The global status of dugongs in the context

of their biology

The global status of dugongs in the context

of their biology

Helene Marsh Helene Marsh

Conservation Significance of Dugongs

Only member of family Dugongidae

Class: MammaliaOrder: SireniaFamily: Dugongidae

Species: Dugong Dugong dugonSteller’s sea cow Hydrodamalis gigas

(extinct)

Family: Trichechidae

Species: Antillean manatee Trichechus manatus

Amazonian manatee Trichechus inunguis

West African manatee Trichechus senegalensis

All extant sirenians vulnerable to extinction

Conservation Significance of Dugongs

• Only strictly marine herbivorousmammal

• Largest population size (>100,000) and range of extant sirenians

Seagrass Specialists

• Mouth specialised for bottom feeding

• Remove all parts of plant (leaves and rhizomes)

• Leave feeding trails and feeding plumes

• Like nutritious new growth – possibly farming large areas when in large herds

Life History

• Lifespan 70+ yrs• Age first breeding 6-17 yrs• Litter size 1• Calving interval 2.5 yrs• Gestation period 13-15

months• Lactation ~ 1.5 yrs• Adult survivorship > 95%• Max rate of increase 5%• Sustainable harvest ~ 2%

If Vietnamese waters support only a small population e.g. 100 dugongs then ANY mortality from human causes is potentially unsustainable

If Vietnamese waters support only a small population e.g. 100 dugongs then ANY mortality from human causes is potentially unsustainable

Movements

• Have tracked > 60 animals• Most movements local• Several animals made long-distance

movements• Longest movement ~800 km in few days

Movements of satellite- tracked dugongs

– 42 dugongs tracked on the east coast•5-551 days

– Covered over 80% of coast

– 1/3 moved linear distances >80km

– 1/6 moved >150km

1500km

280km

400km

Shoalwater Bay

Cooktown

Townsville

Preliminary information on genetics of female dugongs• Genetic types of female dugongs from

Southeast Asia (Indonesia, Thailand, Philippines) generally distinct from those from Australia

• Two partially overlapping maternal lineages in Australian coastal waters

• Pattern reflects ancient separation due to sea level change with modern incomplete mixing– Dugongs colonise new areas only rarely– Males only may be moving and breeding

Main Conclusions from Biology

• Rate of change in dugong numbers is very sensitive to changes in adult survivorship

• 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 or move - this reduces the level of mortality that is sustainable

•Dugong conservation MUST address the issues of habitat conservation AND mortality reduction

•Dugong conservation in Vietnam should be developed in consultation with neighbouring countries - China, Thailand and Cambodia

Conclusions for Vietnam

The Global Status of Dugongs

• UNEP status report and action plan

• Covers 37 countries of dugongs’ range

Methods• Evaluation based on published information and

expert opinions of about 100 scientists and managers regarding 37 countries and territories in dugong’s range

• Snowball sampling technique

• Informants contacted 1997 - 2001 during process leading to development of global status and action plan

• Information reviewed by 60 in-country experts.

Evidence for Decline

• Numbers have declined in at least 21 countries

• Dugongs are extinct in 3 island groups• No evidence of reduction in extent of

range - reduction of area of occupancy within range

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1970 1980 1990 2000

0.0

0.5

1.0

1.5

2.0

2.5

3.0

1960

Dug

ongs

ca

ught

per

mon

th /

beac

h

Dugong catch per unit effort in shark control nets on the urban coast of Queensland

Threats to Dugongs:Overview From 37 Countries

THREAT NO. OF COUNTRIES

Fishing mortality (gill netting) at least 35Habitat loss / Degradation allHunting / Poaching at least 16

(27?)Boat Impacts at least 11

Dugongs tangle in mesh nets and accidentally drown in many countries

Habitat loss, especially due to extreme weather events

1000 km2 of seagrass habitatwere lost in Hervey Bay Queenslandafter two floods and a cyclone (hurricane)

Hunters and poachers kill dugongs in many countries

Medical use of dugong products is a major reason for poaching and for killing animals which are caught accidentally in nets or fish traps

high risk of extinction

reasonable prospects for survival

uncertain prospects for survival high risk of

extinction

uncertain – likely extinction in Japan

uncertain – likely extinction in Palausecure – except

urban coast of Queensland

Prospects in SE Asia uncertain: likely extinction in Japan

Populations small and fragmented but cover vast areas with potential for recruitment from other areasPressure from gill-netting, dynamite fishing, habitat destruction, boat impacts and hunting likely to increaseFew effective conservation initiatives

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