Eh?
"That," explained our highly qualified scientist-guide Dr James Watson, "is to keep the Hooker's sea lions at bay."
He had to be joking.
Well, that's what I thought until Rambo, a 100kg teenage male sea lion - when he's grown up he'll weigh in at 400kg - turned up on the beach.
We were clustered round the Zodiacs preparing to explore the foreshore area where the schooner Grafton was wrecked in 1864 when young Rambo bustled out of the sea and tried to chase off these interlopers into his territory.
When his charges got a bit too close for comfort Guillaume Bouteloupe, another guide, whipped up his stick like a rapier and flicked the sea lion on the nose with the flag, stopping him in his tracks.
"This is what you do," he said.
"When you hit them on the nose they are very surprised and they stop to work out what has happened.
"After that they keep their distance."
Rambo continued to pursue us as we walked down the beach, from time to charging someone - rising to his full height, making roaring noises and displaying his tusklike teeth - but as soon as Guillaume arrived with his trusty flagstick the sea lion reluctantly backed away.
Then a fresh challenge arrived in the shape of another young male who did not have a flagstick so Rambo rushed headlong into battle.
There was much roaring, wrestling and snapping of jaws without any actual damage being done - which is the norm on such occasions - and the newcomer retreated leaving Rambo to continue to stalk us.
By now we were pretty much regarding him as a bit of a clown, an amiable blowhard, but what we didn't know was that he had already drawn blood.
When the guides from the expedition ship Orion arrived on the beach to prepare the way for our group of 35 tourists, Rambo had surprised them by suddenly emerging from the water and charging Department of Conservation observer Rebecca Hiscock.
"I'd only just got out of the boat and hadn't had time to look around or get one of the poles when suddenly this sea lion charged me and grabbed my leg.
"It wasn't much of a bite, more of a nip really, and he let go straight away.
"But it still gave me a fright and it hurt a bit so I'm afraid I gave a girly scream and ran behind the others."
Later investigation showed the bite hadn't torn Rebecca's protective clothing but it had broken the skin and caused a bit of bruising.
Auckland Island is the largest of a cluster of islands 465km south of Bluff, set in the great Southern Ocean which roars in a giant circle around the Antarctic, but with a reasonably temperate climate.
These days the islands are uninhabited - though there have been a few unsuccessful attempts to settle there - and with most introduced pests having been removed the wildlife is prolific and improving all the time.
The islands are, for instance, home to the yellow-eyed penguin, the rarest in the world, and while we didn't see any at Carnley Harbour we did find the skeleton of one on the beach.
Even more amazing, we also found the skeleton of the Grafton, its wooden ribs sticking out of a stream bed, still surviving 146 years after being wrecked there.
Near this beach the five crew built themselves a hut where they scraped out an existence for 18 months.
During that time they managed to construct a small boat which three of the party sailed to Stewart Island and all were eventually rescued.
"I don't think there's anything left of the hut, but somewhere round here there's a cairn of stones which was the forge they used to make nails for their boat," Rebecca said.
We searched for the cairn - even briefly following what looked like a path into the rata forest which covers most of the island - but couldn't find anything.
The forest itself was fascinating - twisted, gnarly and dripping with mosses and lichens - inspiring one of our group to comment "You'd expect to find hobbits living here".
I forebore pointing out that in fact hobbits like grassy spaces, and instead enjoyed the birds that were living there, tiny Auckland Island tomtits, bellbirds and tui and, down on the shore, red-eyed Auckland Island shags, a black duck and even a rare flightless teal.
While we were checking out this beach, Doc restrictions on numbers meant the other half of our expedition group was exploring a different part of the island where they found a tumbledown hut and a newer replacement shelter, apparently one of several built as a refuge for shipwrecked mariners following the loss of the Grafton, and stocked with supplies.
Such huts must have done a busy trade because it's reckoned that there have been at least 10 shipwrecks on the islands.
Along the way the other group also came across lots more seabirds but the thing that had the greatest impact was the sight of a Hooker's sea lion - they're also known as New Zealand sea lions - munching on something.
At first everyone thought it was a cute little penguin.
But later study of photos indicated the sea lion was in fact eating a furry mammal, possibly one of the New Zealand fur seals also found here, but from the look of the pelt most likely one of its own kind.
"Interesting," said our expedition leader Mick Fogg.
"I've never heard that sea lions resort to cannibalism. But that's certainly what it looks like."
Our naturalist guides were equally intrigued by Rambo's behaviour.
James commented afterwards, "I've never seen a sea lion that was so curious."
Guillaume was less flattering, "In the five years I've been coming here I've never seen such aggression."
And Mick summed it up, "That was a very unusual sea lion.
"Most of them are very comfortable around humans. That one was a psychopath."
• Jim Eagles travelled to the Subantarctic islands as a guest of Orion Expedition Cruises.
