Transcript AMSA2003 - Dugong hunting presentation

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 sociopolitical 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
10
104
105
106
101
102
103
110
111
216
217
218
219
220
2A
2B
221
222
223
224
212
305
1A
214
215
208
209
210
211
304
107
108
109
303
114
203
204
205
206
207
302
112
118
116
120
122
124
126
130
132
201
202
301
3
128
136
134
138
142
140
144
146
1B
0
225
213
3051
306
401
3061
402
307
308
403
309
404
310
311
405
312
4
406
313
407
408
314
315
409
410
411
412
501
5041
502
503
504
505
5
Sustainable annual catch for Torres Strait
Estimated for range of values for rate of
increase RF=0.5
31
61
92
123
154
Best estimate of current catch ~ 1000
dugong p.a.
Fieldwork Mabuiag Is
September 1997
-November 1999
40
Oct-Mar 1998
35
30
25
Factors affecting
hunting
20
15
• Environmental
10
•Social
5
0
Jan
Feb
Mar
Apr
May
50
Jun
Aug
Sep
Oct
Nov
Dec
•Biological
Months
Oct-Mar 1999
45
Number
Jul
40
# Hunters
35
Total trips
Total catch
30
25
20
15
10
5
0
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
General linear models
used to explore
relationships
Probability of Hunting
1.0
Season 1 (SE) May-Oct
Season 2 (NW) Nov-Apr
Season 3 Doldrums
Season 4 Variable
Probability of hunting
0.8
0.6
0.4
0.2
0.0
1
2
3
Season
4
Dugong hunting most likely to occur in south-east
(windy) season when animals sheltering in shallow
water close to the island
Effect of moon phase on hunting
1.0
Probability of hunting
0.8
0.6
0.4
0.2
0.0
0
5
10
15
Lunar day
20
25
30
Dugong hunting most likely to occur at New Moon
(Lunar day =1): dugongs on reef tops at high tide
Hunting statistics
• Effort – number of hunting trips
– 1998 March–Oct
– 1999 March Oct
149 trips
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
Effect of crayfish landings on hunting
1.0
Probability of hunting
0.8
0.6
0.4
0.2
0.0
2
3
4
5
Daily crayfish catch (log kg)
Low probability of hunting if crayfish catch high
Local influence- availability
Global influence - price
6
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
1987 1991
Block 2B
1996
Block 3
2001
1987 1991
1996
Block 5
Block 4
2001
2.5
2.0
1.5
Density
1.0
0.5
0.0
2.5
Block 0
Block 1A
Block 1B
Block 2A
2.0
1.5
1.0
0.5
0.0
1987 1991
1996
2001
1987 1991
Survey year
1996
2001
Dugong Density (animals per km2 +95% .C.I.)
Dugongs in Torres Strait
1987-2001
~12-25 thousand
0.6
0.5
0.4
0.3
0.2
0.1
0.0
1987
1991
1996
Year of survey
2001
25
Impact of seagrass dieback 25 years ago
apparent in age structure of sample 1998-99
Number of dugongs
20
Apparent Pregnancy Rate
TS dugongs
15
10
1978-79
0.09+ 0.03
1979-80
0.19+ 0.06
1980-81
0.24+ 0.08
1998-99
0.38+ 0.06
5
0
0-5
6-10
11-15
16-20
21-25
26-30
31-35
36-40
Age Class (years)
41-45
46-50
51-55
>55
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
ACKNOWLEDGEMENTS
Funded by Australian Fisheries Management Authority
Supported by:
Island Coordinating Council
Torres Strait Regional Authority
Mabuiag, Badu & Boigu Island
Councils
Hospitality, cooperation and support of:
Mabuiag, Badu and Boigu Island Communities
Hunters at Mabuiag, Badu and Boigu Islands