract

Assessing the abundance of wildlife populations is essential to their effective conservation and management. Concerns have been raised over the vulnerability of tropical inshore dolphins in waters off northern Australia to anthropogenic impacts on local populations, yet a lack of abundance data precludes assessment of their conservation status and the management of threats. Using small vessels as cost-effective research platforms, photo-identification surveys and capture-recapture models were applied to provide the first quantitative abundance data for Australian snubfin (Orcaella heinsohni), Australian humpback (Sousa sahulensis), and Indo-Pacific bottlenose dolphins (Tursiops aduncus) at five sites in the Kimberley region of north-western Australia. The abundance of each species was highly variable between different sites, likely reflecting species-specific habitat preferences. Within the c. 130 km 2 study sites, the estimated abundance of most species was ≤60 individuals (excluding calves), and fewer than 20 humpback dolphins were identified at each site in any one 3-5 week sampling period. However, larger estimates of c. 130 snubfin and c. 160 bottlenose dolphins were obtained at two different sites. Several local populations showed evidence of site fidelity, particularly snubfin dolphins. By implementing a standardized, multi-site approach, data on local populations were provided within a broader, regional context, and indicated that each species is patchily distributed in the region. This highlights the need for site-specific baseline data collection using appropriate survey techniques to quantitatively assess the potential impacts of threatening activities to local populations. These findings further illustrate the need to gain a greater understanding of known and potential threats to inshore dolphin populations, their relative impacts, and to mitigate where necessary. An ideal candidate site for a long-term study of snubfin dolphin population dynamics is identified, where trends in abundance and their influencing factors could be investigated. The methods employed herein provide an example of rigorous, site-specific population assessments of inshore dolphins that are broadly applicable to such studies elsewhere.

Demographic characteristics of Australian humpback dolphins reveal important habitat toward the southwestern limit of their range

“…About 60 to 100 animals used the NWC study area at any one time, with a total of ~129 individuals (95% CI 117 to 141 individuals) using the area over the 3-yr study. These estimates are broadly comparable to those reported for other humpback dolphin populations across northern Australia, which average 54 to 89 individuals and 0.1 to 0.19 individuals km −2 (see reviews in Brown et al 2016, Parra & Cagnazzi 2016). …”

“…Reviews of available data from across their range suggest that density is generally lower elsewhere, considerably so in most cases (Brown et al 2016, Parra & Cagnazzi 2016, suggesting this NWC population is of high conservation value. For example, less than 20 individually identifiable humpback dolphins were recorded in each of five ~130 km 2 study sites across the adjacent Kimberley region of WA (Brown et al 2016).…”

“…Parra et al 2006a, Brown et al 2016. Two of the 3 species of tropical inshore dolphins inhabiting the northern Australian coastline, the Australian snubfin dolphin Orcaella heinsohni and the Australian humpback dolphin Sousa sahulensis are endemic to the region, and coastal development has been identified as a major threat to the species' persistence (DE-WHA 2010, Parra & Cagnazzi 2016.…”

A Full-Capture Hierarchical Bayesian Model of Pollock's Closed Robust Design and Application to Dolphins

“…The PCRD is an increasingly popular study design and model (Nicholson et al, 2012;Smith et al, 2013;Brown et al, 2016). The PCRD utilizes a hierarchical sampling strategy, including widely-spaced "primary periods, " between which the population is open to birth, death and temporary migration, and tightly-spaced "secondary periods, " between which the population is assumed closed to population changes.…”

Evaluating monitoring methods for cetaceans

“…Although we have concentrated on the estimation of abundance and precision from different scenarios, survival and immigration/emigration have also been estimated using the data collected from this approach (Tyne et al 2014). Delphinid sighting data have also been collected systematically along transects to estimate abundance and other demographic parameters, such as temporary immigration/emigration (Smith et al 2013;Brown et al 2016;Sprogis et al 2016) using Pollock's Robust Design (Pollock et al 1990). The data from these studies could be used to estimate the power to detect change and evaluate alternative sampling strategies for monitoring in a similar manner to the current study by varying the number of transect cycles.…”