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 km2 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. In particular, the level of interactions with inshore gillnet fisheries requires urgent assessment. 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.
Highlights
Estimating abundance is a key element of wildlife management strategies and important to assessing the conservation status of a species or population
Through our study design and selected analytical procedures, we consider the assumptions of the capture-recapture models used to estimate abundance to be largely met, and these estimates subject to minimal bias (Table 2)
Our estimates of snubfin dolphin abundance fell within the range of those reported elsewhere, despite our study sites being appreciably smaller (Table 1)
Summary
Estimating abundance is a key element of wildlife management strategies and important to assessing the conservation status of a species or population. Cetaceans are long-lived species, with late maturation, low reproductive rates and often occupy high trophic levels; they are inherently vulnerable to human impacts and often in particular need of conservation action (Taylor, 2002; Lewison et al, 2004) Due to their traits (highly mobile, problematic to observe), obtaining unbiased and precise abundance estimates of cetaceans can be difficult, expensive and time-consuming, for species which are sparsely distributed across large and remote areas (Taylor and Gerrodette, 1993; Williams and Thomas, 2009; Peel et al, 2015). Due to their overlap with areas of considerable human activity, cetacean populations occupying near-shore coastal habitats are among the most threatened (e.g., Rojas-Bracho et al, 2006; Slooten et al, 2006, 2013) and in most need of quantitative data to inform management (Wilson et al, 1999; Parra et al, 2006a)
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