Evidence that the dugong harvest in Torres Strait is too high to be sustainable

Helene Marsh School of Tropical Environment Studies & Geography

•Torres Strait region supports world’s largest known dugong population

• Paramount cultural, social, nutritional & economic significance to Islanders

•Dugongs integral to Ailan Kastom: a symbol of socio-political aspirations for Islander self-determination

• Sustainable hunting rights international obligation enshrined in Torres Strait Treaty

•‘Dugongs for our children’ –Islander imperative

How can we tell if human impacts are unsustainable ?

• Determine if population is declining– Slow – takes many years to prove

– Insensitive – decline may not be able to be proved statistically until numbers are really low

– Showing that there is a decline does not determine the cause

• Calculate a safe harvest and compare this number with the actual harvest– Reliable

– F

Best estimate of current catch ~ 1000 dugong p.a.

 

31 61 92 123 154

Sustainable annual catch for Torres Strait

307

305 213

306

312

212

304

211210209208207206205204203202201

315

401

303

302

314

405

404

403

402

311

310

308

309

224223222221220219218217216215214

101

103

313

102

105104106

107108109

128

301

110

146

407

406

111

130

134

132

138

112

140

126

144

136

142

124

114

122

10

116

505

409408

120

501

410504

225

118

503502

4115041412

3061

3051

3

4

0

2A

1B

2B

1A

5

Estimated for range of values for rate of increase RF=0.5

Fieldwork Mabuiag IsSeptember 1997-November 1999

Factors affectinghunting

0

5

10

15

20

25

30

35

40

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Months

0

5

10

15

20

25

30

35

40

45

50

Jan Feb Mar Apr May Jun Jul Aug Sep Oct

Nu

mb

er

# Hunters

Total trips

Total catch

Oct-Mar 1998

Oct-Mar 1999

• Environmental

•Social

•Biological

General linear models used to explore relationships

Season 1 (SE) May-OctSeason 2 (NW) Nov-AprSeason 3 DoldrumsSeason 4 Variable

Probability of Hunting

Season

Pro

babi

lity

of h

untin

g

0.0

0.2

0.4

0.6

0.8

1.0

1 2 3 4

Dugong hunting most likely to occur in south-east (windy) season when animals sheltering

in shallow water close to the island

Lunar day

Pro

babili

ty o

f hunt

ing

0 5 10 15 20 25 30

0.0

0.2

0.4

0.6

0.8

1.0

Dugong hunting most likely to occur at New Moon (Lunar day =1): dugongs on reef tops at

high tide

Effect of moon phase on hunting

Hunting statistics•Effort – number of hunting trips

– 1998 March–Oct 149 trips – 1999 March Oct 153 trips

•Who?– 29 hunters – head of hunting party usually harpooner– 4-5 master hunters

•Where?– Traditional sea country 66-71% trips– Home reefs <5km island 29-34%

•How many – 1998 - 145 dugongs– 1999 - 160 dugongs– Master hunters caught 75-79% catch

D a ily c ra y fis h c a tc h ( lo g k g )

Pro

ba

bili

ty o

f h

un

tin

g

2 3 4 5 6

0 .0

0 .2

0 .4

0 .6

0 .8

1 .0

Low probability of hunting if crayfish catch high Local influence- availabilityGlobal influence - price

Effect of crayfish landings on hunting

Social factors influencing take of dugongs at Mabuiag in

Torres Strait• # people in community

• # hunters in community – actions of master hunters

• information on dugong distribution & abundance – major hunters operate as scouts

• # functional dinghies

• money for fuel

• price / availability of crayfish

Environmental factors influencing take of dugongs at Mabuiag Is. in

Torres Strait

•Coincidence of favorable weather, tides and dugongs

Dugong population biology factors affecting sustainability of dugong

harvest in Torres Strait

Dugong population factors

• probability of females breeding in next year (% calves)

• natural mortality of adults and juveniles

Habitat factors

• seagrass dieback in favoured dugong habitats

Dugongs in Torres Strait1987-2001~12-25 thousand

0

5

10

15

20

25

0 - 5 6-10 11-15 16-20 21-25 26-30 31-35 36-40 41-45 46-50 51-55 >55

Age Class (years)

Nu

mb

er

of

du

go

ng

sImpact of seagrass dieback 25 years ago

apparent in age structure of sample 1998-99

Apparent Pregnancy Rate TS dugongs

1978-79 0.09+ 0.03

1979-80 0.19+ 0.06

1980-81 0.24+ 0.08

1998-99 0.38+ 0.06

A sustainable dugong fishery is dependent on:

(1)Factors affecting the susceptibility of the stock to overharvesting:

• social, cultural & economic needs of Islanders

• environmental factors that determine hunting pressure

(2) The capacity of the dugong population to recover after depletion:

• the major factors that affect the population dynamics of dugongs, especially seagrass dieback

A sustainable dugong fishery is dependent on:

(1)Factors affecting the susceptibility to overharvesting:

• basic social, cultural & economic needs of Islanders

• environmental factors that determine hunting pressure

(2) The capacity of the dugong population to recover after depletion:

• the major factors that affect the population dynamics of dugongs

All th

ese fa

ctors

must

be co

nsidere

d if th

e

Torres S

trait

dugong fish

ery is

to b

e sust

ainable

ACKNOWLEDGEMENTSFunded by Australian Fisheries Management Authority

Supported by: Island Coordinating Council Torres Strait Regional AuthorityMabuiag, Badu & Boigu Island Councils

Hospitality, cooperation and support of: Mabuiag, Badu and Boigu Island CommunitiesHunters at Mabuiag, Badu and Boigu Islands