292 Monitoring Threatened Species and Ecological Communities
Thus, management intervention should not be delayed until population declines
are detected.
Introduction
Several coastal marine mammals are listed as threatened, and are thus Matters of
National Environmental Significance under the Environment Protection and
Biodiversity Conservation Act 1999 (EPBC Act). In addition, marine mammals are
included under various international agreements including the Convention on the
Conservation of Migratory Species of Wild Animals, under which dugongs Dugong
dugon and some species of dolphins are listed as ‘migratory species’. Consequently,
these species often trigger the EPBC Act when proposed coastal development
projects are evaluated. Some species are important resources for coastal
communities; for example, the harvest of dugongs in the Torres Strait is permitted
by the Torres Strait Treaty between Australia and Papua New Guinea and
Australian native title laws. Monitoring of coastal marine mammals is therefore
required to: (1) allow assessment of the impacts of coastal development; (2) enable
Australia to report against international agreements; and (3) inform the sustainable
management of these resources.
Monitoring trends in the distribution and abundance of coastal marine
mammals is difficult because not all animals are at the surface of the water when
survey counts are attempted. Thus, abundance estimates from survey counts are
often negatively biased. In addition, the proportion of the population that is
available to be counted often varies over fine spatial scales both within and
between surveys. In-water visibility can vary from clear with the bottom visible to
turbid in a few minutes. In addition, the animals often make individual or group
movements, making it difficult to separate changes in abundance from temporary
and permanent immigration.
This chapter illustrates approaches for monitoring coastal marine mammals by
using two case studies at very different spatial scales. The first study is a large-scale
(~30 000 km^2 ), strip transect aerial survey of dugongs in Torres Strait (Fig. 22.1).
The second is a local-scale, mark–recapture study of three species of coastal
dolphins in Darwin Harbour and two adjacent sites (Bynoe Harbour and Shoal
Bay): Australian snubfin Orcaella heinsohni; Australian humpback Sousa
sahulensis; and bottlenose, a coastal Tursiops species. Although there are major
differences between these two sampling techniques, both attempt to estimate the
probability of detection to obtain robust estimates of abundance or density.
Case study 1: aerial surveys for dugongs in Torres Strait
The objective of the aerial surveys of Central and Western Torres Strait is to
provide fisheries with independent information on the abundance, distribution