Our Changing World: Tracking Fiordland’s sharks

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Dripping on to the deck, clutching habitat mapping equipment, Eva Ramey has just hauled herself back onto the Department of Conservation's Southern Winds research vessel after a scientific dive.

It's gone well, everything to plan, but the dive did have a little bit of extra excitement for Eva.

"Next thing I know, right in front of my eyes, his eyes are staring into mine and off he goes over my head. And at first I squealed and then I burst out laughing," she says.

The creature holding her gaze: a large broadnose sevengill shark.

Shark encounters

It's not the first time Eva has dived with sharks, and not the first or last time she'll interact with a sevengill either.

"They're very curious and they get close to divers," she says. This particular dive was part of a habitat mapping project, figuring out what lives where in these very special Fiordland ecosystems. The team come across sevengill sharks frequently, as do other divers and fishers in the area.

These encounters sparked the idea for a research project tracking sevengill sharks, says Dr Alice Rogers. She had heard reports of changes in the environment from the Southern Fiordland Initiative, raising questions about how the sharks - important apex predators in Fiordland's marine ecosystem - are faring.

"More recently there's been a bunch of marine heat waves and big flood events and these kind of extreme weather events that are becoming more common in this area… We really don't have much of an idea how those kinds of events might affect sharks."

First, they have to figure out how the sharks are using the fiord ecosystems - where are they hanging out?

How to spy on sharks

Sharks don't need to surface to breathe and they can spend all their time below water. So how do you keep track of them?

The answer is acoustic tags.

By setting up a grid network of acoustic receivers attached to the sea floor, and by performing a small surgery to insert a tag into the shark, they can track their movements throughout the fiords.

Every time a shark swims by a receiver - within about 400 metres - their tag will ping. The receiver will record who's passed by, as well as the shark's cruising depth and water temperature.

"So we kind of put the receivers out, tag as many sharks as we can, and then we wait," says Alice.

So far, the team, involving researchers from NIWA and the University of the Sunshine Coast, have set up a network of 29 receivers, mostly in Breaksea and Dusky Sounds, and have tagged 20 sevengill sharks.

Many of the receivers are arranged in pairs, to act like a 'gate' to ID the sharks as they swim by, entering a different part of the sound. There are also receivers at the exit, to track when the sharks leave the fiords for the open ocean. But if they leave, the researchers can no longer track them.

This is where Alice hopes three satellite tags they've deployed might help. Attached to the shark's dorsal fin "a bit like an earring", these tags continuously record the depth and location of the shark and are programmed to pop off the shark in a year's time. Once they reach the surface, they will transmit some data to passing satellites, but Alice is hoping to retrieve the tags themselves, to download the whole set.

Next steps

The project is supported by the Save our Seas Foundation, which has just approved funding for a second leg, including expanding tagging to school sharks and spiny dogfish.

As part of her PhD, Eva will use the information from the three different shark species to understand their interactions, and to identify areas within the fiords that might be important for the sharks as nursing or feeding grounds. She will couple the movement data with habitat mapping and baited underwater video that they have used to identify what sharks and fish are in the area.

Alice will return to Fiordland in May to tag these smaller sharks, plus to download the first year of sevengill data from the acoustic receivers.

From there they'll start to build a map, unravelling the secret life of Fiordland's sharks.

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