22 – The challenge of monitoring coastal marine mammals^295zone for three categories of water depth to test the assumption in Pollock et al.
(2006) that the time dugongs spend at or near the surface is homogeneous across
water depths. Hagihara et al. (2016) used diving data from six wild dugongs
captured in Torres Strait. Each animal was fitted with a GPS (global positioning
system)/Argos Systems unit (Telonics, Inpala, USA) and a pop-up archival tag
MiniPAT (Wildlife Computers, Redmond, USA) (Fig. 22.2).
Dugong availability varied with water depth and environmental conditions
(Fig. 22.2). Most of the dugong sightings in Torres Strait (84% in 2006, 88% in 2011
and 89% in 2013; Fig. 22.3) were in turbid water 5 to <20 m deep for which the
estimates of availability bias were much lower than those used by Pollock et al.
(2006). Thus, the estimated dugong population sizes using the Hagihara method
were substantially higher than those obtained using the Pollock method (Fig. 22.3).
The larger abundance size estimates obtained using the Hagihara method
support the conclusions of Marsh et al. (2015). They used several lines of evidence
to demonstrate that Torres Strait dugong population has been stable for at least 30
years despite high levels of Indigenous harvest and concluded that the population
size must be much larger than suggested by the Pollock et al. (2006) estimate. Thus
the Hagihara method is considered to provide improved estimates of dugong
abundance compared with the Pollock method.
Case study 2: mark–recapture techniques for coastal dolphins
based on natural marks
Although both dugongs and coastal dolphins share many common challenges for
monitoring, dolphins have a major advantage over dugongs: a dorsal fin, which is
out of the water and able to be photographed when the animals are at the surface.
Fig. 22.2. Availability detection probabilities (closed circles) and standard errors (vertical lines) estimated
from dugongs tracked in Torres Strait under various levels of the environmental conditions index (ECI).
Horizontal lines represent availability estimates from Pollock et al. (2006) for optimal sea state (solid lines)
and marginal sea state (dotted lines). The value for water >20 m deep was assumed to be one (and not
estimated) because no data were obtained from tracked dugongs in water >20 m. The solid and dotted lines
on the figure on the right (ECI 4) overlap and the dotted line is not visible. Figure reproduced from Hagihara et
al. (2016).