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Dugong Status Report and Action Plans for Countries and Territories

Authors:
Dugong
Status Report and Action Plans for
Countries and Territories
1
UNEP/DEWA/RS.02-1
Early Warning and Assessment Report Series
©Doug Perrine/Seapics.com
Dugong
Status Reports and Action Plans for
Countries and Territories
IUCN
The World Conservation Union
S
PECIES
S
URVIVAL
C
OMMISSION
Compiled by
HELENE MARSH
Action Plan Coordinator
IUCN/SSC Sirenia Specialist Group
HELEN PENROSE,CAROLE EROS,AND JOANNA HUGUES
School of Tropical Environment Studies and Geography
James Cook University, Townsville, Australia
Cooperative Research Centre for the Great Barrier Reef World Heritage Area
James Cook University, Townsville, Australia
United Nations Environment Programme World Conservation Monitoring Centre
Cambridge, United Kingdom
© Doug Perrine/seapics.com
Cover Photograph: Dugong (Dugong dugon)
© Doug Perrine/seapics.com
DISCLAIMER
Information for maps was sourced from the Macquarie Illustrated World Atlas, 1995; the Times Atlas of the World
(9th Comprehensive Edition), 1994 (map of Palau); and the Atlas of the South Pacific, 1986 (map of Malakula).
Overview maps (smallest scale) were created using ArcWorld continental coverage, Environmental Systems
Research Institute, Inc., 1992 (1:3 million). All other maps were created using Digital Chart of the World,
Environmental Systems Research Institute, Inc., 1992 (1:1 million).
The information represented on these maps results from the compilation of a variety of source materials. They are
intended to illustrate the text of this report. The Cartography Centre of the School of Tropical Environment Studies
and Geography, James Cook University, makes no representation or warranty concerning the accuracy of this
information, nor claims it as a legal cartographic resource.
CRICOS Provider No.: 00117J
Dugong Status Report and Action Plans for Countries and Territories
UNEP/DEWA/RS.02-1
ISBN 92-807-2130-5
Contents
Page Page
iii
Executive Summary ........................................1
Acknowledgements..........................................4
Chapter 1 – Introduction................................5
Distribution and Abundance ......................................5
Life History..................................................................6
Diet................................................................................7
Habitat ......................................................................7
Movements ...................................................................7
Genetic Population Structure ....................................8
Threatening Processes ................................................8
Habitat Loss and Degradation.................................9
Fishing Pressure ......................................................9
Indigenous Use and Hunting.................................10
Vessel Strikes.........................................................10
Ecotourism ............................................................10
Acoustic Pollution.................................................11
Chemical Pollutants ..............................................11
Diseases.................................................................12
Susceptibility to Mortality During
Capture for Research.............................................12
Approaches to Management ....................................12
Layout of this Document..........................................13
Chapter 2 – Western Range: East Africa,
Red Sea and the Arabian Gulf ................19
EAST AFRICA
Distribution and Abundance ....................................19
Somalia..................................................................19
Kenya ....................................................................19
Tanzania.................................................................19
Mozambique..........................................................20
Madagascar............................................................21
Comoros Archipelago, Mayotte,
Seychelles and Mauritius ......................................22
Threatening Processes
Habitat Loss and Degradation...............................22
Fishing Pressure ....................................................22
Indigenous Use and Hunting.................................23
Existing Conservation Initiatives
Legislation.............................................................23
Kenya........................................................................23
Mozambique.............................................................24
Management .........................................................24
Kenya........................................................................24
Mozambique.............................................................24
Suggested Conservation Initiatives
Research ................................................................24
Management ..........................................................25
Conclusions................................................................25
RED SEA
Distribution and Abundance ....................................26
Threatening Processes
Habitat Loss and Degradation...............................26
Fishing Pressure....................................................28
Indigenous Use and Hunting.................................28
Boat-related Impacts and Ecotourism...................28
Existing Conservation Initiatives
Legislation.............................................................28
Management ..........................................................28
Suggested Conservation Initiatives
Research ................................................................29
Management ..........................................................29
Conclusions................................................................30
THE ARABIAN GULF
Distribution and Abundance ....................................31
Threatening Processes ..............................................32
Habitat Loss and Degradation...............................32
Fishing Pressure ....................................................32
Indigenous Use and Hunting.................................33
Existing Conservation Initiatives
Legislation.............................................................33
Research ................................................................33
Suggested Conservation Initiatives
Research ................................................................34
Management ..........................................................34
Conclusions................................................................35
Suggested Conservation Initiatives for the
Arabian Region (Red Sea and the Arabian Gulf)
Research ................................................................35
Management ..........................................................35
Chapter 3 – India and Sri Lanka
INDIA WITH THE ANDAMAN AND
NICOBAR ISLANDS, AND SRI LANKA
Distribution and Abundance ....................................36
India (including Andaman and
Nicobar Islands)...............................................36
Sri Lanka ...............................................................38
Threatening Processes
Habitat Loss and Degradation...............................38
Fishing Pressure ....................................................38
Indigenous Use and Hunting.................................39
Conservation Initiatives Currently in Place
Legislation.............................................................39
Research ................................................................39
Management ..........................................................39
Suggested Conservation Initiatives
Research ................................................................39
Management ..........................................................40
Conclusions................................................................40
Chapter 4 – East and Southeast Asia
JAPAN
Distribution and Abundance ....................................41
Threatening Processes
Habitat Loss and Degradation...............................42
Fishing Pressure ....................................................43
Indigenous Use and Hunting.................................43
Existing Conservation Initiatives
Legislation.............................................................43
Research ................................................................43
Management ..........................................................44
Suggested Conservation Initiatives
Research ................................................................44
Management ..........................................................45
Conclusions................................................................46
TAIWAN
Distribution and Abundance ....................................47
Threatening Processes
Habitat Loss and Degradation...............................47
Existing Conservation Initiatives
Suggested Conservation Initiatives
Research ................................................................47
Management ..........................................................47
Conclusions................................................................47
CHINA
Distribution and Abundance ....................................48
Threatening Processes
Habitat Loss and Degradation...............................48
Fishing Pressure ....................................................49
Indigenous Use and Hunting.................................49
Boat-related Impacts and Ecotourism...................49
Existing Conservation Initiatives
Legislation.............................................................49
Research ................................................................49
Management ..........................................................49
Suggested Conservation Initiatives
Research ................................................................50
Management ..........................................................50
Conclusions................................................................51
PHILIPPINES
Distribution and Abundance ....................................52
Threatening Processes
Habitat Loss and Degradation...............................52
Fishing Pressure ....................................................54
Indigenous Use and Hunting.................................54
Boat-related Impacts .............................................54
Conservations Initiatives Currently in Place
Legislation.............................................................55
Research ................................................................55
Management ..........................................................56
Suggested Conservation Initiatives
Research ................................................................57
Management ..........................................................57
Conclusions................................................................58
THAILAND, CAMBODIA, VIETNAM
Distribution and Abundance ....................................59
Thailand....................................................................59
Cambodia and Vietnam ............................................61
Threatening Processes
Habitat Loss and Degradation...............................61
Thailand....................................................................61
Vietnam and Cambodia............................................62
Fishing Pressure ....................................................62
Thailand....................................................................62
Vietnam and Cambodia............................................63
Indigenous Use and Hunting.................................63
Thailand....................................................................63
Vietnam and Cambodia............................................63
Boat-related Impacts .............................................64
Existing Conservation Initiatives
Legislation.............................................................64
Thailand....................................................................64
Vietnam and Cambodia............................................64
Research ................................................................64
Thailand....................................................................64
Vietnam and Cambodia............................................65
Management ..........................................................65
Thailand....................................................................65
Vietnam and Cambodia............................................66
Suggested Conservation Initiatives
Research ................................................................66
Management ..........................................................67
Conclusions................................................................67
MALAYSIA AND SINGAPORE
Distribution and Abundance ....................................68
Peninsular Malaysia and Singapore .........................68
East Malaysia and Brunei ........................................68
Threatening Processes
Habitat Loss and Degradation
Peninsular Malaysia and Singapore .........................69
East Malaysia and Brunei ........................................70
Fishing Pressure ....................................................70
Peninsular Malaysia and Singapore .........................70
East Malaysia and Brunei ........................................70
Indigenous Use and Hunting.................................71
Peninsular Malaysia and Singapore .........................71
East Malaysia and Brunei ........................................71
Boat-related Impacts .............................................72
Peninsular Malaysia and Singapore .........................72
East Malaysia and Brunei ........................................72
Existing Conservation Initiatives
Legislation.............................................................72
Malaysia ...................................................................72
Research ................................................................72
Peninsular Malaysia .................................................72
Singapore..................................................................72
East Malaysia ...........................................................72
Management ..........................................................74
Malaysia ...................................................................74
Suggested Conservation Initiatives .........................74
Research ...................................................................74
Management .............................................................74
Conclusions................................................................74
iv
Contents (continued)
Page Page
INDONESIA
Distribution and Abundance ....................................75
Threatening Processes
Habitat Loss and Degradation...............................77
Fishing Pressure ....................................................79
Indigenous Use and Hunting.................................80
Boat-related Impacts .............................................80
Existing Conservation Initiatives
Legislation.............................................................81
Research ................................................................82
Suggested Conservation Initiatives
Research ................................................................82
Management ..........................................................82
Conclusions................................................................83
Chapter 5 – Pacific Islands
PALAU
Distribution and Abundance ....................................84
Threatening Processes
Habitat Loss and Degradation...............................84
Indigenous Use and Hunting.................................84
Boat-related Impacts and Ecotourism...................86
Existing Conservation Initiatives
Legislation.............................................................86
Research ................................................................86
Management ..........................................................86
Suggested Conservation Initiatives
Research ................................................................87
Management ..........................................................87
Conclusions................................................................87
PAPUA NEW GUINEA, SOLOMON ISLANDS,
NEW CALEDONIA AND VANUATU
Distribution and Abundance ....................................89
Papua New Guinea...................................................89
Solomon Islands .......................................................90
New Caledonia .........................................................90
Vanuatu .....................................................................91
Threatening Processes
Habitat Loss and Degradation...............................91
Papua New Guinea and the Solomon Islands..........91
New Caledonia .........................................................92
Vanuatu .....................................................................92
Fishing Pressure ....................................................92
Papua New Guinea ..................................................92
Solomon Islands .......................................................92
New Caledonia .........................................................92
Vanuatu .....................................................................92
Indigenous Use and Hunting.................................92
Papua New Guinea ..................................................92
Solomon Islands .......................................................93
New Caledonia .........................................................93
Vanuatu .....................................................................93
Boat-related Impacts and Ecotourism...................93
New Caledonia .........................................................93
Vanuatu .....................................................................93
Existing Conservation Initiatives
Legislation.............................................................94
Papua New Guinea ..................................................94
Solomon Islands .......................................................94
New Caledonia .........................................................94
Vanuatu .....................................................................94
Research ................................................................94
Papua New Guinea ..................................................94
Solomon Islands .......................................................94
New Caledonia .........................................................94
Vanuatu .....................................................................94
Management ..........................................................94
Papua New Guinea ..................................................94
Solomon Islands, New Caledonia and Vanuatu .......95
Suggested Conservation Initiatives
Research ................................................................95
Papua New Guinea, Solomon Islands,
New Caledonia and Vanuatu ..............................95
Papua New Guinea...................................................95
New Caledonia .........................................................96
Management ..........................................................96
Papua New Guinea, Solomon Islands,
New Caledonia and Vanuatu ..............................96
Papua New Guinea...................................................96
Vanuatu .....................................................................96
New Caledonia .........................................................96
Conclusions................................................................96
Chapter 6 – Australia
WESTERN AUSTRALIA
Distribution and Abundance ....................................97
Shark Bay .................................................................97
Ningaloo Marine Park and
Exmouth Gulf .....................................................97
Pilbara Coastal and Offshore Region
(Exmouth Gulf to De Grey River)........................97
Eighty Mile Beach and
Kimberley Coast .................................................98
Threatening Processes
Habitat Loss and Degradation...............................99
Shark Bay .................................................................99
Exmouth Gulf and Ningaloo....................................99
Pilbara Coastal and Offshore Region
(Exmouth Gulf to De Grey River)......................99
Eighty Mile Beach and
Kimberley Coast ...............................................100
Fishing Pressure ..................................................100
Shark Bay ...............................................................100
Exmouth Gulf and Ningaloo..................................100
Pilbara Coastal and Offshore Region
(Exmouth Gulf to De Grey River)
and Eighty Mile Beach and
Kimberley Coast ...............................................100
Indigenous Use and Hunting...............................100
Shark Bay ...............................................................100
Exmouth Gulf and Ningaloo..................................100
Pilbara Coastal and Offshore Region
(Exmouth Gulf to De Grey River)....................100
v
Contents (continued)
Page Page
Eighty Mile Beach and
Kimberley Coast ...............................................101
Boat-related Impacts and Ecotourism.................101
Shark Bay ...............................................................101
Exmouth Gulf and Ningaloo..................................101
Pilbara Coastal and Offshore Region
(Exmouth Gulf to De Grey River)....................101
Eighty Mile Beach and
Kimberley Coast ...............................................101
Existing Conservation Initiatives
Research ..............................................................102
Management ........................................................102
Shark Bay ...............................................................102
Exmouth Gulf and Ningaloo..................................102
Pilbara Coastal and Offshore Region
(Exmouth Gulf to De Grey River),
Eighty Mile Beach and Kimberley Coast) .......103
Suggested Conservation Initiatives
Research ..............................................................103
Management ........................................................103
Habitat Protection...................................................103
Indigenous Management ........................................104
Fishery Interactions................................................104
Ecotourism .............................................................104
Conclusions..............................................................104
NORTHERN TERRITORY AND THE
QUEENSLAND COAST OF THE
GULF OF CARPENTARIA
Distribution and Abundance ..................................105
Cape Londonderry to Daly River...........................105
Northern Coast (Daly River to Milingimbi) ..........105
Gulf of Carpentaria Coast of the
Northern Territory.............................................105
Gulf of Carpentaria Coast of Queensland..............105
Threatening Processes
Habitat Loss and Degradation.............................107
Northern Coast (Daly River to Milingimbi) ..........107
Gulf of Carpentaria Coast of the
Northern Territory.............................................108
Gulf of Carpentaria Coast of Queensland..............108
Fishing Pressure ..................................................108
Northern Coast (Daly River to Milingimbi) ..........108
Gulf of Carpentaria Coast of the
Northern Territory.............................................108
Gulf of Carpentaria Coast of Queensland..............108
Indigenous Use and Hunting...............................109
Northern Coast (Daly River to Milingimbi) ..........109
Gulf of Carpentaria Coast of the
Northern Territory.............................................109
Gulf of Carpentaria Coast of Queensland..............109
Boat-related Impacts and Ecotourism.................110
Other Mortalities ....................................................110
Existing Conservation Initiatives
Legislation...........................................................110
Research ..............................................................110
Management ........................................................110
Northern Coast (Daly River to Milingimbi) ..........110
Habitat Protection ............................................110
Fishery Interactions..........................................111
Indigenous Management..................................111
Gulf of Carpentaria Coast of the
Northern Territory.............................................111
Habitat Protection ............................................111
Fishery Interactions..........................................111
Gulf of Carpentaria Coast of Queensland..............112
Habitat Protection ............................................112
Fishery Interactions..........................................112
Suggested Conservation Initiatives
Research ..............................................................112
Management ........................................................113
Northern Territory..................................................113
Habitat Protection ............................................113
Fishery Interactions..........................................113
Indigenous Management..................................113
Gulf of Carpentaria Coast of Queensland..............113
Habitat Protection ............................................113
Indigenous Management..................................113
Fishery Interactions..........................................113
Conclusions..............................................................114
TORRES STRAIT AND NORTHERN
GREAT BARRIER REEF
Distribution and Abundance ..................................115
Torres Strait ...................................................115
Northern Great Barrier Reef Region
(Hunter Point to Cape Bedford
near Cooktown) .............................................115
Threatening Processes
Habitat Loss and Degradation.............................115
Torres Strait ...................................................115
Northern Great Barrier Reef Region .............118
Fishing Pressure ..................................................118
Torres Strait ...................................................118
Northern Great Barrier Reef Region .............118
Indigenous Use and Hunting...............................118
Torres Strait ...................................................118
Northern Great Barrier Reef Region .............119
Boat-related Impacts and Ecotourism.................119
Existing Conservation Initiatives
Legislation...........................................................119
Torres Strait ...................................................119
Torres Strait and the Northern
Great Barrier Reef Region .........................119
Research ..............................................................119
Torres Strait ...................................................120
Northern Great Barrier Reef Region .............120
Management ........................................................120
Torres Strait ...................................................121
Northern Great Barrier Reef Region .............121
Habitat Protection ....................................121
Fishery Interactions..................................121
Indigenous Management ...............................121
Suggested Conservation Initiatives
Research ..............................................................122
Management ........................................................122
Torres Strait ...................................................122
Northern Great Barrier Reef Region .............123
vi
Contents (continued)
Page Page
Indigenous Management .....................................123
Fishery Interactions .......................................123
Conclusions..............................................................123
URBAN COAST OF QUEENSLAND
Distribution and Abundance ..................................124
Threatening Processes
Habitat Loss and Degradation.............................126
Fishing Pressure ..................................................127
Mesh Netting..........................................................127
The Queensland Shark
Control Program (QSCP)....................................128
Indigenous Use and Hunting...............................128
Boat-related Impacts and Ecotourism.................128
Chemical Pollution.................................................128
Disease ...................................................................129
Existing Conservation Initiatives
Legislation...........................................................129
Research ..............................................................129
Management ........................................................129
Suggested Conservation Initiatives
Research ..............................................................130
Management ........................................................131
Habitat Protection...................................................131
Indigenous Management ........................................131
Fishery Interactions................................................131
Further Initiatives ...................................................132
Conclusions..............................................................132
References...........................................................................133
Personal Communications.................................................147
Reviewers.............................................................................156
List of Tables
Table 1.1 – Summary of information on
dugong status and management
within its range. .......................................14
Table 1.2 – Summary of information regarding
threats to dugongs ....................................16
vii
Contents (continued)
Page
Chapter 1 – Introduction
Figure 1.1 – The known range of the dugong..........5
Chapter 2 – Western Range: East Africa,
Red Sea and the Arabian Gulf
Figure 2.1 – East Africa.
Inset: The Bazaruto Archipelago........20
Figure 2.2 – East Africa.
Inset: The Lamu Archipelago.............21
Figure 2.3 – The Red Sea.
Inset: Dahlac Islands region, Eritrea;
Inset: Sinai Peninsula and adjacent
islands and marine parks ..........27
Figure 2.4 – The Arabian Gulf.
Inset: Bahrain and Qatar.....................31
Chapter 3 – India and Sri Lanka
Figure 3.1 – Indian Subcontinent and Islands.........36
Figure 3.2 – Sri Lanka and the Gulf of Mannar .....36
Figure 3.3 – Andaman and Nicobar Islands ...........37
Chapter 4 – East and Southeast Asia
Figure 4.1 – East and Southeast Asia.
Inset: Okinawa Island;
Inset: Sakishima Shoto Islands...........41
Figure 4.2 – Hainan Island and the southern coast
of mainland China. .............................48
Figure 4.3 – The Philippines.
Inset: Busuanga Island region;
Inset: The Bicol region. ......................53
Figure 4.4 – Thailand, Cambodia and Vietnam.
Inset: The South Andaman region
of Thailand.................................60
Figure 4.5 – Malaysia, Singapore and Brunei. .......69
Figure 4.6 – East Malaysia and Brunei...................69
Figure 4.7 – Peninsular Malaysia
and Singapore. ....................................70
Figure 4.8 – Peninsular Malaysia............................71
Figure 4.9 – Eastern Indonesia.
Inset: North Sulawesi;
Inset: Seram and adjacent islands. ....76
Figure 4.10 – Central Indonesia................................77
Figure 4.11 – Indonesia.
Inset: Sunda Strait and
Banten Bay, Java;
Inset: The Bukit Peninsula, Bali.........78
Figure 4.12 – The Seas and Oceans of
Indonesian waters ...............................79
Chapter 5 – Pacific Islands
Figure 5.1 – The Palau region in relation to the
Philippines and Indonesia. .................85
Figure 5.2 – Palau ...................................................86
Figure 5.3 – Papua New Guinea, Solomon Islands,
New Caledonia and Vanuatu...............89
Figure 5.4 – Papua New Guinea .............................89
Figure 5.5 – Solomon Islands .................................90
Figure 5.6 – The New Caledonia and
Vanuatu region ....................................91
Chapter 6 – Australia
Figure 6.1 – Western Australia
Inset: Exmouth Gulf and the Northwest
Shelf region;
Inset: Shark Bay region and
Ningaloo Reef...........................98
Figure 6.2 – The Northern Territory Coast of
the Gulf of Carpentaria.
Inset: The northwest coast of the
Northern Territory and the
Melville Island region.............106
Figure 6.3 – The Queensland coast of the
Gulf of Carpentaria...........................107
Figure 6.4 – Torres Strait ......................................116
Figure 6.5 – The northern section of the
Great Barrier Reef World
Heritage Area, Australia ...................117
Figure 6.6 – The urbanised coast of Queensland.
Inset: Moreton Bay region,
southeast Queensland..............125
viii
List of Figures
Page Page
The dugong (Dugong dugon) is the only herbivorous
mammal that is strictly marine, and is the only extant
species in the Family Dugongidae. It is listed as
vulnerable to extinction at a global scale by The World
Conservation Union (IUCN). The dugong has a large
range that spans some 37 countries and territories and
includes tropical and subtropical coastal and island
waters from East Africa to Vanuatu, between about
26° north and south of the Equator.
The purpose of this document is to present a global
overview of the status of the dugong and its
management in the various countries in its range. We
aimed to provide comparative information that will
enable individual countries to develop their own, more
detailed, conservation plans.
This document contains information on dugong
distribution and abundance, threatening processes,
legislation, and existing and suggested research and
management initiatives for 37 countries and territories
in the dugong’s known range. The report is organised
in a geographical sequence from the Western Indian
Ocean region, through to the South West Pacific.
Chapter One introduces the Dugong; Chapter 2
comprises information on East Africa; the Red Sea
and the Arabian Gulf. Chapter 3 discusses India and
Sri Lanka; Chapter 4 presents data from Southeast
Asia including Japan, Taiwan, China, the Philippines,
Vietnam, Cambodia and Thailand; Malaysia,
Singapore and Indonesia; Chapter 5 discusses Palau,
Papua New Guinea, the Solomon Islands, New
Caledonia and Vanuatu; and Chapter 6 presents
information from Australia.
Throughout much of its range, the dugong is believed
to be represented by relict populations separated by
large areas where its numbers have been greatly
reduced or where it is already extirpated. However,
the degree to which dugong numbers have dwindled,
and its range has been fragmented, is not known for
any country in its range. The dugong is still present at
the historical limits of its global range, although there
is evidence of a reduction in its area of occupancy
within this range.
In most countries in the dugong’s range, our
knowledge of dugong distribution and abundance is
known only from anecdotal information. In ten or so
countries, some information on dugong distribution
and abundance has been obtained from spatially and
temporally limited surveys generally conducted
parallel to the shoreline. These surveys provide
minimum counts only. Extensive quantitative aerial
surveys using transects across the shoreline depth
gradient have resulted in a more comprehensive
knowledge of dugong distribution and abundance in
the coastal waters of most (but not all) of the dugong’s
range in northern Australia and the Arabian region.
However, even in these regions, the information is not
comprehensive enough to establish trends in
abundance for most areas, especially as there is
increasing evidence that dugongs undertake large-
scale movements.
It is inappropriate to compare the abundance of
dugongs estimated using shoreline and quantitative
surveys. We believe that most of the estimates of
dugong population size recorded in this document are
underestimates, probably major underestimates.
Nonetheless in most parts of its range the anecdotal
evidence suggest that dugong numbers are declining.
Dugongs are long-lived with a low reproductive rate,
long generation time, and a high investment in each
offspring. Population simulations indicate that even
with the most optimistic combinations of life-history
parameters (e.g. low natural mortality and no human-
induced mortality) a dugong population is unlikely to
increase at more than about 5% per year. This makes
the dugong vulnerable to over-exploitation. The rate
of change of a dugong population is most sensitive to
changes in adult survivorship. Even a slight reduction
in adult survivorship as a result of habitat loss,
disease, hunting or incidental drowning in nets, can
cause a chronic decline.
Dugongs are seagrass specialists and frequent coastal
waters. Major concentrations of dugongs tend to occur
in wide shallow protected bays, wide shallow
mangrove channels and in the lee of large inshore
islands. Dugongs are also regularly observed in
deeper water farther offshore in areas where the
continental shelf is wide, shallow and protected. The
dugong’s fecundity is very sensitive to the availability
of its seagrass food. When dugongs do not have
enough to eat they delay breeding, making habitat
conservation a critical issue.
Dugongs are vulnerable to anthropogenic influences
because of their life history and their dependence on
seagrasses that are restricted to coastal habitats and
1
Executive Summary
are often under pressure from human activities. The
seagrass ecosystems on which dugongs depend are
very sensitive to human influence. Seagrass beds may
be destroyed directly by mining and trawling or lost
through the effects of disturbances such as dredging,
land clearing and land reclamation. These activities
cause increases in sedimentation and turbidity which,
in turn, lead to degradation of seagrass extent, density
and productivity through smothering and lack of light.
Episodic losses of hundreds of square kilometres of
seagrass are associated with extreme weather events
such as some cyclones, and floods. Most losses, both
natural and anthropogenic, are attributed to reduced
light intensity due to sedimentation and/or increased
epiphytic growth caused by nutrient enrichment. In
some cases, factors such as poor catchment
management and sediment instability interact to make
the processes more complex so that it is often difficult
to separate natural and anthropogenic causes of
seagrass loss. In addition, herbicide runoff from
agricultural lands presents a potential risk to seagrass
growth adjacent to sugarcane production areas.
Accidental entangling in mesh nets and traps set by
fishers is a major, but largely unquantified, cause of
dugong mortality in many countries and was
identified as a major concern in most of the countries
covered by this document.
Dugongs are culturally significant to communities
throughout their range. In this document, we record
information about the indigenous use of dugong
products in most countries in the dugong’s range.
Dugongs are caught for meat, oil, medicaments,
amulets and other products. In many countries
hunting dugongs is banned and they are no longer
hunted deliberately, however, dugong products are
still highly valued and stimulate direct takes.
Australia’s indigenous peoples consider dugong
hunting to be an important expression of their identity.
Although there are few records of dugong deaths
resulting from vessel strikes, increasing vessel traffic
in the dugong’s range increases the likelihood of
strikes. Extensive shallow areas used by regionally
important populations of dugongs and situated close
to areas of high boat traffic, are particularly at risk.
The expansion of ecotourism has resulted in the
establishment of tourism operations involving
dugong-watching cruises and/or swim with dugong
opportunities in several countries, including Australia,
the Philippines and Vanuatu.
There are socio-political impediments to dugong
conservation, particularly in developing countries.
The displacement and urbanisation of rural human
populations has led to the loss of traditional values
and taboos to resource exploitation. The nearshore
areas where dugongs occur have become an easy and
convenient source of food and income. The provision
of philanthropic aid from ‘developed countries’
increases the efficiency and level of exploitation. The
situation is exacerbated by an absence of adequate
legislation, enforcement and management.
Unless human values change dramatically, we believe
that it will be impossible to reduce anthropogenic
impacts on the dugong throughout its vast and remote
range. Detecting trends in dugong abundance is
difficult, particularly at low densities. The objectives
of maintaining dugong numbers at present or higher
levels and facilitating the recovery of depleted
populations will not be achieved if the only trigger for
management intervention in an area is a
demonstratively declining population.
A survey by the World Resources Institute rates the
risks from coastal development as medium to high for
much of the dugong’s range outside Australia because
of high levels of human population growth and rapid
rates of industrialisation. In view of the multiple
impacts to which dugong populations are subjected,
we consider that the optimum conservation strategies
are to:
1. identify areas that still support significant
numbers of dugongs;
2. extensively involve the community and jointly
consider how the adverse impacts on dugongs can
be minimised and their habitat protected (Ideally
this should be done in the context of
comprehensive plans for coastal zone
management, perhaps using the dugong as a
“Flagship” Species.).
Control of direct mortality on dugongs in these key
areas should reduce dugong mortality provided the
areas chosen consistently support high numbers of
animals (even though individual dugongs will move in
and out of the areas). The long-term effectiveness of
these areas will depend on community support and the
maintenance of high-quality dugong habitat. This will
hinge on the capacity to control land-based inputs.
Candidate areas for dugong conservation have been
identified in much of the dugong’s range and in this
2
document we have emphasised strategies for
identifying additional areas. However, we
acknowledge that in most situations there will be
multiple demands on these areas necessitating
complex tradeoffs, the solution to which will be
location specific. In all cases, it will be essential for
the socio-economic impediments to dugong
conservation to be addressed.
Individual dugongs can undertake long-distance
movements of up to several hundred kilometres in a
few days. Given the dugong’s capacity to move across
jurisdictional boundaries, it will be important to
coordinate management initiatives for dugongs across
jurisdictions.
3
4
The development of this document was funded by
The World Conservation Union (IUCN) through a grant
from the Peter Scott Foundation, the United Nations
Environment Programme Division of Early Warning and
Assessment (UNEP-DEWA), UNEP World Conservation
Monitoring Centre (UNEP-WCMC) Cambridge, the
Cooperative Research Centre (CRC) for the Great Barrier
Reef World Heritage Area, the Pew Foundation, and the
School of Tropical Environment Studies and Geography
(TESAG), James Cook University (JCU), Townsville
Australia.
Checking information and finding place names for the
maps was an extracting task in which we were ably
assisted by Mr Gill Carr, Mrs Rebecca Hamilton, Amanda
Hodgson,Ben Pearson and Kellie Wilson. Adella Edwards
and Matthias Hoefner developed the maps and Lois
Laivins and Maree Hawken typeset the document. These
tasks were performed with flair, skill and humour.
Doug Perrine generously allowed us to use his superb
underwater photograph of a dugong stirring up mud as
the frontispiece.
Data for Figure (6.5) was reproduced with the
permission of the Cooperative Research Centre (CRC) for
the Great Barrier Reef World Heritage Area.
Many people assisted in the compilation of this
document and generously provided access to unpublished
information and/or commented on drafts, often at
unreasonably short notice. Details of our sources,
including references, personal communications and
reviewers are listed in full at the back of the report. We
also greatly appreciate the very helpful contributions from
many others who provided information on dugong
sightings, local programs, reference material, legislation
and local cultures; reviewer suggestions and editorial
comments. We thank Ameer Abdulla, Sylvia Adam,
Kanjana Adulyanukosol, Saif al-Ghais, Terry Aquino,
Lem Aragones, Rohan Arthur, Shiro Asano, Winny Astuti,
Australian Fisheries Management Authority (AFMA),
Brad Barton, Isabel Beasley, Carla Benoldi,
Tony Bigwood, Mick Bishop, Rooney Buising,
Jacqui Burgess, Chris Callogeras, Darren Cameron,
Louise Chilvers, Lindsay Chong-Seng,
Lien-Sang Chou, Vic Cockcroft, Raoul Cola, Rob Coles,
Peter Corkeron, Junne Cosmas, Brian Crowford,
Nick Cox, Rahman Dako, Himansu Das,
Alain Jeudy de Grissac, Mark Deters, Keith Dewar,
Geoff Dews, Salif Diop, Kirstin Dobbs, Etienne Douaze,
Paul Dutton, Fisheries Department of Western Australia,
Tim Foresman, Bill Freeland, Nick Gales, Rod Garrett,
Claire Garrigue, Sapta Putra Ginting, James Hall,
ShaneHansford, Donna Hart, David Haynes,
Peter Hellyer, Max Herriman, John
Hicks,Tran Minh Hien, Chris Hillman, Ellen Hines,
Shannon Hogan, Ivan Holland, David Holly,
Noah Idechong, Beth Ingraham, Saifullah Jaaman, Tom
Jefferson, Alex Jefremov, Lindsay Joll, Kay Kale,
Choomjet Karnchanakesorn, Toshio Kasuya,
Darlene Ketten, Benjamin Kahn, David King,
Hugh Kirkman, John Kuo, Mohd Rajuddin Kushairi,
Donna Kwan, Ally Lankester, Ivan Lawler,
David Lawson, Stephanie Lemm, KanJanapaj
Lewmanomont, Dennis Lisbjerg, Ulla Lohmann,
Dominggus Maitimu, Stephanus Mandagi,
Eddy Mantjoro, Mansor Mat Isa, Billy Matindas,
Peter Mawson, Rob McCauley, Brenda McDonald,
Len McKenzie, Patrick M’mayi, John Mitchell, R.J.
Moermanto, James Monkivitch, Nyawira Muthiga, Hani
Nusantari, Masa Onishi, Ei Lin Ooi, Tom O’Shea,
Stephanie Palley, Pietro Pecchioni, Michael Phelan,
Suwan Pitaksintorn, Colin Poole, Sombat Poovachiranon,
Lindsay Porter, Tony Preen, Bob Prince, Queensland
Tropical Public Health Unit, Ridzwan Abdul Rahman,
John Reynolds, Hans Rotauscher, Keith Saalfeld, Charles
Santiapillai, Tipawan Sethapun, Seychelles Islands
Foundtion, Kath Shurcliffe, Andrew Smith, Brian
Smith,Tony Stokes, Johannes Subijanto,
Chatcharee Supanwanid, Purbasari Surjadi, Ismu Sutanto
Suwelo, Fadli Syamsudin, Edward Tangon, Ricardo
Tapilatu, Elizabeth Taylor, Gary Thornton, Larisa
Vanstien, Michelle Waycott, Western Australian
Department of Conservation and Land Management
(CALM), Scott Whiting, Bruce Wiggan, Andrew Willson,
Steve Winderlich, John Wong, Stan Wright,
Arnel Andrew Yaptinchay, and Kaiya Zhou. Special
thanks to Bob Prince of CALM and John Reynolds, co-
chair of the IUCN/SSC Sirenia Specialist Group, who
reviewed the document.
This is a long list and we apologise for any inadvertent
omissions.
Acknowledgements
5
Chapter 1
Introduction
Figure 1.1 – The known range of the dugong.
The dugong (Dugong dugon) is the only herbivorous
mammal that is strictly marine and is the only extant
species in the Family Dugongidae. The other members of
the Order Sirenia, the three species of manatee, all use
fresh water to varying degrees (Reynolds & Odell 1991).
The only other recent Sirenian, Steller’s sea cow
Hydrodamalis gigas, was hunted to extinction within 27
years of its discovery in the eighteenth century (Stejneger
1887). All extant members of Order Sirenia (including the
dugong) are listed as vulnerable to extinction (Anon.
1996a). All populations of the dugong are also listed
on Appendix 1 of the Convention on International
Trade in Endangered Species of Wild Fauna and Flora
(CITES). Prospects for the survival of the dugong
are the best, because each manatee species has a more
localized distribution than that of the dugong (Reynolds &
Odell 1991). In addition, the estimates of dugong
abundance in Australia (Chapter 6) are much greater
than have been recorded or suggested for any species
of manatee.
Distribution and Abundance
As summarised in this document, the dugong has a
large range (Figure 1.1) that spans at least 37 countries
and territories and includes tropical and subtropical
coastal and island waters from east Africa to Vanuatu,
between about 26° and 27° north and south of the equator
(Nishiwaki & Marsh 1985). The dugong’s historic
distribution is believed to have been broadly coincident
with the tropical Indo–Pacific distribution of its food
plants, the phanerogamous seagrasses of the families
Potamogetonaceae and Hydrocharitaceae (Husar 1978).
As detailed in this document, it is generally believed
that throughout much of its range, the dugong is
represented by relict populations separated by large areas
where its numbers have been greatly reduced or it is
already extirpated. However, the degree to which dugong
numbers have dwindled, and their range fragmented, is
not known for any country in its range. We are encouraged
that dugongs are still present at the historical limits of
6
their range (Figure 1.1, see also Chapters 2, 4 and 5),
although as discussed in this document there is evidence
of a reduction in the dugong’s area of occupancy within
this range. Nonetheless, we consider that it is likely that
dugong numbers are higher than previously supposed in
many areas. For example, following the death of at least
37 dugongs during the 1983 Nowruz oil spill, it was
feared that dugongs might have been extirpated from the
Arabian Gulf. Aerial surveys in the region established that
these fears were unjustified and that the estimated Arabian
Gulf population was 7310 (± s.e. 1300) dugongs (Preen
1989a). Recent fixed-wing aircraft and helicopter surveys
have confirmed that dugongs still occur in two areas
where they had been presumed to be locally extinct:
Ryukyu Islands in southwestern Japan and in a marine
park in eastern Malaysia (Chapter 4).
In most of the 37 countries and territories in the
dugong’s range for which we have any information, our
knowledge of their distribution and abundance is known
only from incidental sightings, accidental drownings, and
the anecdotal reports of fishers (Table 1.1). In another
fourteen countries, the dugong is known only from
spatially and temporally limited surveys generally
conducted parallel to the shoreline. These surveys provide
minimum counts only. Extensive quantitative aerial
surveys using transects across the shoreline depth gradient
have resulted in a more comprehensive knowledge of
dugong distribution and abundance in the coastal waters
of most (but not all) of the dugong’s range in the Arabian
region (Chapter 2) and northern Australia (Chapter 6).
However, even in these countries, the information is not
comprehensive enough to establish trends in abundance
for most areas, especially as there is increasing evidence
that dugongs undertake large-scale movements at scales
greater than those covered by individual surveys (even
though the areas covered by these surveys is often very
large (~30,000km2)). In addition, even the population
estimates resulting from transect surveys are probably
underestimates because the correction for availability bias
(the dugongs that are unavailable to observers because of
water turbidity) are conservative (Marsh &
Sinclair 1989a).
It is inappropriate to compare the abundance of
dugongs estimated using shoreline and quantitative
surveys. We believe that most of the estimates of dugong
population size reported in this document are
underestimates, probably major underestimates.
Nonetheless in most parts of its range, the anecdotal
evidence suggests that dugong numbers are declining.
In most of the developing countries that comprise the
dugong’s range, it may be impractical to use aerial
surveys to improve knowledge of dugong distribution and
relative abundance. More economical survey and research
techniques, such as interviews, should be used at least
initially, in areas where there is little or no information as
suggested by Marsh & Lefebvre (1994) and detailed in
Aragones et al. (1997).
Life History
Almost all information on dugong life history has
been obtained from the analysis of animals accidentally
drowned in shark nets or killed by native hunters in
northern Australia. The age-determination method
developed for pinnipeds and toothed cetaceans was
adapted for dugongs enabling their age to be estimated
from the number of growth layer groups in the tusks (see
Marsh 1980). Life-history parameters are summarized in
Marsh (1995a, 1999). Dugongs are long-lived with a low
reproductive rate, long generation time, and a high
investment in each offspring. The oldest dugong whose
tusks have been examined for age determination was
estimated to be 73 years old when she died. Marsh’s
research suggested that females do not bear their first calf
until they are at least ten and up to 17 years old. Using the
same age determination technique, Kwan (unpublished
data) has recent evidence from Torres Strait of dugongs
producing their first calf as early as age 6 years. Gestation
is in the region of 13-15 months. The usual litter size is
one. The calf suckles for 14-18 months or so, and the
period between successive calvings is spatially and
temporally variable; estimates range from 2.4 (Kwan
unpublished data) to seven years. Dugongs start eating
seagrasses soon after birth, but they grow rapidly during
the suckling period when they also receive milk from their
mothers. Population simulations indicate that even with
the most optimistic combinations of life-history
parameters (e.g. low natural mortality and no human-
induced mortality) a dugong population is unlikely to
increase more than 5% per year (Marsh 1995a, 1999).
Although this figure will be revised slightly upward as a
result of Kwan's unpublished data, the conclusion that its
life history makes the dugong vulnerable to over-
exploitation remains.
Dugong mating behaviour appears to vary spatially.
Preen (1989b) observed mating herds in Moreton Bay,
Queensland (Figure 6.6), where male dugongs violently
compete for oestrous females, and similar herds have been
observed in two localities in northern Queensland
(Marsh et al. 1999). Several presumed males attempt to
embrace the presumed female, each attempting to mate
with her (Preen 1989b). In contrast, resident dugongs in
South Cove in Shark Bay in Western Australia
(Figure 6.1) exhibit mating behaviour consistent with the
definition of a lek. “In a classic lek males aggregate on
7
mutually exclusive display areas at a traditional site that
lacks resources required by females. Females visit this
site only in order to mate” (Anderson 1997). Anderson
(1997) observed male dugongs defending mutually
exclusive territories in which unique behaviours were
displayed in order to attract females. It is not known
whether lekking occurs elsewhere in the dugong’s range.
Diet
Dugongs are seagrass specialists, uprooting whole
plants when they are accessible, but feeding only on
leaves when the whole plant cannot be uprooted
(Anderson 1982a; Marsh et al. 1982, 1999). Anderson
(1998b) claims that his observations in North Cove, Shark
Bay, Western Australia (Figure 6.1), suggest that dugongs
selectively forage for Halodule rhizomes. Dugongs prefer
seagrasses that are lower seral or ‘pioneer’ species (Preen
& Marsh 1995), especially species of the genera
Halophila and Halodule. Diet selection is correlated with
the chemical and structural composition of seagrass
(Lanyon 1991; Aragones 1996). The most frequently
selected species are lowest in fibre and highest in
available nitrogen and digestibility (Lanyon 1991;
Aragones 1996). Selection for the species that are highly
digestible (Halophila) and have high nutrients (Halodule)
means that dugongs maximize the intake of nutrients
rather than bulk (Aragones 1996). Marine algae are also
eaten (Marsh et al. 1982), but this is believed to occur
only when seagrass is scarce (Spain & Heinsohn 1973).
Anderson (1989) and Preen (1995) have evidence that
dugongs may deliberately forage for macro-invertebrates
near the southern limits of their range in both western and
eastern Australia. However, examination of stomach and
faecal samples (Preen & Marsh 1995) suggests that
dugongs do not deliberately forage on macro-
invertebrates in more tropical areas in Australia. This
conclusion must be regarded as tentative because of the
differential digestibility of plant and animal material in
the mammalian alimentary canal (Sheffield et al. 2001).
The highly specialized dietary requirements of the
dugong suggest that only certain seagrass meadows may
be suitable as dugong habitat (Preen et al. 1995). Preen
et al. (1995), de Iongh (1996) and Aragones and Marsh
(2000) suggest that grazing activities by dugongs alter the
species composition of seagrass communities at a local
scale. Thus, areas that support sizeable numbers of
dugongs may have the capacity to provide better ‘quality’
food than areas that support few or no dugongs and rely
only on natural turnover rates for recycling and
redistribution of nutrients (Aragones & Marsh 2000).
Habitat
Dugongs generally frequent coastal waters. Major
concentrations tend to occur in wide shallow protected
bays, wide shallow mangrove channels and in the lee of
large inshore islands (Heinsohn et al. 1979). These areas
are coincident with sizeable seagrass beds. Dugongs are
also regularly observed in deeper water further offshore in
areas where the continental shelf is wide, shallow and
protected. For example, in Torres Strait between Australia
and Papua New Guinea, significant numbers of dugongs
are seen more than 10km from land (Marsh & Saalfeld
1989, 1991). Marsh and Saalfeld (1989) have also sighted
dugongs ~58km from the north Queensland coast, in
water up to 37m deep. This distribution reflects that of
deepwater seagrasses such as Halophila spinulosa (Lee
Long et al. 1993). Dugong feeding trails have been
observed at depths of up to 33m off north-eastern
Queensland (Lee Long & Coles 1997). Whiting (1999)
reported dugongs including calves at Ashmore Reef (12o
15'S, 123o05'E) on the Sahul Banks on the edge of the
Australian continental shelf. Although Ashmore Reef is
only 140km from the Indonesian Island of Roti, these
locations are separated by the Timor Trough which is
2000m deep.
There is evidence that dugongs use specialised
habitats for various activities. Shallow waters, such as on
tidal sandbanks (Marsh et al. 1984c) and estuaries
(Hughes & Oxley-Oxland 1971), have been reported as
sites for calving. Anderson (1981) suggested that this may
be a strategy to minimise the risk of shark predation.
The physical characteristics of South Cove in Shark
Bay, Western Australia (Figure 6.1) may make it
especially suitable for the lek mating behaviour observed
by Anderson (1997). At the higher latitudinal limits to
their range, deeper waters may be used as a thermal refuge
from cooler inshore waters (Anderson 1986; Marsh et al.
1990; Preen 1992) in winter.
Life history attributes of dugongs are likely to vary
across such a huge range of habitat types (Reynolds pers
comm. 2001).
Movements
Most movements of the more than 60 dugongs that
have been tracked by means of VHF or satellite
transmitters in Indonesian and Australian waters have
been localised to the vicinity of seagrass beds (Marsh &
Rathbun 1990; Preen 1992; de Iongh 1996; de Iongh et al.
1998; Preen 1999, 2001). Animals caught in the same
region tend to show individualistic patterns of movement.
8
Daily movements depend on tidal amplitude. At localities
where the tidal range is large (e.g. up to 8.5m in
Shoalwater Bay in Queensland; Anderson & Birtles 1978)
(Figure 6.6), dugongs can gain access to their inshore
feeding areas only when water depth is 1m or more. In
areas with low tidal amplitude such as Shark Bay in
Western Australia (Anderson 1982b) (Figure 6.1) or in
areas where seagrass grows subtidally, daily movements
are not dictated by tides. At the high-latitude limits of
their range, dugongs make seasonal movements to
warmer waters. In winter in Moreton Bay, Queensland
(Figure 6.6), many dugongs regularly make a round trip of
15–40km between their foraging grounds inside the bay
and oceanic waters, which average up to 5°C warmer
(Preen 1992). Dugongs also undertake winter movements
of the order of 100km across Shark Bay Western Australia
(Figure 6.1) to warmer waters in the westward part of that
bay (Anderson 1982b, Marsh et al. 1994a; Gales, Holly &
Lawler unpublished data). At least some individual
dugongs undertake long-distance movements. An adult
female moved 600km between two sites in the Gulf of
Carpentaria in Australia (Figure 6.2) over about five days
(Preen 1995). Another male travelled between two
localities, in the Central Section of the Great Barrier Reef
Australia (Figure 6.1), a straight-line distance of 140km,
three times in six weeks (Marsh & Rathbun 1990). Of the
ten dugongs fitted with satellite transmitters in Shoalwater
Bay, Queensland (Figure 6.6) by Preen (1999), four made
substantial trips out of that bay. Two made return trips:
one 100km north, the other 220km north. Two other
animals journeyed 400km south to Hervey Bay (Figure
6.1) where their transmitters came off. Thirteen dugongs
were tracked between the Townsville and Hinchinbrook
Island region in Queensland (Figure 6.6). Twelve trips
were made of more than 30km beyond the area regularly
used by these animals, six trips of more than 100km and
one trip of more than 600km (Preen 2001). Most of these
movements were return trips. For example, the animal that
moved more than 600km north returned to her capture
point after five months and almost immediately moved
another 165km south along the coast. The movements of
this dugong thus spanned about 800km of coast. Reports
in 2001 of dugongs at Aldabra Atoll (Chong-Seng pers
comm. 2001) 425km from Madagascar confirm their
capacity to cross deep ocean trenches (up to 4km in depth)
as dugongs have not been seen in this region for many
years (Cockcroft et al. 1994; Cockcroft & Young 1998).
This capacity of dugongs to undertake long-distance
movements indicates that the management of dugongs is
an international issue in most parts of their range.
Genetic Population Structure
Molecular techniques are being used to investigate the
genetic population structure of dugongs (Tikel 1998;
McDonald unpublished data). Results to date are based
on mitochondrial DNA which evolves relatively quickly
and is considered a good index of population structure. It
is transmitted only in the female lineage and can only be
used to estimate female-mediated gene flow. Results are
based on very small sample sizes from outside Australia
and for many areas in the dugong’s range within Australia.
In addition in mammals, male-mediated gene flow is often
markedly greater than female-mediated flow. To test for
this in dugongs, a nuclear marker, or markers, will be used
to make a more complete assessment of the genetic
population structure. Thus the conclusions outlined below
are tentative.
The results suggest that the haplotypes of dugongs
from parts of Southeast Asia (Indonesia, Thailand and the
Philippines) are generally distinct from those from
Australia with overlap at Ashmore Reef between Western
Australia and Timor, suggesting that there is (or has been
in the past) limited genetic exchange between Australia
and Asia. The genetic structure of dugong populations
around the Australian coast appears to comprise two
maternal lineages one of which has also been recorded
from dugongs from Kenya and the Arabian Gulf
(Tikel 1998). Torres Strait between Australia and Papua
New Guinea is a major zone of overlap between the
two lineages.
Threatening Processes
Dugongs are vulnerable to anthropogenic influences
because of their life history and their dependence on
seagrasses that are restricted to coastal habitats, and
which are often under pressure from human activities.
There are a number of socio-political impediments to
dugong conservation, particularly in developing countries.
The displacement and urbanisation of rural human
populations has led to the loss of traditional values and
taboos to resource exploitation. The nearshore areas
where dugongs occur have become an easy and
convenient source of food and income. The provision
of philanthropic aid from ‘developed countries’
increases the efficiency and level of exploitation. The
situation is exacerbated by an absence of adequate
legislation, enforcement and management (Cockcroft
pers comm. 2001).
The rate of population change is most sensitive to
changes in adult survivorship. Even a slight reduction in
adult survivorship as a result of habitat loss, disease,
9
hunting or incidental drowning in nets, can cause a
chronic decline in a dugong population. As explained
above, Marsh (1995a, 1999) suggested that the maximum
rate of increase under optimum conditions would be on
the order of 5% per year even when natural mortality is
low (<5% per year). The sustainable harvest is likely to be
in the order of 2% of the female population per year. The
sustainable harvest rate will be lower in areas where the
pre-reproductive period and/or calving interval are
lengthened by food shortage (Marsh 1999). Dugongs may
be short of food for several reasons including habitat loss,
seagrass dieback, decline in the nutrient quality of
available seagrass or a reduction in the time available for
feeding because of acoustic disturbance such as
boat traffic.
Habitat Loss and Degradation
Seagrass ecosystems are very sensitive to human
influence (Fonseca 1987; Shepherd et al. 1989; Poiner &
Peterken 1996). Seagrass beds may be destroyed directly
by mining and trawling (Silas & Bastion-Fernando 1985),
or lost through the effects of disturbances such as
dredging, inland and coastal clearing, land reclamation
and boat propeller scarring. These activities cause
increases in sedimentation and turbidity which, in turn,
lead to degradation through smothering and lack of light.
Other threats include sewage, detergents, heavy metals,
hypersaline water from desalination plants and other
waste products. Most losses, both natural and
anthropogenic, are attributed to reduced light intensity
due to sedimentation and/or increased epiphytic growth
caused by nutrient enrichment. In some cases, factors
such as poor catchment management and sediment
instability interact to make the processes more complex so
that it is often difficult to separate natural and
anthropogenic causes of seagrass loss. In addition,
herbicide runoff from agricultural lands also presents a
potential risk to seagrass functioning adjacent to
sugarcane production areas (Haynes et al. 1998).
Episodic losses of hundreds of square kilometres of
seagrass are associated with extreme weather events such
as some cyclones, hurricanes and floods (Poiner &
Peterken 1996). Jones (1967) reported the widespread
loss of seagrass in the Gulf of Mannar–Palk Bay area
between India and Sri-Lanka in 1954 as a result of a
cyclone accompanied by very heavy rains. Large numbers
of dugongs were reportedly washed ashore a few days
after the cyclone. In the Gulf of Carpentaria in Northern
Australia (Figure 6.2 and 6.3) in 1985, cyclone Sandy
caused the loss of 151km2of seagrass, representing ~20%
of the entire Gulf’s seagrass area. In 1991–92 several
hundred square kilometres of seagrass disappeared from
Torres Strait between Australia and Papua New Guinea
(Figure 6.4), possibly because of high turbidities resulting
from flooding of river(s) in Papua New Guinea.
Furthermore, more than 1000km2of seagrass was lost in
Hervey Bay, Queensland (Figure 6.6) in 1992–93,
possibly because of high turbidities resulting from
flooding of local rivers, and runoff turbulence from a
cyclone three weeks later (Preen & Marsh 1995). Such
events can cause extensive damage to seagrass
communities through severe wave action, shifting sand,
adverse changes in salinity and light reduction (Heinsohn
& Spain 1974; Kenyon & Poiner 1987; Preen & Marsh
1995; Preen et al. 1995). Recovery and recolonization
after large-scale losses of tropical seagrasses may take a
decade or more (Poiner & Peterken 1996).
Halophila ovalis, one of the preferred food species of
dugongs, appears to be particularly sensitive to light
reduction, with the duration and frequency of light-
deprivation events apparently being the primary factors
affecting the survival of this seagrass in environments that
experience transient light deprivation (Longstaff et al.
1999). During light-deprivation experiments the biomass
of H. ovalis declined rapidly and recovered slowly, with a
complete die-off occurring after 30 days of deprivation
(Longstaff et al. 1999). Members of the genus Halophila
occur at greater depths than other species of tropical
seagrasses and this sensitivity to light reduction is a
plausible explanation of the large-scale loss of deep-water
seagrasses in Torres Strait (Poiner & Peterken 1996)
(Figure 6.4) and Hervey Bay (Preen et al. 1995)
(Figure 6.6).
To date, the approach to seagrass protection has
largely been through marine parks and fishing industry
closures to prevent structural damage to seagrass beds
through trawling. There have been few attempts to protect
seagrass beds from adverse impacts on ecosystem
processes associated with land use, even though such
impacts have been recorded as of concern in most of the
countries for which we obtained information for this
document (Table 1.2). Localities that provide shelter and
water conditions ideal for seagrasses are often the target
for port developments and at the down-stream
end of severely affected catchments (Lee Long & Coles
1997). As identified by Lee Long and Coles (1997),
research is urgently required to describe the response of
seagrasses to natural and human factors and to
establish: (1) acceptable levels of change in response to
such factors, and (2) the water-quality conditions that lead
to these changes.
Fishing Pressure
Accidental entangling in gill and mesh nets or traps
set by fishers is considered a major, but largely
unquantified, cause of dugong mortality in many
10
countries (Perrin et al. 1996) and was identified as a major
concern in virtually all countries covered by this
document (Table 1.2). Throughout most of the dugong’s
range, this pressure comes from locally-based artisanal
fisheries. Of more concern, are the industrial scale gill net
fisheries which have developed in some areas. Fortunately
for dugongs, these are in offshore waters which are not
major dugong habitats. To our knowledge, the systematic
collation of data on the incidence of dugong by-catch in
fisheries has not been attempted by observer programs in
any country in the dugong’s range. No data are available
on the take of dugongs by lost or discarded nets, although
drowning in these nets presumably occurs.
The relationship between tides, bottom topography,
turbidity and patterns of netting needs investigation.
In relatively shallow bays with large tidal
fluctuations and high turbidity, seagrass meadows are
largely intertidal. In such circumstances, dugongs and
netters are all forced to use intertidal areas on
the high tide, increasing the chances that dugongs
will be caught.
Acoustic alarms (pingers) are proving effective at
reducing the mortality of the harbour porpoise, Phocoena
phocoena, in gill nets (Trippel et al. 1999). These alarms
are increasingly seen as a possible solution to the problem
of marine mammals drowning in nets in developed
countries, although the associated costs are likely to
preclude their use throughout most of the dugong’s range.
The auditory capabilities of the West Indian manatee
range from 0.4 to 46 KHz (Gerstein et al. 1999), spanning
the range of acoustic alarms (10–12 kHz) (Trippel et al.
1999). The effectiveness of the use of acoustic alarms in
reducing the mortality of dugongs in nets has not been
tested. Given the dugong’s specialised habitat
requirements, it is important to test whether their use
reduces the habitat available to dugongs before they are
widely adopted in countries such as Australia and Japan.
Shark nets set for bather protection can be another
source of dugong mortality. Between 1962 and 1995,
shark nets set on swimming beaches in Queensland netted
837 dugongs (Anon. 1992). Most animals caught in shark
nets die (Paterson 1990). In response to a Ministerial
Committee of Enquiry (Anon. 1992), initiatives were
begun in 1992 (Gribble et al. 1998) to reduce the capture
of non-target species. Baited hooks have replaced shark
nets in many localities and the mortality associated with
this program is now low.
Indigenous Use and Hunting
Dugongs are culturally significant to communities
throughout their range. In this document, we record
information about the indigenous use of dugong products
in most of the countries for which we have information
(Table 1.2). Dugongs are caught for meat, oil,
medicaments, amulets and other products.
In many countries dugong hunting is now banned
and animals are no longer hunted deliberately
however, dugong products from indirect takes are still
highly valued. Australia’s indigenous peoples consider
dugong hunting to be an important expression of their
identity. In the Western Islands of Torres Strait, the
dugong harvest in the 1990s has been estimated to be on
the order of 1000 per year (Marsh et al. 1997a).
Indigenous peoples’ rights to utilise their sea country
(for activities including hunting) have recently been
recognised by the High Court of Australia (The
Commonwealth v. Yarmirr; Yarmirr v. Northern Territory
[2001] HCA 56 [11 October 2001] D72000 and D92000).
This decision confirms that the indigenous people of
Australia hold Native Title over the sea beyond the low
water mark. These rights are non-commercial and non-
exclusive. The Torres Strait Treaty between Australia and
Papua New Guinea explicitly protects the traditional way
of life of the local indigenous peoples including the right
to hunt dugongs.
Vessel Strikes
Vessel strikes are a major cause of mortality for
Florida manatees (Wright et al. 1995). Although
manatees possess the intellectual and physical ability to
recognise and avoid boats (Hartman 1979; Gerstein 1994,
1995), the results of Gerstein et al. (1999) suggest that the
West Indian manatee possesses a limited low-frequency
hearing sensitivity and therefore has difficulty detecting,
as well as locating, approaching boats from safe distances.
The relevance of these results to dugongs is unknown
because the anatomy of the dugong ear differs from that
of the manatee (Ketten pers comm. 2001). Although there
are few documented dugong deaths due to vessel strikes
(Table 1.2), increasing vessel traffic in the dugong’s range
increases the likelihood of strikes. Areas where there are
extensive shallow areas used by regionally important
populations of dugongs close to areas of high boat traffic
are particularly at risk.
Ecotourism
The expansion of ecotourism has resulted in
the establishment of tourism operations involving
dugong-watching cruises at several locations in
Australia (Chapter 6) and swim with dugong
operations in the Philippines (Chapter 4) and Vanuatu
(Chapter 5). The effect of these activities on the animals is
unknown, although it is under investigation in Western
11
Australia. We are unaware of similar operations outside
these regions.
Acoustic Pollution
Despite consistent anecdotal reports of dugongs
ceasing to use areas with high boat traffic, there has been
no formal attempt to study the effect of acoustic pollution
from boat traffic on the dugong. Such a study has
commenced in Queensland, Australia. Acoustic pollution
could be particularly important in areas with large tidal
ranges and little seagrass below the low tide mark.
Presumably, high levels of vessel traffic in such areas
could prevent dugongs from using available intertidal
seagrass meadows. Defence Force exercises are
conducted at several localities within the dugong’s range
in Australia. The exercises include surface and underwater
explosion of bombs, amphibious landings, and firing of