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The challenge of monitoring coastal

marine mammals

Helene Marsh, Lyndon Brooks and Rie Hagihara

Summary

Monitoring trends in the distribution and abundance of coastal marine mammals
is challenging because their detectability below the water surface is variable and
they are highly mobile, generally occupying vast areas. These attributes diminish
the precision and accuracy of abundance estimates, both of which are essential to
detect trends and trigger management actions. However, many survey techniques
are available that improve the precision and accuracy of abundance estimates by
explicitly accounting for the factors that inf luence our ability to spot individual
animals during surveys. This chapter demonstrates two approaches that account
for such biases by using strip transect sampling and mark–recapture surveys to
estimate the abundance of dugongs and coastal dolphins in northern Australia.
Although these approaches require considerable ecological knowledge, a high level
of technical expertise and significant resources, they provide the best estimates of
abundance to date for these species. Nonetheless, trend detection is still difficult
given the current survey frequency and precision of the abundance estimates.