Seal success could be downfall
NZ fur seals have recovered so well that their numbers might need to be cut back. Will Harvie reports.
The researchers carried riot shields when collecting biological samples from New Zealand fur seals on Banks Peninsula. And they used them. ‘‘It was an exciting project,’’ says Dr Adrian Paterson, with the sort of Kiwi-scientist deadpan that minimises risk.
The associate professor in the Department of Pest Management and Conservation at Lincoln University says seals are ‘‘not to be trifled with’’.
‘‘Getting bitten by a seal is not great.’’
Seals have particularly unhygienic mouths and a bitten human needs quick attention.
Paterson and colleagues, especially former PhD student Arsalan Emami-Khoyi, took small boats to seal colonies on rocky outcroppings on the peninsula. They jumped ashore, prepared to defend themselves.
‘‘If any of the big guys come lunging at you ... yeah, we did use those [shields],’’ he says.
It turned out that briefly capturing pups was best. Even that was ‘‘fairly iffy’’.
The point was to collect biological samples – blubber and faeces mostly – for DNA analysis back in the lab.
The results have recently been published in two scientific papers that detail the remarkable comeback from near extinction for the indigenous seals and why there will likely be calls to cull the animals in about 10 years.
Emami-Khoyi, Paterson and colleagues reconstructed the demographic history of New Zealand fur seals in using maternally inherited, mitochondrial DNA sequences collected from seals around New Zealand and the subantarctic Bounty Islands.
Their analyses show that there were probably 2.5 million to 3m fur seals on New Zealand islands before the arrival of Polynesians in the late 13th century.
These Polynesian-Maori avidly hunted seals and about the time of Captain Cook’s arrival in the late 18th century there were an estimated 1.5m to1.8m seals, about a 40 per cent decline.
That loss was nothing compared to the effect of European sealing, which peaked between about 1790-1820. Paterson estimates this sealing reduced the population to about 10,000 animals, or about 0.4 per cent of the prehuman population.
Today, the Department of Conservation estimates the NZ fur seal population is about 200,000 animals, about 5-10 per cent of prehuman numbers. So the recovery has been fairly remarkable.
Fur seals are among the most successful recovering native marine mammals, Paterson says.
In part, this is because fur seals live in contiguous ocean environments. They can travel easily to breed and establish new colonies.
Compare this to a native species trapped in a forest remnant. They don’t have as much opportunity to travel and intermingle genes with other populations.
And that points to another aspect of Emami-Khoyi and Paterson’s findings: genetic diversity within the surviving population is relatively high.
Although there isn’t much difference between fur seals on the West Coast or the Catlins or Banks Peninsula, there is a lot of gene diversity within these colonies. (Most fur seals live on the South Island, although fossil records show more lived on the North Island before human arrival).
The relatively high diversity is explained, Paterson thinks, by the ‘‘bottleneck’’ – the moment of fewest animals. It lasted about two or three seal generations.
‘‘There’s increasing evidence suggesting that when populations go through a really quick bottleneck ... they still retain a lot of their genetic variation,’’ he says in an interview.
‘‘Imagine you have a swimming pool full of genes which springs a leak,’’ Paterson explains in his blog.
‘‘Usually this will only leave a few puddles at the bottom. The fur seals are more like they have retained water up to your ankles, useful for paddling about in [and] better than a puddle.’’
So fur seals are doing relatively well.
But well enough that ‘‘conflicts between humans and seals are going to increase’’, Paterson says.
The population may not reach into the millions but it will increase markedly. ‘‘The key message from our research is that the population, unless checked, will continue to grow. There’s nothing that will obviously limit it.’’
Seals will start to occupy beaches in front of million-dollar baches, he says. Which sounds OK, but seal colonies stink. Swimmers will find themselves among seals in the water. Bites, recall, are nasty.
‘‘I would say that within a decade there will be calls to cull seals.
‘‘They are already starting to generate issues because they’re coming into more contact with humans and there is only going to be more contact.’’
And they’ll probably return to the North Island in larger numbers.
In the same blog, Paterson wrote about anecdotal evidence of increased seal numbers. He was boogie-boarding at Kaka Point in the Catlins. A seal surfed in ‘‘beside me all the way to the beach. This never happened to me as a child’’ growing up in the area, he writes.
‘‘When I ask my first year classes if they have been up close to a fur seal, almost 100 per cent of them have had this experience.’’
It’s worth noting that in their peer-reviewed science papers, Paterson and colleagues report on ‘‘effective population’’, which is about an order of magnitude less than the ‘‘estimated population’’.
This difference reflects that big, strong males dominate mating and keep harems of females. Some males will never mate and some males and females are too young or old to mate, or don’t mate for some other reason.
'There's increasing evidence suggesting that when populations go through a really quick bottleneck ... they still retain a lot of their genetic variation.' Associate professor Adrian Paterson