SHARK TRACK

Tracking Porirua's rig sharks

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Science

Tracking rig sharks in Porirua Harbour

I'm using a new tracking technique to expose the secrets of rig…

Inventing a new tracking method

Because it’s quite shallow in the estuary I can tie a small, temporary float to a shark using nylon cord. The float stays on the surface and is easily towed along by the rig shark.

Inside the float is a device I developed with Peter De Joux at NIWA. It transmits GPS data to a mesh network of routers, and a computer collects the shark locations instantly.

The trackers constantly transmit their position - this lets me build a detailed map of real-time shark locations and because the information is accurate I can tell what habitat the shark is in and what they are probably feeding on.

Shark positions on phone map
I can use real-time shark positions to find and track rig sharks

Our new tracking system is a lot more accurate than the standard systems of tracking bottom-dwelling sharks for long periods. Instead of identifying an area hundreds of metres across our system is accurate to around 10 meters.

When a shark is about to leave the harbour I get an email from the system and go out to remove the tracker. The nylon is thin enough to break if it gets snagged. I even get an alert when the rechargeable batteries in the tracker are running low so I can replace them.

A shark tracking maps tells a story

Shark tracks
I can monitor where rig sharks go at different times of day and tide levels.

Now I can see where the sharks are at what time and date I can start to answer questions about their behavioural patterns:

  • How do rig sharks behave at night compared to day?
  • How do rig sharks behave at high tide compared to low tide?
  • What do rig sharks choose to feed on and when?
  • Where and when do rig sharks mate?
  • Do weather events and land water run-off affect rig behaviour?

As the data comes in I hope to identify rig shark behaviour so far unknown to science. This will help us understand them better and can be useful in many ways, and it will likely assist other studies that can utilise the findings and methods.

Results so far

It’s early in my research but I’ve already found some surprising results - and I’ve got more questions than I started with! It’s very exciting.

Latest blog post

Further research

  • As part of my PhD I also tag each rig shark with a small identification number so that when a shark is found, anywhere around New Zealand, the person who fished or found it can report the ID number. This adds to a bank of tracking data which helps scientists understand rig behaviour and estimate population numbers.
  • Later in my research I plan to monitor the way rig move their bodies as they feed, mate and travel using Inertial Measurement Units (IMUs). This could allow me to form a more detailed picture of rig behaviour beyond their location.
  • Rig belong to a group of sharks (elasmobranchs) with electric sense, meaning they can detect very small changes in electric fields. I will use a transplant and monitor technique to test if rig sharks use the earth’s magnetic fields to navigate, as some other elasmobranches do. 

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