Large-scale movements of dugongs: evidence, reasons

and policy implicationsHelene Marsh, Ivan Lawler,

Donna Kwan

Direct evidence for large-scale movements

–>80 animals satellite tracked–most movements local–several animals made long-distance movements–longest movement ~600 km in few days

Movements of satellite- tracked dugongs

– movements individualistic– 2/5 moved >80km– 1/5 moved >150km– 3 animals covered >80%

of coast

1500km

400km

Shoalwater Bay

Cooktown

Townsville

Aerial surveys: indirect evidence for movements

Standardised technique– Standard survey design– Corrections for perception and

availability bias– Strict ceiling on weather– Two aircraft minimize effects of

movement within surveys

- Large spatial scales (30,000 km2)

every 5 years– Different survey regions surveyed

in different years

Time series since mid 1980s for much of east coast of Qld

Variation between surveys: population size

NN

S

Constant hotspots within surveys: population desnity

Reasons for movements: large-scale seagrass dieback

e.g. 1000 km2 of seagrass lost in Hervey Bayafter two floods and a cyclone. Similar diebacks recorded in pristine and remote areas in Qld and WA

Impact of dugong habitat loss in Hervey Bay 1992

When their habitat is lost dugongs:

postpone breeding and starve or move

Locations of dugong carcasses

DateDu

go

ng

nu

mb

ers

A

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)

Num

ber o

f dug

ongs

Impact of seagrass dieback 25 years before on age structure of females sampled 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

Policy implications:

zonal protectionfor persistent hotspots but need to spread risk

Dugong protection areas

Policy implications:Indigenous catch quotas must recognize variability in size & potential rate of increase of target population

Policy implications:Population trends not reliable index of population status at local or regional scales except overlong time periods: PBR provides more information

Sustainable annual catch for Torres Strait using PBR 32-154

Best estimate of catch ~1000

Conclusions

• Dugong habitats are subject to intermittent large-scale diebacks even in remote and relatively pristine areas

• Large numbers of dugongs postpone breeding or move as a result

• This represent considerable policy challenge to managers