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How it works: the drumline setup consists of an anchor, two buoys and a satellite-linked GPS communications unit attached to a hook baited with one sea mullet. A triggering magnet is attached to the communications unit and when a shark takes the bait and puts pressure on the line, the magnet is released, alerting the DPI team that there is an animal on the line. Once alerted, a DPI contractor responds within 30 minutes to tag and release the shark ensuring minimal harm to the shark and an increase of the tagged shark population. Image: Contributed

A few questions about shark behaviour

Geoff Helisma

The Independent spoke with the NSW Department of Primary Industries’ principal research scientist, Paul Butcher.

GH: There was a two-year trial in Western Australia, a place notorious for its white shark attacks, and only two white sharks were caught on drumlines; is there a scientific explanation for why that would happen?

PB: Not really, only that we do know in WA that it is a completely different population to what we have on the east coast of Australia – so, two genetically distinct populations. Over in the west, you’re mainly looking at large white sharks; adults that are 3.5 to six metres in length; where the majority of what we see here are from 1.5 to nearly four metres in length, what we call juveniles, sub adults, basically teenagers, and this gear is showing that they can catch those animals.

GH: You release the sharks a kilometre offshore; do you have data that shows taking them one kilometre offshore is effective, given that a white shark can travel hundreds of kilometres in one day?

PB: It’s basically a fight and flight response. With our satellite tags that we put on the animals every time it comes to the surface, we get a GPS location. We did that with nearly 70 white sharks, as part of the original trials, and could show that they were 15 to 25 kilometres offshore the following day. They eventually come back to the coast. Obviously, we don’t want to … disrupt the total movements of the species. We do know that they come back into the coast in the weeks after the initial capture, and some recaptures we’ve had up along the coast, as well.

GH: So how does that boil down to having safer beaches?

PB: A two-pronged approach. I won’t say we’re moving [them]. We’re capturing animals that may interact with surfers or beach users at that point of time. We catch the animals; we know they have that flight response. They move offshore, so we’re removing that potential interaction with surfing groups at that time the animals are moving offshore. There’s no chance of interaction while those animals are offshore.