Dugong comeback
Southern Queensland dugong populations are rebounding after being devastated in 2011 by Cyclone Yasi.
FOLLOWING MASS bleaching events that killed off huge areas of coral in both 2016 and 2017 (see AG 139), positive stories about life in the usually stunning Great Barrier Reef Marine Park have been hard to come by.
That’s why it is so exciting to hear that dugong numbers are recovering in southern areas of the park that were pounded by 2011’s Cyclone Yasi.
The Great Barrier Reef Marine
Park Authority (GBRMPA) has announced that aerial surveys – which are conducted every five years and were most recently completed last November – show a huge increase in both dugong calf numbers and overall population size in the park’s southern region. According to the GBRMPA, 5500 animals were recorded, 10 per cent of which were calves.
“The large increase in dugong calf production is very positive,” says Professor Helene Marsh of James Cook University in Townsville, Australia’s leading expert on the species and part of the team that completed the survey. “Dugongs play an important ecological role in coastal marine ecosystems… The substantial increase in dugong calves can be linked to the better food sources available for adult dugongs, as the condition of intertidal seagrass cover has improved.”
Dugongs, which are close relatives of the manatees of the northern hemisphere, are large herbivorous marine mammals that feed on seagrass.
They are found around the coastlines of the Indo-Pacific region, and the northern coast of Australia plays host to the world’s largest population, found all the way from the Pilbara in the west to southern Queensland (see opposite) in the east.
When aerial surveys of dugongs in the southern region of the GBR Marine Park were completed in November 2011, they showed that powerful Yasi – which battered the coastline as a Category 5 severe tropical cyclone in February that year – had taken a heavy toll on marine life, which included stripping seagrass meadows of vegetation.
Even more damaging was the massive flooding of coastal regions that followed, washing large volumes of sediments and pollution into the water.
This increased turbidity meant seagrass meadows were obscured from the sunlight they need to thrive and large areas of these marine plants were killed, leading to a dearth of food for dugongs. Breeding females particularly require a lot of food when calving.
Only 500–600 dugongs were sighted in 2011 in the region south from Hinchinbrook Island to the Queensland–NSW border, none of which were calves.This was the lowest number recorded since surveys first began in the mid-1980s. In 2011 there were reports of 187 dugongs found dead or dying, stranded along the coast.
Other experts have commented that, while the increase in dugong numbers in the latest survey is certainly a very positive development, it may be partly due to the migration of animals from further north; dugongs are very slow breeders, so the increase in population size cannot be down to reproduction alone. Nevertheless, Helene says, the fact dugongs are returning and breeding in these areas means that seagrass meadows have recovered to the point where they can support pregnant females and calves, a positive development in itself.
According to a GBRMPA spokesperson, the effect on seagrass of Cyclone Debbie, which struck Queensland in March this year, has not yet been quantified. But it’s known that, compared with the aftermath of Yasi, Debbie was accompanied by much less coastal flooding and associated damage to marine environments.
Dugongs, which are listed as vulnerable on the International Union for Conservation of Nature’s Red List of Threatened Species, have suffered from development along southern Queensland’s coast since the 1960s.
They’ve also been heavily impacted by the installation at bathing beaches of shark nets, in which they can become entangled and drown.
Dugongs also suffer from being trapped in fishing nets and are killed in collisions with boats.
JOHN PICKRELL
is a former AUSTRALIAN GEOGRAPHIC editor. Follow him on Twitter: @john_pickrell