Tagging at Stewart Island

‘Marble Tail’ at Stewart Island

Annually from March through to April, a small team from NIWA (National Institute of Water and Atmospheric Research) and DoC (Department of Conservation) embark to Stewart Island to continue ongoing research of the white shark population that gathers there. The sharks are there all year round, but in greatest abundance through late summer. The exact reasons are not known as to why this is, but a fairly reasonable hypothesis is the abundance of seal pups taking to the water. Aside from the seal population, the waters around Stewart Island are rich in sea life.

Map of Paterson Inlet, Stewart Island

We have been invited to participate for a couple of years, but this year was the first time we have been able to attend. We joined this year’s expedition at the end of March to observe and assist in the work, and spent three days on the water. This year, the work continued with the deployment of acoustic tags that was initiated in 2011. Acoustic receivers have been placed around Paterson Inlet at various locations to monitor the movement of white sharks within this localised area. White shark movements at Stewart Island are not limited to this area alone; however the cost and logistics of monitoring a larger area are considerable – way beyond the reach of the current available funding. The studies at Paterson Inlet (see map above) have revealed some interesting information so far, and are proving a great location for continued observation and monitoring research of white sharks.

An acoustic receiver is cleaned after retrieval

The work started with servicing three acoustic moorings. All the steel ‘D’ links that attached the acoustic receiver to the mooring anchor and buoy were replaced to ensure the links did not fail due to sea salt corrosion. The acoustic receivers were also renewed (this is done every three months) so the data recorded can be downloaded and analysed. One of the acoustic receivers we recovered and renewed is located close to a salmon farm in Big Glory Bay, and much to my amazement, no shark activity has been recorded anywhere near the salmon cages. I was expecting to hear that juveniles were hanging around the cages picking off escapees or attempting to help themselves to a free meal, but this is not the case: a fact I am sure the salmon farmers are extremely happy with! Once this job was completed we headed out to the sharkie ‘hot spots’ such as Bunker and Bench island (see map below) to continue tagging sharks.

Bunker Island and Bench Island, near Stewart Island

Acoustic tags are deployed in the same way a sport fishing tag or PAT tag is – the shark is attracted to the boat and encouraged to swim close enough to the boat, at the surface, so the tag can be darted into the thick skin near the dorsal fin. Encouraging a shark to swim where you want it so you can deploy a tag is not as simple as it sounds, and the team has spent long hours trying to develop ways to make the job more simple. One team member, who is the on-board camera operator, has developed a video pole-camera which he holds over the side of the boat to film the sharks.

Kina Scollay keeps an eye on sharks using a pole-cam

White sharks, however, are not the dumb animal that so many think, and as the team’s methods have improved and advanced, so the sharks have learned to do exactly the opposite to what the team want – with skipjack tuna throw baits at $2.40 per kg and each weighing in at close to 10 kg, this quickly becomes a costly lesson to learn.

Tia (female 3.2TL) learns to steal baits

The weather is another obstacle the team are forced to work around, and the three days spent on the water proved just how changeable it can be. We were basking in calm, warm conditions on the first day when Bruce, a 4m TL male, was given his acoustic tag (below) and in return, he snapped both trim tabs on the boat in thanks for our efforts! The following two days were spent running between Bench Island and Edwards Island trying to dodge rising swells, icy winds and horizontal rain! The day before my arrival a timid 3.2m TL female was observed keeping distance as larger sharks dominated the throw baits.

An Acoustic Tag is deployed on Bruce (4.0mTL)

Finally toward the end of the day, the timid female was coaxed close enough to be offered a bite of skipjack throw bait and be given an acoustic tag. At that point, her demeanour changed, and she was affectionately given the name ‘Tia.’ She was first on the scene at every location the following three days I was there, dispelling any theory that size is the main factor that dominates shark-to-shark behaviour. At 3.2m, she was far from the largest, but she dominated the throw baits over the larger sharks (except when Bruce arrived on the scene and the two of them kept other sharks at a distance as they joined forces, it seemed). Just as Ella and Miranda have done in previous years, Tia learned quickly to make vertical approaches on the bait (see above photo), and the only reason she was not successful in her attempts to steal meals was because she had not worked out that if she kept her back to the boat she was basically invisible to us until the last minute – and we could not retrieve the bait in time. As clear as the water is, the rock and weed covered bottom made the water appear almost black, and the sharks’ backs were as good as the same colour (see below). It’s little wonder seals become prey.

A white shark passes under the boat, almost invisible

Four tags were deployed over the three days, three acoustic and the fourth a PAT tag. This seems – on the face of it – to be a pretty low success rate, considering how busy the water was around us with sharks (in fact, in the days after I left they recorded a maximum of seven around the boat on one afternoon); however, as I mentioned, it’s one thing to attract the sharks to the boat, it’s quite another to encourage them into a position where deploying a tag is possible. Furthermore, and as I observed firsthand on all three days, it is the more dominant sharks that tend to be tagged first as they are the ones close to the baits. The less dominant and/or more cautious animals keep a distance, and because of this they are far harder to tag.

‘Mini Phred’ makes a close pass at Kina Scollay’s pole-cam

Special thanks to:

Dr Malcolm Francis (NIWA)

Clinton Duffy (DoC)

Kina Scollay (Ocean Answers Ltd)

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