Including identification and evaluation of the key threats and
evaluation of available management options to improve its
status
Research Report
Helene Marsh, Amanda Hodgson, Ivan Lawler, Alana Grech and
Steven Delean
School of Earth and Environmental Sciences, James Cook
University
ISBN 9781921359057
Published April 2008
MTSRF Project 1.4.1 -
Condition trends and projected futures of marine species of
conservation concern
Executive Summary
This survey is the second of an integrated series of three
aerial surveys which aim to survey the entire Queensland coast for
dugongs over three field seasons. It provides the first synopsis of
the distribution and abundance of the dugong on the remote coast of
Queensland from Cooktown north including Torres Strait. The results
of previous surveys of sections of this region have been difficult
to interpret because of the potentially confounding influences of
unpredictable dugong movements between areas within the region.
The results of the 2006 survey of the whole region of almost
56,000 km2 suggest a total population of some 23500
+ 2900 dugongs close to the estimate of some 23000
+2600 for the combined 2000 survey of the Northern Great
Barrier Reef (GBR) and 2001 surveys of Torres Strait. Both
estimates are based on the method of Pollock et al.
(2006).
The time series of surveys since the mid 1980s suggests
considerable movement of dugongs between survey blocks within the
Northern GBR region. Population movement between the Northern and
Southern GBR are also likely and may also explain some of the
variation in the dugong population estimates of both regions.
The overall result of the time series of aerial surveys for the
whole Northern GBR/Torres Strait region suggests that the
fluctuations in the estimates in the size of the Torres Strait
dugong population are unlikely to result from significant movements
at a population level between the Northern GBR and Torres Strait.
This result accords with new genetic evidence.
A likely reason for the movement of dugongs within the Torres
Strait region is the susceptibility of the region to episodic
seagrass diebacks, which are now believed to be largely natural
events, the frequency of which may be exacerbated by climate
change.
The aerial surveys of the Northern GBR and Torres Strait since
the mid 1980s for dugongs have not demonstrated a significant
decline in dugong numbers, despite concern about the sustainability
of the traditional harvest of dugongs in this region and the
limited arrangements to regulate this harvest to date. However,
given the difficulty in detecting declines in marine mammal stocks,
we caution about using this result as a reason for postponing
community-based management initiatives, especially as: (1) the
whole area of dugong habitat in Torres Strait has not been surveyed
because the region to the west of the survey area is inaccessible
from light aircraft based in Australia; (2) there is considerable
uncertainty surrounding the impact of climate change on the
frequency of seagrass diebacks; (3) there is evidence that the life
history and reproductive rate of female dugongs are reduced by
seagrass diebacks; and (4) the fact that dugongs in Torres Strait
are breeding at younger ages, smaller sizes and more often than has
been recorded elsewhere may be a density-dependent response to
declining population size.
The discrepancy between the estimated sustainable catch and the
anecdotal catch estimates, particularly for Torres Strait, suggests
that one of the following may be true: (1) the aerial surveys
underestimate the actual dugong population size, probably because:
(a) the availability correction factor is underestimated; and/or
(b) the assumption of full independence between the two observers
in a tandem team is violated; (2) dugongs are breeding faster than
estimated, either because of a density-dependent response to
declining population size and/or environmental conditions that have
improved the food supply; and/or (3) the anecdotal estimates of the
harvest rate are too high.
The data generated using PBR modelling suggest annual
sustainable anthropogenic mortality limits of 56-112 dugongs in the
Northern GBR. We suggest that an annual sustainable anthropogenic
mortality limit of 56 would be a prudent interim management target
given the World Heritage Status of the region and the management
objective of population recovery for dugongs in the Great Barrier
Reef Marine Park.
PBR modelling suggests annual sustainable anthropogenic
mortality limits of about 100-200 dugongs per year for Torres
Strait depending on the value given to the Recovery Factor. The
latter is a policy decision linked to the objective of the
management arrangements.
The dugong population in the Northern GBR/Torres Strait
region is substantial (>20,000 individuals) and is genetically
healthy.
We believe that there is time to work with local
Traditional Owners and commercial fishers to develop appropriate
management arrangements without dugongs becoming locally extinct
within this region. This approach accords with the Torres Strait
Treaty 1985 between Australia and Papua New Guinea. The Treaty
recognizes the importance of: (1) ‘protecting the traditional
way of life and livelihood of Australians who are Torres Strait
Islanders and of Papua New Guineans who live in the coastal area of
Papua New Guinea in and adjacent to the Torres Strait’; and
(2) ‘the marine environment’ of the region.
Index Blocks in Hervey Bay in southern Queensland were
also surveyed to provide a context for the survey at a larger
spatial scale. Despite the high correlation between
the population estimates for the Index Blocks and their total
region, the Index Blocks were not particularly robust
indices of the dugong population in their region because they
represent an unknown and probably variable proportion of the
population. Index blocks external to a survey region add
considerably to the expense and logistical problems associated with
a regional survey and we consider that the practice of surveying
Index Blocks should be discontinued.
