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Dugongs in Japanese waters: an international perspective. Helene Marsh James Cook University Australia Coordinator IUCN Dugong Action Plan. The modern sirenia (seacows)- dugongs and manatees. Biodiversity importance of Order Sirenia. Taxonomically unique 2 families Trichechidae

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dugongs in japanese waters an international perspective

Dugongs in Japanese waters: an international perspective

Helene Marsh

James Cook University Australia

Coordinator IUCN Dugong Action Plan

biodiversity importance of order sirenia
Biodiversity importance of Order Sirenia
  • Taxonomically unique
  • 2 families
    • Trichechidae
      • Antillean manatee Trichechus manatus
      • Amazonian manatee Trichechus inunguis
      • West African manatee Trichechus senegalensis
    • Dugongidae
      • Dugong Dugong dugon
      • Steller’s sea cow Hydrodamalis gigas
global conservation status of modern sirenia
Global conservation status of modern sirenia

Antillean manatee vulnerable

Amazonian manatee vulnerable

West African manatee vulnerable

Dugong vulnerable

Steller’s sea cowextinct

conservation significance of dugongs
Conservation significance of dugongs
  • One of only four members of order Sirenia
  • Only member of family Dugongidae
  • Only strictly marine herbivorous mammal
  • Largest population size (>100,000) and range of extant Sirenians
habitat partitioning extant sirenia
Habitat partitioning: extant Sirenia
  • Dugong
    • strictly marine
    • coastal and estuarine
  • Antillean and West African manatees
    • coastal, estuarine and freshwater (may be dependent on access to freshwater)
  • Amazonian manatee
    • freshwater in Amazon River system

All restricted to tropics and sub-tropics

dietary partitioning modern sirenia
Dietary partitioning: modern Sirenia
  • Dugong
    • seagrass specialist, eats invertebrates at higher latitudeends of range
    • obligate bottom feeder
  • Manatees
    • generalist feeders on salt & freshwater vegetation
    • feed throughout water column
    • opportunistic omnivores
  • Steller’s sea cow
    • surface feeder on cold-water algae

Dugong digging up seagrass like a bulldozer

Conclusion: Seagrass conservation is essential to dugong conservation

  • Dugong
    • >60 animals satellite tracked
    • most movements local
    • several animals made long-distance movements
    • longest movement ~800 km in few days
dugong movements
Dugong Movements



















Maximum movement

Conclusion: Dugongs could travel from other parts of their range to

Okinawa but would be expected to do so only rarely

preliminary information on genetics of female dugongs
Preliminary information on genetics of female dugongs
  • Genetic types of female dugongs from Southeast Asia (Indonesia, Thailand, Philippines generally distinct from those from Australia) - overlap at Ashmore Reef between Australia and Timor
  • Two maternal lineages in Australian coastal waters - overlap in Torres Strait between Australia and Papua New Guinea
  • One Australian lineage also recorded from East Africa and the Arabian Gulf
dugong life history
Dugong Life history
  • Lifespan < 70 yr
  • Age first breeding 6-17yr
  • Gestation period 13-15 mth
  • Calving interval > 2.5 yr
  • Lactation ~ 1.5 yr
  • Adult survivorship >95%
  • Max rate of increase < 5%
  • Sustainable harvest ~2%
  • Rate of change in dugong numbers is very sensitive to changes in adult survivorship
  • The impact of mortality (drowning in nets, boat strikes, hunting) on dugongs is serious
  • When dugongs don’t have enough to eat because of habitat loss, they delay breeding - this reduces the level of mortality that is sustainable

Dugong conservation MUST address the issues of

habitat conservation AND mortality reduction

habitat loss due to extreme weather events
Habitat loss due to extreme weather events

1000 km2 of seagrass habitat

were lost in Hervey Bay Queensland

after two floods and a cyclone


Hunting and

poaching kill dugongs

in at least 26 countries


Propellors kill and maim

manatees and some dugongs

evidence for dugong decline 37 countries
Evidence for dugong decline- 37 countries
  • Anecdotal evidence suggests that dugong numbers have declined in at least 17 countries and that dugongs are extinct in 3 more
  • No evidence of reduction in extent of range - reduction of area of occupancy within range
  • Situation particularly serious in East Africa, Indian region and parts of Southeast Asia
  • Quantitative evidence of decline available only for Queensland, Australia
trends in dugong numbers qld shark control program
Trends in dugong numbers Qld Shark Control Program




Total dugongs caught per year









response to dugong decline in queensland australia
Response to dugong decline in Queensland, Australia
  • Moratorium on hunting on urban coast
  • 16 Dugong Protection Areas (6353 km2)
    • no gill-netting (red)
    • gill-netting with restrictions (yellow)
  • Restructuring of fishing industry - buyout of fishers
suggested approaches to dugong conservation
Suggested approaches to dugong conservation
  • Identify areas that still support significant numbers of dugongs
  • Consider with extensive local involvement how dugong mortality can be minimised and their habitat protected
  • If possible, protect dugongs in the context of comprehensive plans for coastal zone management using the dugong as a flagship species
situation in okinawa
Situation in Okinawa
  • Small isolated dugong population
  • Evidence of decline - dugongs now present only around Okinawa Island in contrast to former wide distribution in Nansei Shoto
  • Threatened by:
    • habitat loss and disturbance- exacerbated by proposed construction of offshore base
    • fishing mortality (trap nets and gill nets)

These threats need to be addressed in the context of the

best scientific information if the dugong is to be conserved