TOWARDS A SUSTAINABLE INDIGENOUS FISHERY FOR DUGONGS IN TORRES STRAIT: A CONTRIBUTION OF EMPIRICAL DATA ANALYSIS AND

PROCESS

Thesis Submitted By

Donna Kwan B.Sc., M.Sc.

February 2002

For the degree of

Doctor of Philosophy

School of Tropical Environment and Geography

James Cook University

Townsville

Australia.

ii

DEDICATION

This work completes another one of my life circles and is dedicated in memory of my two greatest heroes:

my mother, Josephine Rose Kwan

and

my son, Morgan Henry Hoedt

'Thy firmness makes my circle just

and makes me end where I've begun''

Randolph Stow. 1985. Meny-Go-Roundin the Sea. Penguin Books, Middlesex.

STATEMENT OF ACCESS

I, the undersigned, the author of this thesis, understand that James Cook University will make it available

for use within the University library and, by microfilm or other photographic means, allow access to users

in other approved libraries. All users consulting this thesis will have to sign the following statement:

"in consulting this thesis I agree not to copy or closely paraphrase it in whole or in part without consent of

the author; and to make proper and written acknowledgement for any assistance which I have obtained

from it."

Beyond this, I do not wish to place any restrictions on the access to this thesis.

Donna Kwan

February 2002

DECLARATION

I declare that this is my own work and has not been submitted in any form for another degree or diploma

at any university or institution of tertiary education. Information derived from the published or unpublished

work of others has been acknowledged in the text and a list of references is given.

Donna Kwan

February 2002

ACKNOWLEDGEMENTS

Without dedicating an entire chapter to my acknowledgements, I will not do justice to all those who

contributed. However, I would like to thank the Australian Fisheries Management Authority (AFMA) who

funded my project and provided a PhD scholarship at James Cook University.

I am most grateful to the hunters, Chairmen, Councils, Elders and members of lsland communities of

Boigu, and Badu and most especially Mabuiag, for their hospitality, friendship, tolerance and commitment

to me and my project in proper Ailan Pasin (Island Way). A Big Esso (thank you) to: Edward Dau,

Chairman of Boigu lsland Council and Councillor Fred Anau, Jack Ahmat, Chairman of Badu lsland

Council and Councillors William Bowie and Titom Nona and especially Terrence Whap, Chairman of

Mabuiag Island, Councillors Edmund Bani and Thowa Whap and council staff, Holly Banasa and Kelsie

Whap. I am also very grateful for the assistance of the following people who worked as research

assistants in Torres Strait: Ishmael Gibuma, Nelson Gibuma, Greg Toby, John Coburn, Michael (Ninja)

Hankin and Naomi Paipai. Esso Garr(greatest thank you) to the hunters at Boigu Island: especially (in

alphabetical order) Ishmael Gibuma, Nelson Gibuma, Ray Marama and Gary Tom; at Mabuiag lsland

Bertie Amber, Edmund Bani, John Coburn, Frank Genai, William Gizu, Amos Joe, Kenya Joe, Charlie

Hankin, Koza Hankin, Tabithia Joseph, John Kris, lona Mooka, Kenya Mooka, Matthew Paipai, Allan Repu

and Aaron Whap; at Badu: James Ahmat, Jack Nai, Harry Nona, Paiwi Nona, Dintu Tamwoy. Without their

contribution, this thesis would not have been possible.

I owe a special Big Esso to Dr Judith Fitzpatrick (Ama Judy, Aunty Judy) for sharing her special extended

family at Mabuiag lsland with me and for being a kindred spirit. I would also like to thank my other

extended families in Torres Strait who felt compelled to adopt, feed and console an otherwise ' lone

dugong researcher' at various times especially: Helen, Jim and Luke Heery at Malu Kiwai State School at

Boigu Island; Jim and Linda, Rachael and Sarah Ballantyne at Mabuiag State School; Matty and Loralie

Night (thanks Loua for having the only bottle of gin on Boigu lsland at my time of greatest need; will take

care of Matt and Sapheira) and Pedro, Traci and Loren Thornton, for their friendship and support during

my fieldwork.

I would like to especially thank the following AFMA Torres Strait Fisheries staff in Thursday lsland for

adopting this 'forlorn dugong researcher' in Torres Strait: Mick Bishop, Johanna Dorante (Esso Garr J.oh,

shall always miss you your friendship), Jillian Kennedy, Tony Kingston, Fred Mills and Nancy Mosby. I am

grateful to the Torres Strait Regional Authority (TSRA) and Chairman Getano Lui and the lsland Co-

ordinating Council (ICC) and Chairman Henry Garnierfor their support. I thank the following TSRA and

ICC staff for their valuable guidance and advice: Geoff Dews, Paul Hayes, Douglas Jacobs, Kevin Murphy

ABSTRACT

Given the globally threatened status of dugongs and the fact that Torres Strait supports the world's largest

dugong population, there is international, national, regional and local interest in ensuring the sustainability

of the Torres Strait traditional dugong fishery. This fishery is endorsed by the Torres Strait Treafy between

Australia and Papua New Guinea (PNG).

This study investigated, within a biological context, the major factors (i.e., those additional to biological)

that determine sustainability of the traditional dugong fishery in Torres Strait.

My thesis had the following specific aims:

(1) To provide information relevant to the development of community-based management for the dugong

fishery in Torres Strait by:

describing and quantifying the major factors that affect hunting pattern, hunting effort, hunting

success and harvest levels in the major dugong hunting community of Mabuiag Island, and

estimating the life history parameters of Torres Strait dugongs at the time of my study, and (i)

comparing them with parameters obtained from parallel studies of dugong life history and

reproductive biology in order to gain insights into the factors influencing these aspects of dugong

ecology, and (ii) incorporating them into population models to predict the natural rate of change

in the size of the population in the absence of hunting mortality.

(2) To assist in the development of community-based management in Torres Strait through contributing to

capacity building by actively involving Torres Strait Islanders in my research and training them in the

collection of catch statistics and biological samples from dugongs.

The fieldwork for this project was undertaken primarily at Mabuiag Island between September 1997 and

November 1999 inclusive. All data and specimens from 127 female and 51 male dugongs were collected

from animals butchered for food by Torres Strait Islander hunters.

Being able to live and work within the Mabuiag Island community provided me with the rare opportunity to

develop a relationship with Islanders built upon the mutual trust, co-operation and commitment. Involving

Islanders as active participants in my research allowed me to collect the valuable specimens on which my

information is based. The active involvement of hunters and community members enhanced their

development of a sound understanding of and trust in the information I obtained. By training Torres Strait

Islanders as research assistants, 1 also increased the capacity of the community to actively participate in

future research, monitoring and management programs for dugongs.

This study provided the opportunity to identify the main factors which impact catch rates of dugongs by

Islanders. Only four to five hunters were responsible for 79% and 75% of the 145 and 170 dugongs taken

at Mabuiag lsland during March to October in 1998 and 1999, respectively. The magnitude of the dugong

catch was influenced by ecological, economic environmental and social factors that affected the temporal

and spatial nature of hunting effort and hunting success.

When my results were compared with those of other studies using similar techniques, I showed that the

age and size at sexual maturity and first reproduction, pregnancy rates of female dugongs and the sex

ratio of their young exhibit considerable variability in both space and time. The age and size at sexual

maturity and first reproduction of male dugongs exhibit similar variability. At Mabuiag lsland in 1997-99,

female dugongs reached sexual maturity between six or seven and fourteen years and at body lengths of

between 2.05 and 2.58 m. Male dugongs reached sexual maturity between four and thirteen years and at

body lengths between 1.90 and 2.57 m. Calving intervals of adult female dugongs from Mabuiag lsland in

1998-99 ranged between 2.4 f s.e. 0.3 to 3.3 f s.e. 0.5 years. The parameters from Mabuiag lsland in

1997-98 are thus the lowest recorded for dugongs. At Mabuiag lsland in 1997-98, almost all sexually

mature nulliparous females were either pregnant andlor lactating and the sex ratio of foetuses was 1:3 in

favour of males.

Variability in life history parameters such as age at sexual maturity and first reproduction, pregnancy rates

and calving intervals in dugongs is likely to result from spatial and temporal variation in seagrass

community composition and biomass. Thus, food quality and availability influence dugong population

dynamics. When their seagrass food disappears as a result of episodic disturbance, some dugongs

suspend breeding. It is likely that the effects of food supply on life history parameters and hence fecundity

in dugongs includes both density dependent and density independent components. The gaps in the age

composition of the catch from Mabuiag in 1997-99 together with the low pregnancy rates of animals

caught in the Daru market at that time suggest that a seagrass dieback in Torres Strait in the late 1970s

had significant effects on dugong recruitment. These links between seagrass food availability and dugong

life history parameters have very important implications for management given the dugong's susceptibility

to large-scale episodic losses of seagrass.

Several findings in this study highlight that a regional approach to management will be required because

the potential for episodic or longer-tern over-harvesting can be exacerbated in the event of a combination

of: low dugong fecundity as a result of extensive seagrass dieback events such as that reported in the

1970s; movements of dugongs close to hunting communities to feed on intertidal seagrass in response to

dieback of deepwater communities exacerbating high hunting pressure; increase in hunting pressure in

response to reduction in important sources of income such as the cray fishery.

Management coordinated at a regional scale must include the Northern Peninsula Area of Cape York and

PNG. Management will need to build on established processes which incorporate Ailan Kast'om (the

customary way of life of Torres Strait Islanders) and institutional structures in the Torres Strait region

which include the: Torres Sfraif Treafy, Torres Sfraif Fisheries Acf (1984), Native Title and regional

Islander initiatives such as MaSTERS (Marine Strategy for Torres Strait).

A community-based management approach, which builds on the research processes, and the empirical

information obtained in this study, has considerable potential to ensure that the Torres Strait dugong

fishery is sustainable. The challenge now is to build on information and trust generated by this project by

enabling Islanders to use these results to develop and implement appropriate management plans for their

dugong resources.

Table of Contents

DECLARATION ................................................................................................................................. 111

ACKNOWLEDGEMENTS ................................................................................................................. IV

......................................................................................................... ABSTRACT ................... ..... VI

.............................................................................................................................. LIST OF TABLES XVI

...................................................................................................................... LIST OF FIGURES XIX

...................................................................................................................................... CHAPTER 1 ..... 1

1.1 INTRODUCTION ................................................................................................................... 2

1.2 STATUS OF AND THREATS TO THE DUGONG ................................................................ 4

.............................................. 1.3 CHALLENGES FOR CONSERVATION AND MANAGEMENT 6 ............................................................................................... 1.3.1 The Need for Conservation 6

....................... 1.3.2 Co-management Initiatives: the Community-Based Management Approach 8 .......................................................... 1.3.3 Life History Characteristics and Sustainable Hunting I 1

1.4 THESIS AIMS ...................... ... ............................................................................................ 13

1.5 THESIS OUTLINE ........................................................... .......... ........................................... 14

CHAPTER 2 .................................................................................................................................. 17

FACTORS AFFECTING TH E POPULATION DYNAMICS AND REPRODUCTIVE ECOLOGY OF LARGE HERBlVO RES. WITH PARTICULAR REFERENCE TO DUGONGS ............................ 17

.................................................................................................................... 2.1 INTRODUCTION 18

......... 2.2 MECHANISMS THAT AFFECT THE POPULATION DYNAMICS OF LARGE MAMMALS 19 ..... 2.2.7 Relationship Between the Parameters That Determine the Rate of Population Change 19

2.2.2 Effects of Environmental Stochasticity and Density Dependence on Nutrition in Large .................................................................................................................................... Herbivores 79

2.2.3 The Influence of Environmental Stochasticity on Populations: When it is likely to be more .................................................. important than Density Dependence on population dynamics? 21

................................................................ 2.2.4 Importance of Nutrition to Reproductive Success 22 .......................................................................... 2.2.5 Effect of Nutrition on Population Dynamics 23

2.3 SEASONALITY IN BREEDING ................... .. ................................................................... 24

................................................................................................ 2.4 DUGONGS AND SEAGRASSES 26 .................................................................................... 2.4.1 Dugongs as Seagrass Specialists 2 6

..... 2.4.2 Indo-Pacific Tropical Seagrass Communities with Particular Reference to Torres Strait 27 ............................................ 2.4.3 Tropical Seagrasses as Resources for a Specialist Herbivore 29

............................ 2.4.4 Effects of Large-Scale Seagrass Loss on Dugongs .... ............................ 33 .................................................................................................................................................... 2.4.4.1 Introduction 33

2.4.4.2 Loss of body condition and risk of mortality .................................................................................................. 35 2.4.4.3 Reduction in fecundity ................................................................................................................................... 36 2.4.4.4 Environmental factors influencing movements in sirenians ........................................................................... 37

..................................................................................................... 2.5 CONCLUDING REMARKS 42

2.6 SUMMARY .................................... ... ........................................................................................... 42

............................................................................................... ...................................... CHAPTER 3 .. 44

AN OVERVIEW OF THE TORRES STRAIT REGION FROM THE PERSPECTIVE OF THIS RESEARCH AND RATIONALE FOR THE SELECTION OF STUDY SITES .................................... 44

............................................................................................ 3.1 THE TORRES STRAl T REGION 4 5

......................................................................................................... 3.2 DEMOGRAPHIC PROFILE 47

........................... 3.3 INSTITUTIONAL STRUCTURES AND GOVERNANCE IN TORRES STRAIT 49

..................................... 3.4 ISLANDER PARTICIPATION IN ENVIRONMENTAL MANAGEMENT 51 3.4.1 Marine Sfrafegy for Torres Strait ........................................................................................ 52

................................................................................. 3.5 TORRES STRAIT AS DUGONG HABITAT 53

.............................................................................. 3.6 THE TORRES STRAI T DUGONG FISHERY 54

3.7 SELECTION OF STUDY SITES WITHIN TORRES STRAIT ..................................... 3.8 DESCRIPTIONS OF STUDY SITES ........................................................................................... 56

3.9 SUMMARY ............................................................................................................................... 5 7

CHAPTER 4 ....................................................................................................................................... 5 9

BUILDING RELATIONSHIPS BETWEEN THE SCIENTIST AND COMMUNITY ............................ 59

4.1 INTRODUCTION ......................................................................................................................... 60

....................... 4.2 SAMPLING PROTOCOL: CULTURAL AND COMMUNITY CONSIDERATIONS 61

4.3 SAMPLING REGIME ................................................................................................................... 63

............... ........................................................................... 4.4 COLLECTION OF SPECIMENS .. 65

4.5 BUTCHERING PROCE DURE ...................... .. .................................................................... 6 5

4.6 CHALLENGES DURING FIELD WORK ...................................................................................... 67 4.6.1 Community Perceptions ..................................................................................................... 67

4.6.1.1 Boigu Island ................................................................................................................................................... 67 4.6.1.2 Impacts from external perceptions ................................................................................................................ 68

4.7 TRAINING OF TORRES STRAIT ISLANDERS ......................................................................... 69

4.8 BENEFITS OF THE RELATIONSHIP BETWEEN HUNTERS AND SCIENTIST ........................ 71

4.9 SUMMARY ................................................................................................................................ 7 2

........................................................................................................................................... CHAPTER 5 74

THE PRACTICE OF HUNTING DUGONG AT MABUIAG ISLAND IN 1997-99 ................................ 74

...................................................................................................................... 5.1 INTRODUCTION 7 5

5.2 METHODS ................................................................................................................................... 76 5.2.1 Collecfion of Data ................................................................................................................ 76

5.3 RESULTS AN D DISCUSSION ............................................................................................... 76 ............................................................................................... 5.3. 1 . Hunters and Hunting Parties 76

5.3.2 Hunting Mefhods ...................... ... .................................................................................. 79 ................................................................................................................ 5.3.3 Hunting L ocafions 81

......................................................................................................... 5.3.4 Seasonality of Hunting 82 ...................................................................... 5.3.5 Maintenance of fhe dugong hunting tradifion 83

5.3.6 Consumption of Dugong Meat ............................................................................................. 84 5.3.7 Issues of Consumpfion ..................................................................................................... 85 5.3.8 lmplicafions for Moniforing and Management ...................................................................... 86

5.4 SUMMARY .............................................................................................................................. 88

CHAPTER 6 ..................... .................. .................................................................................................... 90

FACTORS AFFECTING THE TRADITIONAL HARVEST OF THE DUGONG FISHERY AT MABUIAG ISLAND IN 1997-99 ........................................................................................................................ 9 0

6.1 INTRODUCTION ....................................................................................................................... 91

6.2 METHODS .................................................................................................................................. 92 . . 6.2.1 Probabrlity of Hunting .......................................................................................................... 92 6.2.2 Catch Monitoring .................................................................................................................. 94

.................................................................................................................................... 6.3 RESULTS 95 ............................................................................................ 6.3.1 Probability of Dugong Hunting 95

6.3.2 Environmental and Temporal Factors Affecting Hunting ...................................................... 95 6.3.3 Interactions Between Dugong Hunting and the Commercial Crayfish Fishery ................. 100 6.3.4 Factors Affecting Dugong Hunting Effort .................. ..... ................................................. 103

6.3.4.1 Total dugong catch per trip ......................................................................................................................... 103 6.3.4.2 Catch per hunting hour .................................................................................................................................. 104 6.3.4.3 Sex Ratio ....................................................................................................................................................... 105 6.3.4.4 Sex and Size Distributions of Dugongs Caught by Hunters ........................................................................ 107 6.3.4.5 Hunter Selectivity ........................................................................................................................................... 107

6.3.5 Total Dugong Catch ......................................................................................................... 108 6.3.6 Catch Monitoring ............................................................................................................. 708

6.4 DISCUSSION .............................................................................................................................. 114 6.4.1 Biophysical Factors A ffecfing Dugong Hunting ................................................................ 714

............................................................................................................................................................. 6.4.1.1 Tides 114 6.4.1.2 Hunting at night ............................................................................................................................................. 115

.............................................................................................................................................. 6.4.1.3 Effects of boats 115 6.4.1.4 Distribution and abundance of dugongs ........................................................................................................ 116

6.4.2 The Catch ...................................................................................................................... 118 6.4.2.1 Variability in the annual catch ........................................................................................................................ 118 6.4.2.2 Catch Selectivity ............................................................................................................................................ 119

.............................................................................................................. 6.4.3 Catch Monitoring 719 6.4.4 Socio-Economic Effects on the Catch .................................................................................. 720

..................... 6.4.5 Human populations, hunting and the distribution and abundance of dugongs 722

6.5 SUMMARY ................... .. ....................................................................................................... 123

CHAPTER 7 ...................................................................................................................................... 1 2 6

LIFE HISTORY AND REPRODUCTIVE BIOLOGY OF FEMALE DUGONGS .................................. 126

7.1 INTRODUCTION .......................... .. ...................................................................................... 127

7.2 MATERIALS AND METHODS ..................................................................................................... 128 7.2.1 Collection of Specimens ............................ .. .................................................................... 128

7.2.1 . 1 Morphometric information .............................................................................................................................. 128 7.2.1.2 Tusks ............................................................................................................................................................. 129 7.2.1.3 Eye lenses ..................................................................................................................................................... 129 7.2.1.4 Fat .................................................................................................................................................................. 129 7.2.1.5 Ovaries and uteri ........................................................................................................................................... 129 7.2.1.6 Mammary glands ........................................................................................................................................... 130

7.2.2 Age Determination .............................................................................................................. 130 .................................................................................. 7.2.3 Examination of Reproductive Organs 131

7.2.3.1 Gross and histological examination of ovaries ............................................................................................ 131 7.2.3.2 Gross and histological examination of uteri ................................................................................................... 131 7.2.3.3 Mammary glands ........................................................................................................................................... 132

7.2.4 Classification of Reproductive Status ................................................................................ 132 7.2.5 Reproductive Activity ......................................................................................................... 732 7.2.6 Fat Analyses and Assessment of Body Condition5 .............................................................. 133 7.2.7 Populafion Modelling ............................................................................................................ 734 7.2.8 Sampling Biases ...................................... .. .......................................................................... 735

7.3 RESULTS ................................................................................................................................... 138 7.3.1 Age Composition ............................................................................................................... 138

7.3.1.1 This study ...................................................................................................................................................... 138 7.3.1.2 Comparison with other dugong populations ................................................................................................. 140

7.3.2 Assessment of Reproductive Status ................................................................................. 141 .................................................................... 7.3.3 Age at Sexual Mafurity and First Reproduction 143

...................................................................................................................................................... 7.3.3.1 Thisstudy 143 ................................................................................................. 7.3.3.2 Comparison with other dugong populations 144

7.3.3.3 Comparison with Florida manatees ............................................................................................................... 145 ................................................................. 7.3.4 Size af Sexual Maturity and First Reproduction 145

7.3.4.1 This study ...................................................................................................................................................... 145 ............................................................................................... 7.3.4.2 Comparison with other dugong populations 145

7.3.4.3 Comparison with Florida manatees ......................................................................................................... 145 ............................................................................................................................. 7.3.5 Lifter Size 145

7.3.5.1 This study ..................................................................................................................................................... 145 .................................................................................................. 7.3.5.2 Comparison with other dugong populations 146

.......................................................................................................................... 7.3.5.3 Comparison with manatees 146 ............................................................................................... 7.3.6 Size and Sex Ratio at Birth 146

7.3.6.1 This study .................................................................................................................................................... 146 ................................................................................................. 7.3.6.2 Comparison with other dugong populations 146

7.3.6.3 Comparison with manatees .......................................................................................................................... 147 .................................................................. ........................................... 7.3.7 Gestation Period ... 148

7.3.7.1 This study ..................................... .... ............................................................................................................... 148 7.3.7.2 Comparison with Florida manatees ............................................................................................................. 149

...................................................................................... 7.3.8 Pregnancy Rate .................. ..... 149 7.3.8.1 This study ..................................................................................................................................................... 149

................................................................................................. 7.3.8.2 Comparison with other dugong populations 151 .................................... ........................................................................ 7.3.8.3 Comparison with Florida manatees ... 152

................................................................................................................... 7.3.9 Calving Interval 153 ..................................................................................................................................................... 7.3.9.1 This study 153

7.3.9.2 Comparison with other dugong populations .................................................................................................. 153 7.3.9.3 Comparison with Florida manatees .............................................................................................................. 153

........................................................................................................................ 7.3.10 Lactation 154 7.3.10.1 This study .................................................................................................................................................... 154

.............................................................................................. 7.3.10.2 Comparison with other dugong populations 156 ............................................................................................................. 7.3.10.3 Comparison with Florida manatees 156

.................................................... 7.3.11 Incidence of Gonadal Activity and Breeding in Females 157 7.3.1 1 . 1 Observations of ovarian activity .............................................................................................................. 157

........................................................................................................ 7.3.1 1.2 Seasonal patterns in gonadal activity 158 ................................................................................................ 7.3.12 Body Condition in Females 160

......................................................................................................... 7.3.12.1 Comparison with Florida manatees 162 .......................................................................................................... 7.3.13 Population Modelling 162

.................................................................................................................................................... 7.3.13.1 This study 162 ................................................................................................ 7.3.13.2 Comparison with other dugong populations 163

............................................................................................................ 7.3.13.3 Comparison with Florida manatees 163

............................................................................................................................ 7.4 DISCUSSION 167 .................................................................................................................. 7.4.1 Major Findings 167

......................... ............................. 7.4.2 Attainment of Sexual Maturity and First Reproduction ... 167 ............................................................................................ 7.4.3 Size at Birth and Lifter Size 168

............................................................................................................................ 7.4.4 Sex Ratio 169 ........................................................................................................... 7.4.5 Length of Gestation 169

................................................................................................................ 7.4.6 Pregnancy Rate 169 ............................................................................................................ 7.4.7 Calving Intervals 170

.................................................................................................................. 7.4.8 Body Condition 171 .................................................................................................... 7.4.9 Seasonality of Breeding 172

....................................................................................................... 7.4.10 Population Dynamics 173

7.5 SUMMARY ...................... ... ............................................................................................... 174

CHAPTER 8 ....................................................................................................................................... 176

LIFE HISTORY AND REPRODUCTIVE BIOLOGY OF MALE DUGONGS ....................................... 176

8.1 INTRODUCTION ........................................................................................................................ 177

................................................................................................ 8.2 MATERIALS AND METHODS 1 78 ................................................................................................... 8.2.1 Collection of Specimens 178

......................................................................................................... 8.2.3 Age Determination 178 ...................... 8.2.4 Histological Preparation and Examination of Male Reproductive Specimens 178

.................................................................. 8.2.5 Classification of Reproductive Status in Males 178 ..................................................................................................... 8.2.6 Reproductive Activity 179

............................................................ 8.2.7 Fat Analyses and Assessment of Body Condition6 179 .................................................................................................... 8.2.8 Phase Analysis of Testes 179

.................................................................................................................................... 8.3 RESULTS 180 ................................................................................................... 8.3.1 Age Composition in Males 180

..................................................................................................................................................... 8.3.1.1 This study 180 .................................................................................................. 8.3.1.2 Comparison with other dugong populations 182 ............................................................................. 8.3.2 Aff ainment of Sexual Maturity in Males 182

...................................................................................................................................................... 8.3.2.1 This study 182 .................................................................................................. 8.3.2.2 Comparison with other dugong populations 185

............................................................................................................... 8.3.2.3 Comparison with Florida manatees 185 ............................................................................................. 8.3.4 Reproductive Status of Males 185

8.3.4.1 This study ...................................................................................................................................................... 185 ................................................................................................ 8.3.4.2 Comparison with other dugong populations 186

............................................................................................................... 8.3.4.3 Comparison with Florida manatees 188 ................... ............................... 8.3.5 Incidence of Gonadal Activity and Breeding in Males ... 188

8.3.5.1 Observations of testicular activity .................................................................................................................. 188 ............................................................................................... 8.3.5.2 Seasonality of changes in testicular histology 189

.................................................................................................. 8.3.5.3 Comparison with other dugong populations 189 ............................................................................................................... 8.3.5.4 Comparison with Florida manatees 189 . . .................................................................................................. 8.3.6 Body Condrfron in Males 190

...................................................................................................................................................... 8.3.6.1 This study 190

.............................................................................................. 8.4 DISCUSSION .......................... .. 192 ................................................................................ 8.4.1 Major Findings .............................. .. 192

......................................................... 8.4.2 Aft ainment of Sexual Maturity and First Reproduction 192 ...................................................................................................... 8.4.3 Seasonality of Breeding 193

.................................................................................................................................. 8.5 SUMMARY 195

......................................................................................................................................... CHAPTER 9 196

REASONS FOR THE VARIABILITY IN DUGONG LlFE HISTORY PARAMETERS: IMPLICATIONS ............................................................................... FOR CONSERVATION AND MANAGEMENT 196

9.1 VARIABILITY IN LlFE HISTORY PARAMETERS OF DUGONGS: EVIDENCE OF RESOURCE DEPENDENCY? ........................................................................................................................... 197

9 . 1 . 1 Aff ainment of Sexual Maturity and Age at First Reproduction ............................................. 197 ......................................................................................................................... 9.1.2 Fecundity 200

.......................................................................................................... 9.1.3 Skewed Sex Ratios 201

9.2 LlFE HISTORY PARAMETERS AND REPRODUCTION OF DUGONGS: POSSIBLE REASONS FOR THE OBSERVED DIFFERENCES BETWEEN DARU IN THE 1970s AND MABUIAG IN THE

............................................................................................................................................... 1990s 2 0 2 ........................................................................................ 9.2.1 Evidence for Genetic Differences 203

....................................................................... ......... 9.2.2 Evidence for Density Dependence .. 2 0 3 ......................................................................... 9.2.3 Evidence for Densify Independent Factors 205

9.3 IMPLICATIONS FOR MANAGEMENT ............................... .. ................................................... 206 ............................................................................................................ 9.3.1 Habitat Protection 206

.................................................................................................... 9.3.2 Monitoring of Populations 207 .................................................................................................................. 9.3.3 Catch Monitoring 208 ................................................................................................................. 9.3.4 Estimates of PBR 209

9.4 SUMMARY ........................................................................................................................... 210

......... HISTOLOGICAL CHARACTERISTICS USED IN PHASE ANALYSES OF MALE DUGONGS 275

............................................................................................................................ APPENDIX 8.2 276

DETAILS OF MALE DUGONGS ASSESSED FOR REPRODUCTIVE STATUS ON THE BASIS OF PHASE ANALYSES (SENSU MARSH ETAL . 1984C; HERNANDEZ ETAL . 1995) ........................ 276

APPENDIX 8.3 ................................................................................................................................ 280

CHARACTERISTICS OF SEMINIFEROUS TUBULE USED IN PHASE ANALYSES ....................... 280

LIST OF ACRONYMS ..................... .. ............................................................................................ 282

LIST OF TABLES

Table 2.1. The changes in dugong abundances in various populations in Australia evident from standardised aerial surveys (adapted from Marsh and Lawler 2001 a). ................ ...... ...................................................................................... 4 0

Table 3.1. Historical estimates of the population of Islanders residing in the Torres Strait region. .................... ... ..................................................................................... 48

Table 3.2.The annual catch of dugongs in Torres Strait communities obtained by various methods (adapted from Marsh 1998). ........................................................................ 55

Table 3.3. Population estimates of the major dugong hunting communities of Badu, Boigu and Mabuiag Islands where this study was based. ........................................... 57

Table 5.1.Total catch, total number of hunting trips and CPUE (number of dugongsltrip) for individual hunters during March to October in: (a) 1998 and (b) 1999. .................................... .... .................................................................................... 7 7

Table 5.2. Details of hunters operating from Mabuiag lsland in 1998 and 1999. ...................................................................................................................................... 78

Table 5.3. Numbers of hunting trips (n = 173) to reefs shown in Figure 5.1 in 1998 and 1999 ......................... ... .......................................................................................... 81

Table 6.1. Summary of the results of the generalised additive models showing the main variables that affect the probability of hunting for dugongs by hunters based in Mabuiag in 1998-99 (the final model is bolded, see text for explanation of the AIC). ........................... ... ................................................................... 97

Table 6.2. Summary of the results of the generalised additive models showing the main variables that affect the mean total dugong catch per trip of hunters based in Mabuiag lsland hunting dugongs in 1998-99 the final model is bolded, see text for explanation of the AIC). ................... .... ................................. 105

Table 6.3. Summary of the results of generalised additive models showing the main variables that affect the proportion of females caught (i.e. ratio of number of females to total catch) by hunters based in Mabuiag lsland hunting dugongs in 1998-99 the final model is bolded, see text for explanation of the AIC). ....................... .. ................................................................................ 106

Table 6.4 Monthly catch statistics and hunting CPUE* (number dugongsltrip) of dugongs landed at Mabuiag Island in (a) 1998 and (b) 1999. ............................................... 109

Table 6.5. Sampling by CSlRO from several islands in Torres Strait between 21 January 1999 and 20 November 1999 showing sampling effort and recorded dugong catch. ............................................................................................................... 11 1

Table 6.6. Comparison of observations of dugong catches by an AFMA monitoring observer and this study at Mabuiag Island in 1999 ................................................ 11 2

Table 6.7. Percentage frequency of samples resulting in observed mean values that are more than 50% outside the true value. ........................................................ 113

Table 6.8. Percentage frequency of samples resulting in observed mean values that are more than 50% outside the true value based on repetitive samples repeated after varying intervals. .................................................................................... 11 3

Table 6.9. The results of the simulated three by fifteen-day samples showing a considerable spread of results either side of the expected mean number of 21.2 dugongs .............................................................................................................. 114

Table 7.1. Criteria used to assess the reproductive status of female dugongs modified from Marsh (1984a) and Boyd et a/. (1999). Note some of the

........................... .................................................. stages below are not mutually exclusive. .. 133

Table 7.2. Mortality schedules used for the various life stages of dugongs used in population models based on available data from the Florida manatee ....................................................................................................................................... 135

Table 7.3. Assessment of the potential biases on key life history parameters of dugongs and their impact on population dynamics ............................................................... 137

Table 7.4. The numbers of reproductive specimens collected from female dugongs from September 1997 to October 1999. ....................................................................... 141

Table 7.5. Details of the reproductive status of all females (n = 149) sampled at Mabuiag lsland in 1997-99 assessed according to the criteria described in Tables 7.1 and 7.2. ............................................................................................... 143

Table 7.6. Number of adult females (n = 105) in the various reproductive states assessed according to the criteria described in Table 7.1. ......................................... 143

Table 7.7. Pre-reproductive periods of female dugongs from Mabuiag lsland (bolded) compared with dugongs from other areas and the Florida manatee (adapted from Boyd et a/. 1999). ................................................................................. 144

Table 7.8. Details of foetuses recorded from pregnant female dugongs caught near Mabuiag Island during 1997-1999. ...................................................................................... 147

Table 7.9. Details of various reproductive parameters based on: (a) confirmed pregnancies or (b) all possible pregnancies of female dugongs sampled in 1997-99 at Mabuiag Island. The annual pregnancy rates, calving intervals and lactation periods are shown for three possible gestation

............................................................................................... periods (13, 14 and 15 months) 150

Table 7.10. Apparent pregnancy rate (& S.E) from females sampled from Daru in 1978-1982 compared with that from Mabuiag lsland during 1997- 1999 (bolded). .......................................................................................................................... 151

Table 7.1 1. Annual pregnancy rates (k S.E ) calculated using three gestation periods to compare data from dugongs harvested from Mabuiag lsland in 1997-99 with specimens obtained from other areas and the Florida manatee ....................................................................................................................................... 152

Table 7.12. Estimates of calving intervals of various dugong populations compared with those of the Florida manatee based on the annual pregnancy rate and three possible gestation periods 13, 14 and 15

months. Estimates are based on (a) confirmed pregnancies and (b) all ............................................................................................................ possible pregnancies. 154

Table 7.1 3. Total number of females assessed for lactation by histological and .................................................................................................. macroscopic examinations. 155

Table 7.14. Size of foetuses of pregnant females with active and proliferating .................................................................................................................... mammary glands. 156

Table 7.1 5. The ovarian activity of primigravid and primiparous females. .................. ...... ............. 157

Table 7.16 The effect of sex ratio at birth on the annual rate of change of the female component of a stable dugong population assuming a mortality schedule based on data from the Florida manatee for various combinations of calving intervals and age of first reproduction for female dugongs ................................................................................................................................... 162

Table 7.17 Annual rate of change of stable sirenian populations (this study bolded) estimated using a stage-based Leslie Matrix population model, POPSIM (Crouse et a/. 1987; Somers 1994) for various combinations of

........................... ....... age of first reproduction and calving intervals (after Boyd et a/. 1999) .. 163

Table 8.1. Reproductive status of all males assessed according to histological criteria (sensu Marsh et a/. 1984b; Hernandez eta/. 1995), erupted

....................................................... tusks or from age (GLGs) andlor body length information. 183

Table 8.2. Pre-reproductive periods of male dugongs from Mabuiag Island compared with other areas (after Marsh 1995a) .............................. .... ........................................ 185

Table 8.3 Comparison of various reproductive stages of adult males harvested at Mabuiag Island with data from Marsh ef a/. (1984~). .............................................. 187

Table 9.1.The variability of the life history parameters of dugongs from various .................................... location and times evident from data summarised in Chapters 7 and 8. 198

LIST OF FIGURES

Figure 1 .I. A conceptual model of my research and the contents of this thesis. The current chapter is coloured yellow. .................................................................................... 14

Figure 2.1. Areas of inter-reefal seagrass habitats (stippled) in Torres Strait based on surveys undertaken by CSlRO in Westem, Central and Eastern Torres Strait (adapted from NSR 1997). Note areas to the west of 142" have not been comparatively sampled. ....................................................................... 28

Figure 2.2.The results of standardised aerial surveys conducted in 1987, 1991, 1996 and 2001 showing the temporal and spatial variability the estimates of the dugong population in Torres Strait (Marsh et al. 1997; Marsh, Lawler and Kwan, unpublished data 2002) ...................................................................... 41

Figure 3.1. The Torres Strait region showing the boundary of the Torres Strait Protected Zone, the dugong sanctuary and the main communities in the Inner, Western, Central, Eastern Islands groups and the two Islander communities Bamaga and Siesia in the Northem Peninsula Area (NPA) of Cape York ........................ .. ............................................................................................... 46

Figure 4.1 .Conceptual diagram showing the contribution to effective management of a sustainable dugong fishery in Torres Strait from the research process which obtained both information and established a relationship between hunters and the scientist. ........................ ... ....................................... 61

Figure 4.2. The traditional method of 'cutting' a dugong by hunters from Mabuiag Island. ............................ .... ................................................................................... 66

Figure 5.1 .The major hunting areas of dugongs used by hunters from .......................................................................................... Mabuiag and adjacent communities. 79

Figure 5.2. Hunting platforms once used to hunt dugongs were built in dugong feeding areas from which dugongs were harpooned. ................... ..................... .......................... 84

Figure 6.1. The linear relationship between lunar day and the probability of dugong hunting in Mabuiag Island in 1998-99 evident from the generalised additive models. The solid line represents smooth spline, dashed lines represent approximate 95% confidence intervals. .................................................. 98

Figure 6.2. The linear relationship between daily crayfish catch and the probability of dugong hunting in Mabuiag Island in 1998-99 evident from the generalised additive models. The solid line represents smooth spline, dashed lines represent approximate 95% confidence intervals ................................................... 98

Figure 6.3. The linear relationship between the probability of dugong hunting and wind velocity in Mabuiag Island in 1998-99 evident from the generalised additive models. The solid line represents smooth spline, dashed lines represent approximate 95% confidence intervals ................................................... 99

Figure 6.4. The relative probability (5 s.e) of dugong hunting in the months from March to October in 1998 and 1999. .............................................................................. 99

Figure 6.5. The relative probability (+ s.e) of dugong hunting and season in 1998-99. ...................................................................................................................................... 100

Figure 6.6. The pattern of hunting activity of hunters based at Mabuiag lsland in (a) 1998 and (b) 1999. ........................ ... ........................................................................... 101

Figure 6.7. The timing of hunting trips undertaken by hunters based at Mabuiag Island in (a) 1998 and (b) 1999. .................................................................................... 102

Figure 6.8. The relationship between the total dugong catch per trip and (a) 'lunar day', (b) 'wind velocity', (c) 'tidal difference', and (d) 'mean tidal difference'. The solid lines represent smoothing spline fits, dashed lines represent approximate 95% confidence intervals. ............................................................... 104

Figure 6.9. Proportion of female dugongs in the catch from Mabuiag lsland in different months over two years (1998 and 1999). Error bars represent approximate 95% confidence intervals. ................ ........ .......................................................... 107

Figure 6.10. Distribution of dugongs caught in the 635 samples of fiveday intervals at Mabuiag Island during 1997-99. .............................................................................. I I I

Figure 6.1 1. Distribution of dugongs caught in 153 hypothetical samples of ............................................................... 20-day intervals at Mabuiag Island during 1997-99. 112

Figure 6.12. Comparison of the dugong catch from Mabuiag lsland during 1977-78 (Nietschmann 1984) and 1998-99. ................................................................................ 1 18

Figure 7.1. Uterus of a female dugong (MD56) showing the left and right ovaries, bicornuate (left and right) horns and placental scars. The white lined scar is more recent than the green lined scar in the right hom. ....................................... 130

Figure 7.2. Age distribution of female dugongs (n = 92) sampled from traditional hunting at Mabuiag Island during 1997-99. ................................................................ 139

Figure 7.3. Relationship between age determined by GLGs and mean dry weight of eye lens in females (square root transformed) (95% C.1). ........................................ 140

Figure 7.4. Age distribution of female dugongs (a) drowned in shark nets in Townsville and (b) harvested from the Momington lsland fishery in 1969- 81 (Marsh ef a/. 1984a). ......................................................................................................... 142

Figure 7.5. Relationship between body length and date of death of foetuses (n = 24) recorded from pregnant female dugongs caught in Mabuiag lsland during 1997-1999 combined with data from foetuses (n = 26) from Daru

.................................................................................... ......................... by Marsh (1 995a). ... 149

Figure 7.6. The proportions of adult female dugongs in various reproductive states in different months. ................... .. ................................................................................ 159

Figure 7.7. The mean ovarian fat depth in different reproductive classes of female dugongs (estimated mean ? 95% confidence intervals). ............................................... 160

Figure 7.8. The mean fat tissue depth in different reproductive classes of females dugongs (estimated mean k 95% confidence intervals). ................................... 161

Figure 7.9. The mean fat water content in different reproductive classes of female dugongs (estimated mean and 95% confidence intervals). ................................... 161

Figure 7.10. The annual rate of change of a stationary population of dugongs (r = 0) (Caughley and Sinclair 1994) assuming a sex ratio of 1:1, mean ages of first reproduction ranging from 6-16 years and estimated for the calving intervals of: (a) 2.4 years, (b) 4.8 years and (c) 7.2 years. ....................................... 164

Figure 7.1 1.The annual rate of change of a stable population of dugongs, assuming sex ratio of 1 :3 at birth at various ages of first reproduction and estimated for the calving intervals at (a) 2.4 years, (b) 4.8 years and (c)

................................................................................................................................ 7.2 years. 165

Figure 7.12.The effect of calving interval and age at first reproduction on the survivorship for a stationary population (r=O) for various life stages and

............................................................. assuming a sex ratio of 1: 1 at birth. .................... ... 166

Figure 8.1. Age distribution of male dugongs (n = 51) based on GLGs sampled from traditional hunting at Mabuiag Island during 1997-99. ....................................... 181

Figure 8.2. Relationship (95% C.I.) between age determined by GLGs and mean dry weight of eye lens in males with unerupted tusks ..................................................... 182

Figure 8.3. Age distribution of male dugongs (a) drowned in shark nets in Townsville and (b) taken by hunters in the Mornington Island fishery in

................................................................................................. 1969-81 (Marsh et a/. 1984~). 184

Figure 8.4. Proportions of male dugongs classified in various reproductive stages in months sampled from animals harvested from Mabuiag Island

......................................................................................................................... during 1997-99 188

Figure 8.5. Numbers of adult males sampled in various years with active and inactive testes in (a) February - May and (b) June - January. ................................................. 190

Figure 8.6.The mean fat tissue depth in different reproductive classes of male dugongs (estimated mean and 95% confidence intervals). ....................................................... 191

Figure 8.7. The mean fat water content in different reproductive classes of male dugongs (estimated mean and 95% confidence intervals). .................... ... ............. 191

Figure 10.1. A conceptual model outlining the main contribution of my research to providing information to ensure the sustainability of the dugong fishery in Torres Strait (adapted from Stobutzii et a/. 2001). The terms of this model are defined by the results of Chapters 7, 8 and 9. The various components of the model are not ranked. The correlation of the individual components with an increased or decreased risk of unsustainable harvests is indicated. .......................................................................................... 218

Figure 10.2. An adaptive management process by which information (shown in Figure 10.1) could be obtained and used by communities, their fisheries representatives and AFMA to be actively involved in the management and monitoring of the dugong fishery, especially when risks

............................................................................................. of over-harvesting are evident, 221

Figure 10.3. The major cultural, social, political, institutional and biological factors influencing management to ensure a sustainable dugong fishery in Torres Strait. ..................... ... ..... .......... ....................................................... 225