Seagrass: Bringing turtles and dugongs to the Seaway

Try and gauge the age of the green sea turtle that divers rescued in the Gold Coast seaway recently.

This is what we know:

Before the Sea World Rescue and Research team released him, Dale the South Straddy water taxi operator saw the turtle and guessed he was about 70 cm long

According to “Dugong and Marine Turtle Knowledge Handbook” green sea turtles measure a measly 5cm when they are born. As soon as they hatch they head out to the open sea, where it takes them five to seven years to grow to 30 or 40 cm, eating small marine animals near the surface.

Around the young age of seven this little sea turtle that Sea World called Twiggy travelled from the open ocean and began “foraging on the sea floor among habitats containing seagrass or seaweed”. Eventually it appears he found the seagrass in the seaway and made it home. The southern area of the Broadwater has about 340 hectares of seagrass.

Green sea turtles don’t reach breeding age till about 35 years old and they measure a metre when they are fully grown.

So it’s a fair guess that Twiggy is about 20 years old.

Twiggy probably wasn’t born on the Gold Coast either.

He may have been born at one of the turtle nesting sites on South Stradbroke Island, however green sea turtle migrations between breeding seasons average about 400km and can be up to 2600km. He was probably born a part of the decreasing Southern Barrier Reef population, but he could come from even further afield – possibly Torres Strait, or Gulf of Carpentaria or Raine Island in the northern Great Barrier Reef, which has the world’s largest number of breeding green turtles. Sadly the success of nesting on Raine Island is less than 10 percent because a rising water table is drowning the eggs. There are turtles on the Gold Coast that were born as far away as New Caledonia.

Where ever he was born, he’ll be heading back there when he’s 35 to breed. He’ll be away for a few months then come back to the Gold Coast to forage for four to six years before he’s ready to make the journey again.

Turtles eat about 2kg of seagrass a day, the coordinator of local Seagrasswatch monitoring group, Seagrass Gold Coast (SGGC), Daniela Wilken-Jones, said.

SGGC’s monitoring work is important because seagrass is vital for turtles, dugongs and a myriad of species including commercially important fish, plus seagrass is sensitive to impacts, both human and natural, making it like a canary in a coal mine, she said.

“Seagrass habitats are indicators of environmental health and provide an early warning of ecosystem decline,” Ms Wilken Jones said.

Shona Pinkerton from Devocean Dive dives everyday in the Broadwater and always sees turtles sheltering along the seawall and grazing on seagrass at Western Beach adjacent to Marine Parade and Bayview St.

The seagrass beds near Loders Creek were thick but have been damaged by the last round of dredging, she said.

About once a year she sees a dugong.

Last year she saw a one off the sand-pumping pipe at Southport.

“They are pretty amazing underwater, very comfortable swimmers, big whiskers, very nimble, just gorgeous and they’re big, at least a couple of metres.”

SGGC volunteer teams within the Broadwater in 2013 sighted two dugongs and reported one feeding trail. A feeding trail is where dugongs mow lines of seagrass down as they graze.

It’s unclear whether dugongs sighted in the Broadwater are residents or visitors from the Moreton Bay population as there’s no dugong research on the Gold Coast, Dr Janet Lanyon from the University of Queensland Marine Vertebrate Ecology Research Group said.

Dugong sightings in the Broadwater are not an unusual occurrence, Dr Lanyon said.

“It happens quite regularly… always in the summer months… but they are not a migratory species… their genetic structuring is very localised,” the dugong researcher said.

The internationally respected dugong expert said it was likely that dugongs had travelled to the Broadwater from Moreton Bay for foraging after the 2011 floods, as the huge plume of land runoff from the floods had damaged the bay’s seagrass pastures.

In fact the water taxi operator, Dale Little, confirmed that he had seen and heard of several dugongs feeding in the Broadwater at that time. Some of them had metal tags on their flippers which provides some evidence they were from Moreton Bay.

Now that Moreton Bay’s seagrass pastures have repaired, there’s ample seagrass to support the local herd of 750-1000 dugongs, so they wouldn’t need to leave the bay to forage, Dr Lanyon said.

However, like most tourists, the big marine mammals enjoy dining when they visit the Gold Coast.

Dugongs are like cows. They graze constantly around the clock, Dr Lanyon said.

They are usually grazing when they are spotted in the Broadwater, she said.

Another Queensland expert on dugongs is Professor of Environmental Science at James Cook University, Helene Marsh. Prof Marsh helped write the “Dugong and Marine Turtle Knowledge Handbook”.

She said the Moreton Bay population is significant for a number of reasons:

  • It is the southernmost significant population on the east coast of Australia
  • It is the only significant population close to a major city
  • It occurs in relatively oceanic clear water and so is less susceptible to terrestrial runoff and extreme weather events than many other populations on the urban coast
  • From a numerical perspective there are several other places in Australia with many more dugongs than Moreton Bay

Prof. Marsh said dugongs in the Broadwater may not be nationally significant, but they may be “very significant to the locals”.

Dugongs are considered vulnerable on a global scale and in Queensland, she said.

“The major problem in Queensland is along the urban coast from about Port Douglas south where dugongs are subject to multiple cumulative impacts.”

Dr Lanyon said despite the long periods between sightings, it’s possible that it’s always the same dugongs that are sighted at the Broadwater. In other words the Broadwater could have a local dugong or two.

Dugongs are discrete. They spend 98 percent of their time underwater, quite camouflaged in the murky waters they live in and when they breathe their nostrils often barely break the surface.

Some places are well known for lone resident dugongs: In Manly in Redland Bay for instance there was a well-known dugong resident with distinctive scars from boat strikes, she said.

Herds such as in Moreton Bay are not the norm. Dugongs are most often solitary. Mother and calf is the second most common grouping, she said.

Students at UQ have been researching dugong’s vocal repertoire. Underwater sounds like bird trills and chirps are common between mothers and calves. Their new research has revealed a clear distinction between the vocalisations of the populations in Hervey Bay and Moreton Bay.

New data has also shown that dugongs are more vocal in bigger groups, pointing at the possibility of social structures.

“Dugongs have very distinct personalities,” Dr Lanyon said.

“Some are very docile, gentle; some are curious and confident; some are stroppy old dugongs.”

She said the boating in the Broadwater would be a threat to dugongs; anything with a propeller and a hard hull, particularly if it’s going fast. Dredging and land runoff are equally bad. Anything that damages seagrass is bad for dugongs.

No seagrass, no dugongs, no turtles.

If seagrass is damaged like it was in the 2011 floods, dugongs die or leave or suffer health impacts. Natural weather events like storms cause runoff, which kills or damages seagrass until it recovers.

Dredging, particularly sustained dredging, affects water quality, particle suspension and light attenuation, impacting severely on seagrass thus impacting severely on turtles and dugong, she said.

Dr Lanyon said the dugong status should be changed from ‘vulnerable to extinction’ to ‘endangered’. One of the main reasons they need more protection is their special reliance on certain sea grasses. Another reason is their life history.

“Of all marine mammals dugongs are among the slowest breeders and the slowest growers. The breeding age is 17 and the mother has one calf. In Moreton Bay the mother stays with the calf for years. Energetically it takes a lot out of her. The calf is normally chubby while the mother is underweight. They only have a few calves in a lifetime.”

The Moreton Bay population is in good shape, thanks to the ocean buffer between the urban coastline and the seagrass beds, adjacent to North Stradbroke and Moreton Islands. That buffer makes Moreton Bay unique, as most seagrass habitat is in shallow water close to shore, as it is in the Broadwater, exposing the dugongs and turtles that graze on it to human impacts.

Dr Lanyon said she has run out of funding for her research for now and is waiting for more.

Queensland is the dugong capital of the world, she said.

“They should be our iconic and special animal, it’s amazing they get so little attention.”

Image: Divers on the Broadwater see green sea turtles everyday sheltering in caves along the rock wall or feeding on seagrass along the Western Beach area. Photo supplied by Devocean Dive.

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