58 Australian Geographic

entirely of seagrass, and adults can consume

more than 25kg a day. Because of this and their

cumbersome, barrel-like body shape, dugongs

are commonly known as sea cows. But Dave

Holley, Shark Bay Marine Park coordinator,

who’s worked with WA’s dugong populations

since 2000, suggests they’d be better described

as sea pigs. That’s because they prefer to dig up

the rhizomes and roots of seagrass, rather than

graze on its leaves. It’s a habit that creates tell-

tale feeding trails of disturbed marine sediment.

Similar to many herbivores, dugongs cope

with this dif cult-to-digest plant matter with a

very long (30m or more), bacteria-fi lled intes-

tine. Their oddly shaped and proportioned face

is also an adaptation to their seagrass- chomping

lifestyle. They have a distended, trunk-like

upper lip that angles sharply downward, and is

covered in very sensitive bristles that help locate

and manipulate seagrass in murky habitats.

There’s a common assumption that dugongs

are strict vegetarians. But Professor Helene

Marsh – international doyenne of dugongs

who’s been working on the species for four

decades – says that’s not entirely correct. Recent

research has documented these mammals at the

southern extremes of their distribution eating

invertebrates from the ocean fl oor, including

worms and crustaceans.

“It looks like it’s important during winter in

places like Shark Bay and Moreton Bay, although

we don’t yet know just how important,” explains

Helene, who’s based at James Cook University,

Townsville. It might be that these dietary sup-

plements are a vital protein source in waters

where temperatures dip to about 18°C – the

lower limit tolerated by the species.

 T

HE MERMAIDS OF Moreton Bay are at

the south-easterly limit of the dugong’s

distribution. Under the direction of

Dr Janet Lanyon, a marine zoologist at the

University of Queensland, during the past two

decades this population has become the best

studied in the world.

When Janet began her work of Brisbane in

the early ’90s, she was motivated by aerial survey

evidence that suggested dugong populations

along Queensland’s most urbanised coastline

were suf ering signifi cant declines. Over the

years, the biggest threats to dugongs have been

identifi ed as seagrass habitat destruction, boat

strikes, net entanglements and pollution. All

of these factors were taking their toll in the

increasingly busy waterways of Moreton Bay

and Hervey Bay, just 300km to the north.

Moreton Bay’s wild dugong population was

accessible enough to be closely monitored. At

that stage, much of the information on the spe-

cies had been gathered from aerial surveys and

autopsies on animals drowned in fi shing nets

or struck by boats.

“No-one had done much hands-on work

with dugongs and we had little understanding

of population biology and life history para-

meters from wild populations,” Janet explains.

“We also had a limited understanding of their

behaviour and social interactions.”

Her idea was to study the physiology and

behavior of the Moreton Bay population by

capturing, marking and recapturing. But there

was evidence that handling dugongs could be

dangerous for the animals. Work by Janet’s

mentor Helene Marsh and others suggested

dugongs could experience a stress-prompted

physiological phenomenon called “capture

myopathy” – a muscle-wasting condition that

can lead to death.

It took several years, but Janet and her col-

leagues got around the problem by modifying

a form of rodeo-style capture developed for sea

turtles. It meant the animals didn’t need to be

pulled from the water, no nets were involved and

tagging could be performed in a rapid sequence

at the water’s surface.

For the past 14 summers, Janet’s team

has been carrying out a mark-and-recapture

program with huge success. “We’ve now tagged

close to 800 dif erent animals, including some

we’ve recaptured seven or eight times over

the years, and we’re getting some really good

information about how these animals grow and

mature, and their social dynamics.”

Signifi cant fi ndings include the revelation

that dugongs do not appear to aggregate and

travel together in stable family groups, as some

other marine mammals do. The large groups

that form in Moreton Bay Continued page 62

Their oddly shaped face is also an adaptation

to their seagrass- chomping lifestyle.^