22 – The challenge of monitoring coastal marine mammals^299for a population of only around 90 individual humpback dolphins in total and as
few an average of 21 individuals on one of the sites (Shoal Bay). The confidence
intervals were considerably wider for the estimates of apparent survival, temporary
emigration and movements between sites. Nonetheless, despite quite wide
confidence intervals on estimates of these parameters, useful and informative
estimates were obtained, indicating seasonal effects.
Discussion
These case studies demonstrate that estimating population trends for coastal
marine mammals requires considerable knowledge of the species’ ecology, high
technical expertise and significant investment. Precise and accurate abundance
estimates are difficult to obtain, and therefore changes in population size are
difficult to detect. The life histories of these species means that their populations
are unlikely to grow at more than a few per cent per year, usually less (Marsh et al.
2011), and while there are no similar constraints on population decline, in the
absence of catastrophes, most declines are likely to be gradual.
The two case studies show how aerial survey techniques and mark–recapture
studies can correct for availability and detection biases to obtain the best abundance
size estimates. But equally, the underlying assumptions of the sources and extents of
bias, and their corrections, must be verified to avoid reaching erroneous conclusions
about population size and/or trends. In the dugong example, failure to account for
changes in the detectability of dugongs in different water depths, as environmental
conditions changed, led to substantial underestimates of the population size. Such a
result could lead to the erroneous conclusion that the current level of Indigenous
hunting is unsustainable and trigger management actions that adversely affect the
cultural practices of Indigenous communities. In the dolphin case study, the robust
design models are the only way temporary emigration can be modelled, while the
MSCRD allowed us to model the temporary movement of Australian humpback
dolphins between the focal area and other sites. The small size of the dolphin
populations limits our ability to detect population trends, although the abundance
estimates reported here are typical of the sizes of local populations reported
elsewhere for these species. Moreover, the effort expended in this study across all
three species (i.e. four vessels, 60 000 km of transects, 96 588 images, 2823 animals
photographed representing 445 individual dolphins) highlights that detecting
population trends for species such as coastal dolphins is possible, but resource
intensive because these animals occur in small numbers, are sparsely distributed
and difficult to capture at suitable rates.
Assuming that biases associated with the population estimate are constant or
can be estimated, the power of a time series of surveys to detect changes in
population size depends on: (1) the length of the monitoring period; (2) the rate of
change in the population; (3) the frequency of surveys; and (4) the precision of the