How many dugongs could the How many dugongs could the seagrass habitats of the urbanseagrass habitats of the urban

coast of Queenslandcoast of Queensland support?support?

How many dugongs could the How many dugongs could the seagrass habitats of the urbanseagrass habitats of the urban

coast of Queenslandcoast of Queensland support?support?

Helene Marsh, Glenn De’ath, Neil Gribble, Baden Lane

Part of dugong herd in Moreton Bay near Brisbane in 2003 Amanda Hodgson

Marine Parks of urban coast of Queensland -

• > 280,000 km2 of multiple use marine parks

• > 75,000 km2 of no-take MPA

• > 8,000 km2 of Dugong Protection Areas

• politically controversial• realistic recovery targets

required

Recovery target extrapolated from historical anecdotes

Jackson et al’s population estimate of 1.1 to 3 million dugongs for entire east coast of Queensland in 1890s(Science 2001)

touted as realistic recovery target and evidence that Moreton Bay

2/3 way to Ecological Extinction (Pandolfi et al. Science 2003)

Contemporary photograph of dugongs in Moreton Bay

Methodology• Determine rate of decline of dugong population 1960s

– 1990s using change in CPUE in shark nets• Estimate 1990s dugong population using aerial survey• Use shark net decline to hindcast 1990s population

estimate to 1960s• Calculate whether current seagrass meadows could

support hindcast population by: – estimating time required for dugongs to consume seagrass

once – comparing this estimate with time required for seagrass to

recover from dugong grazing taking account of all sources of uncertainty and variability in data

Queensland Shark Protection Program

1962-1999

0-3 nets at 47 beaches at 10 locations

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•

•

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•

•

••

•••••

•••

•

••••••••

••••••••••

1970 1980 1990 2000

0.0

0.5

1.0

1.5

2.0

2.5

3.0

1960

Dug

ong

s ca

ught

pe

r m

onth

/ be

ach

Dugong by-catch in the shark nets declined at rate of 8.7% (95% CI = 7.1, 10.6) p.a.to 3.1% (1.4, 6.1) of initial catch rates by 1999

Aerial surveys to estimate dugong numbers in 1990s

A target for recovery?

• Aerial composite population estimate of 4220 (95% CI 2360, 8360) in 1990s

• Assuming decline in dugong CPUE in shark nets represented change in population

• Dugong population estimated to be 72000 (95% CI 31000, 165000) dugongs in the early 1960s

Captive dugongs gain weight on 28-40 kg wet weight seagrass per day

Dugongs generally dig up whole seagrass plants

Time depth recorders show dugongs rarely feed in >15m

2593-3815 km2 seagrass < 15 m in region including species which are

not preferred food of dugongs

Data from McKenzie et al. and Abal and DennisonPhotograph by Jane Mellors off Townsville C-Bar

Have a drink and watch the dugongs!

Mean seagrass biomass per meadow in region 5-12 g dry weight

m2

Which hindcast population estimate could be supported today?

Estimated number of dugongs in 1960

Minimum number of days

Maximum number of days

31,000 61 299

72,000 26 132

165,000 11 57

1.1 million 1.5 8.5

How long does it take for seagrass to recover from dugong grazing?

Observational and experimental studies suggest:

• <30 - 200 days (clear water ~27 oS)

• several months -year turbid water 18oS

Which hindcast population estimate could be supported today?

Estimated number of dugongs in 1960

Minimum number of days

Maximum number of days

31,000 61 299

72,000 26 132

165,000 11 57

1.1 million 1.5 8.5

Conclusions

• Current carrying capacity probably < 30,000 dugongs

• Jackson et al’s (2001) estimate of 1.1 to 3 million dugongs unrealistic recovery target

• Improved estimates of carrying capacity of seagrass meadows for dugongs required for realistic recovery targets

• Come to the Workshop next Thursday!