Part of dugong herd in Moreton Bay near Brisbane in 2003 Amanda Hodgson
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Part of dugong herd in Moreton Bay near Brisbane in 2003 Amanda Hodgson. How many dugongs could the seagrass habitats of the urban coast of Queensland support?. Helene Marsh, Glenn De’ath, Neil Gribble, Baden Lane. Marine Parks of urban coast of Queensland - .

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

How many dugongs could the

seagrass habitats of the urban

coast of Queensland

support?

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


Marine parks of urban coast of queensland l.jpg
Marine Parks of urban coast of Queensland - Amanda Hodgson

  • > 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 l.jpg
Recovery target extrapolated from historical anecdotes Amanda Hodgson

Contemporary photograph of dugongs in Moreton Bay

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)


Methodology l.jpg
Methodology Amanda Hodgson

  • 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


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Queensland Shark Amanda Hodgson

Protection Program

1962-1999

0-3 nets at

47 beaches at

10 locations


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Dugong by-catch Amanda Hodgson

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


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Aerial surveys Amanda Hodgson

to estimate

dugong

numbers in

1990s


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A target for recovery? Amanda Hodgson

  • 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


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Captive dugongs gain Amanda Hodgson

weight on 28-40 kg

wet weight seagrass per day




2593 3815 km 2 seagrass 15 m in region including species which are not preferred food of dugongs l.jpg
2593-3815 km Amanda Hodgson2 seagrass < 15 m in region including species which are not preferred food of dugongs

Data from McKenzie et al. and Abal and Dennison

Photograph by Jane Mellors off Townsville C-Bar

Have a drink and watch the dugongs!




How long does it take for seagrass to recover from dugong grazing l.jpg
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



Conclusions l.jpg
Conclusions grazing?

  • 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!


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