09 January 2013
The colonies of New Zealand Fur Seals off South Australia's Kangaroo Island are shining examples of successful mammal recovery.
Since they were hunted in the early nineteenth century, the population has recovered and has been growing at around 10% each year for the last 24 years. So steady is the increase that these seals are actually being called a 'pest' by some industry groups and there is local debate about culling the animals.
Our marine mammal expert, Honorary Research Associate, Dr Peter Shaughnessy, has been consulted by the Kangaroo Island Natural Resources Management Board (NRMB) about ideas to control the populations.
Having studied seals for more than 40 years, Dr Shaughnessy is well-qualified to provide expert advice. This month, he will travel to Kangaroo Island with fellow scientists for an annual survey of fur seal numbers.
"There are two colonies – one at Cape Gantheaume and one at Cape du Couedic. When we began at Cape Gantheaume in 1989, there were 457 pups. There are now 10.7 times as many."
New Zealand Fur Seals are slightly different to Australian fur seals in colour, the sounds they make and the way they walk. Contrary to the name of the species, the New Zealand Fur Seal is native to Australia, not just New Zealand. In Australia it is found mainly in South Australia and Western Australia. Dr Shaughnessy says some people think they are an introduced species from New Zealand and therefore should be killed, but that is nonsense.
Kangaroo Island offers easy access for scientists as most seal hotspots are reasonably accessible. Sometimes they have to trek across rocky terrain to get to the populations, but it would not be as difficult as studying seal colonies in other places such as the Neptune Islands that are only accessible by boat.
"There have been two or three dips in the population increase since 1989," says Dr Shaughnessy. "The first one we associated with warmer sea surface temperatures south of Kangaroo Island, which meant less food for the mothers. They didn't die but they didn't produce any pups that year. However the following year they were back up to normal."
Scientists only count the new seal pups each year, as they are easily recognisable. Using a process called 'mark-recapture' the scientists catch the pups and cut part of the hair on their heads to mark them. The number marked is carefully recorded. The next step is to work out the proportion of pups marked, which they do by walking through the colony and recording numbers of marked and un-marked pups in samples. This procedure is more accurate than a direct count of pups, some of which sleep under rocks.
Dr Shaughnessy began the New Zealand fur seal surveys with the CSIRO before later transferring to the South Australian Museum.
"It's really interesting to document the recovery of a mammal species that was harvested. It's very good news for Australia because there are very few mammals that are increasing in numbers," he says.
"Without this study, people would just be guessing the size of the colonies."
Dr Shaughnessy says he was recently consulted by the South Australian Government as some people want permission to cull the animals.
"The tuna operators want to cull them because the New Zealand Fur Seals get into their nets. They want permission to kill animals they claim they can recognise. And we say, well how can you recognise them? And if you do kill them, three or four others are going to come and replace them quick smart. In addition, some people have applied for permission to shoot the seals on the Capes on Kangaroo Island."
Dr Shaughnessy advised the NRMB that culling would be a bad idea. He says after all, the population of New Zealand Fur Seals on Kangaroo Island will not keep multiplying forever.
"There will be limiting factors. Either they'll run out of space or they'll get to the limit of their food." Dr Shaughnessy believes that it'd be a waste of time trying to interfere, as the populations wouldn't change significantly unless enormous numbers were removed, not to mention the impact on the tourism industry.