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

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

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

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  1. Evidence that the dugong harvest in Torres Strait is too high to be sustainable Helene Marsh School of Tropical Environment Studies & Geography

  2. Torres Strait region supports world’s largest known dugong population • Paramount cultural, social, nutritional & economic significance to Islanders

  3. 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

  4. 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

  5. Best estimate of current catch ~ 1000 dugong p.a. Sustainable annual catch for Torres Strait Estimated for range of values for rate of increase RF=0.5

  6. Fieldwork Mabuiag IsSeptember 1997 • November 1999

  7. Oct-Mar 1998 Factors affecting hunting • Environmental • Social • Biological • General linear models used to explore relationships Oct-Mar 1999

  8. Probability of Hunting Season 1 (SE) May-Oct Season 2 (NW) Nov-Apr Season 3 Doldrums Season 4 Variable Dugong hunting most likely to occur in south-east (windy) season when animals sheltering in shallow water close to the island

  9. Effect of moon phase on hunting Dugong hunting most likely to occur at New Moon (Lunar day =1): dugongs on reef tops at high tide

  10. 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

  11. Effect of crayfish landings on hunting Low probability of hunting if crayfish catch high Local influence- availability Global influence - price

  12. 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

  13. Environmental factors influencing take of dugongs at Mabuiag Is. in Torres Strait • Coincidence of favorable weather, tides and dugongs

  14. 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

  15. Dugongs in Torres Strait 1987-2001 ~12-25 thousand

  16. Impact 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

  17. A sustainable dugong fishery is dependent on: • 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

  18. A sustainable dugong fishery is dependent on: • 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 these factors must be considered if the Torres Strait dugong fishery is to be sustainable

  19. 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

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