Life's a battle for endangered NZ sea lion

SIMPLE SCIENCE

ROGER HANSON
Last updated 05:00 08/02/2014

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A few weeks ago I stood with Martin Cawthorn, a marine mammals expert, watching a couple dozen New Zealand (or hooker's) sea lions hauling up onto Sandy Beach on Enderby Island, one of the Auckland Islands and some 460 kilometres south of Bluff.

Martin is a recognised authority on hooker's sea lions, having spent countless hours observing and studying them in locations such as this.

Watching the 400-kilogram males dozing on the beach conjures up a picture of indulgent idleness and ease, but nothing could be further from the truth. Sea lions have a tough life. Every time they go to sea - and they can go out as far as 150km from the coast and dive as deep as 500m - they run the gauntlet of sharks and killer whales. We saw one sea lion, still alive, with the most horrific injuries sustained by a shark attack. It looked like a surgeon's knife had been taken to its body, its internal organs exposed as it lay in a patch of rata forest off the beach, its sad eyes moving but clearly just waiting to die.

The breeding season for hooker's sea lions begins in November when the males fight for territory on the beach and for females.

As Martin pointed out, these fights are brutal, no-holds-barred contests in which blood flows. We saw nasty-looking battle scars on many of the males.

There are six extant species of sea lions (including the Galapagos sea lion), three species in the northern hemisphere and three in the southern hemisphere. The hooker's sea lion is indigenous to New Zealand and was named in 1831 after the British botanist and explorer Joseph Hooker.

These creatures are sexually dimorphic. The males are three times as heavy as the females and about 3.25 metres long, whereas the largest females are about 2m. They have a mane made of a type of hair, hence their name, sea lion. The Maori people have a lovely name for the mane, pureke, which means rain-cape.

Like most of the mammalian species in the Antarctic region, in the past sea lions were hunted almost to extinction for their oil and the hooker's sea lion is now one of the rarest of the sea lion species with a total population of about 10,000. The vast majority of these were born on beaches on Enderby Island, Auckland Island itself, Dundas Island, which is part of the Auckland Islands group, and Campbell Island. The sea lions are generally philopatric, which means they return to their beaches of birth to breed. The upper body of these mammals is dark and the lower body light grey to confuse sharks, which as we saw doesn't always succeed.

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After coming ashore, the males set themselves up on the beach in tear-drop-shaped territories. The tear-drop shape has to do with their field of vision.

When the females come ashore, 80 per cent of them are pregnant. The gestation period is 11 months. It is known that this involves delayed fertilisation, a process whereby the embryo does not immediately implant in the uterus but is kept in a dormant state; the duration of this dormant state in Hooker's sea lions is not known.

The arrival of pups is within a narrow window, seven to 10 days after hauling ashore, at a time when the weather is warmest and their favourite food source, squid, is most plentiful.

The females form harems on the beach, not necessarily because they have been corralled by the males but to serve as a means of protection from the hormone- pumped males who can inflict serious injuries on the females and can crush or even eat the pups. After about three weeks the mothers take the pups away from the rookeries.

Martin says the pups are loathe to leave and it is a captivating sight to see the mothers trying to coax them away. They stay with their mothers for up to a year.

The females are sexually mature at 3 years and have pups after about four years and annually thereafter. The males do not have the bulk required to fight and hold a territory until they are about 8 years old.

The hooker's sea lions allowed us to approach to within a couple of metres. It was fascinating to be able to witness these noble indigenous New Zealand mammals at such close quarters, living in - thanks in some measure to the work of the Department of Conservation - their natural and pristine subantarctic environment.

- Taranaki Daily News

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