The Conversation 10 May 13;
“How many are there?” and “how are they doing?” are the first questions people usually ask about species of conservation concern. These seemingly straightforward questions are tough to answer when it comes to the dugong.
What we do know is that dugongs are generally safer in remote areas, where traditional hunting is the major pressure, than they are around coastal urban areas where they are affected by habitat loss, gill netting, and vessel-strikes, rather than hunting.
We don’t know how many dugongs there are globally or in Australian waters. Estimating dugong numbers is difficult because the animals mostly live in turbid water and tend to surface discreetly, often with only their nostrils breaking the surface. Our best estimates mostly come from aerial surveys combined with sophisticated statistical models.
About one-fifth of the dugong’s range is in Australia. Dugong habitat extends from Shark Bay in Western Australia, along 24,000 km of our northern coastline to Moreton Bay near Brisbane. Our genetically healthy dugongs are the most abundant marine mammals in our northern coastal waters. While aerial survey data indicate more than 70,000 dugongs, the number is certainly higher. Large parts of the remote coasts of Western Australia and the Northern Territory have not been surveyed recently, or at all.
The status of Australian dugongs varies greatly. Shark Bay supports a large dugong population with minimal human pressures, making it the most secure dugong population in the world. On the other hand, the urban coast of the Great Barrier Reef (GBR) region between Cairns and Bundaberg poses many threats to dugongs.
Torres Strait is the world’s largest dugong habitat. Surveys conducted by my group at James Cook University show that the region contains a remarkable 58% of the habitat supporting high densities of dugongs in Queensland, as illustrated by the map below.
Relative density of dugongs along the coast of Queensland and adjacent Northern Territory waters based on 25 years of JCU aerial surveys. Dr Alana Grech
Archaeological research by Ian McNiven’s group at Monash indicates that dugongs have been hunted in Torres Strait for at least 4,000 years and that the harvest has been substantial since well before European settlement. Today dugong hunting is sanctioned by the Torres Strait Treaty between Australia and Papua New Guinea (PNG) and in Australia by the Commonwealth Torres Strait Fisheries Act and the Native Title Act.
The data to compare contemporary and past catch rates are not available. The current total regional dugong catch is unknown although the Torres Strait Regional Authority is attempting to correct this deficiency for Australian communities.
Dugong cow and calf killed by collision with a ferry in Moreton Bay, Queensland. Rachel Groom
In 2004, I was co-author of two modelling papers using different techniques that suggested that the current dugong catch in Torres Strait was not sustainable. I now question this conclusion for several reasons:
Dugong habitat in Torres Strait is much more extensive than we thought. In 2010, the Torres Strait Regional Authority partnered with scientists at Fisheries Queensland to conduct the first seagrass survey of far western Torres Strait. This survey discovered that this very remote region supported the largest continuous seagrass bed in Australia. My group subsequently extended our aerial survey of Torres Strait to cover this area and established that it also supports a sizable dugong population.
Our time series of aerial surveys conducted since the mid-1980s has not demonstrated a significant decline in dugong density in Torres Strait.
Studies of the diving behaviour of wild dugongs fitted with timed-depth recorders and GPS-satellite transmitters indicate that the aerial survey population estimates used in the modelling are significant underestimates.
Studies of hunter behaviour indicate that about two-thirds of the high density dugong habitat in Torres Strait is never hunted.
James Cook University research is being used by the Torres Strait Regional Authority in negotiations with the PNG Government and Islander leaders regarding the management of hunting. The Authority is also working with a veterinarian to address animal welfare concerns.
In the remote GBR region north of Cooktown the dugong situation is similar to Torres Strait. However, dugongs along the urban coast of the GBR, including around Townsville, have to cope with additional challenges. Analysis of the records of dugongs caught in shark nets indicated a precipitous decline in catch rates between the 1960s and 1980s.
The university’s aerial surveys since the mid-1980s indicated that the population had stabilised as a result of significant management interventions by the Commonwealth and Queensland governments.
But the 2011 floods and cyclones reduced the dugong population to the lowest level since surveys began. Worse, the dugongs stopped breeding because of a shortage of food – no calves were seen in the region during our 2011 survey.
Dugong mortalities recorded by the Queensland government’s StrandNet program in 2011 were the highest since reporting began in 1998. Some dugongs migrated from the region and are now returning, but the high level of coastal development is cause for grave concern.
The most serious human impacts on dugongs in the urban GBR are habitat loss, gill netting, and vessel-strikes, rather than hunting. All these impacts have associated animal welfare concerns.
If you were a dugong, where would you rather live: Torres Strait or Townsville?