By Alison Ballance
The rig shark (Mustelus lenticulatus) is also known as spotted dogfish, after the golden 'bambi'-like spots on its back. It feeds on crabs it finds in the mud on the bottom of estuaries, and has grinding plates rather than sharp teeth. Photo: Malcolm Francis / Coastal Fishes of NZ
Shark researcher Warrick Lyon holds a small rig shark. This shark was too small for the GPS tracking system, but has been tagged with a numbered fishery tag. Photo: RNZ / Alison Ballance
Tracking rig sharks in Pauatahanui Inlet is proving to be a difficult job at the best of times for shark researcher Warrick Lyon, let alone when the electronic tags keep getting ‘stolen’ off the sharks. I joined Warrick on a recent research trip into the estuary, and although it was only 8am he had already had an eventful morning.
“A fisherman caught my shark!” he told me as he picked me up from the boat ramp. “It was towing a GPS transmitter that tells me the location of the shark; the transmitter floats on the surface and there’s a 6-metre long nylon tether down to the shark. So the shark swims along the seafloor and that’s where they like to be, and the tag floats on the surface, and it was the tether between the two that the fisherman caught.”
To make matters worse, Warrick was actually talking to the fishers at the time!
As he describes in his Shark Tracker blog, the chances of this happening are incredibly small: ‘I’m guessing that there are between several hundred to several thousand rig sharks in Porirua Harbour during the spring and summer, so what are the chances that the ONE I have tagged gets caught? I’ll do some simple math for you it’s between 0.00002% and 0.000005%, that’s a pretty low. What made it even more unlikely was that I was talking to the fishers at the time, telling them about my research, and if they ever caught one of my tagged sharks could they please return the tag to me. Then one of them caught my rig shark.’
Warrick Lyon has developed a GPS tracking system that uses a floating tag, built into the empty housing of a torch, which is connected to the rig shark by a 6-metre long nylon tether. Photo: RNZ / Alison Ballance
Warrick is a technician at NIWA and is carrying out research into rig sharks for a PhD at the University of Auckland. The first stage of his research is to find out how adult rig use Pauatahanui Inlet during the warmer summer months when they use the estuary as a breeding ground. To do this he has worked with NIWA’s Peter De Joux to develop an innovative electronic tracking system, which uses GPS and allows him to track individual sharks in real time. A system of routers around the estuary act as a smart network that transmit the location of the tag to a base computer sited in Mana, overlooking the estuary. GPS cannot transmit through water, yet rig sharks spend their lives on the bottom of the estuary looking for the mud crabs that make up most of their diet. The solution was to allow the tag to float on the surface. The tag transmits every 30-seconds, which provides a very detailed picture of what the sharks are doing.
This track covers two days, and shows a male rig shark moving around Pauatahanui Inlet. The track ended when a fisher snagged the GPS tag's tether causing it to break. Photo: Warrick Lyon
Warrick Lyon's GPS tracking system for rig shark (right, a small shark being released into the water) in Pauatahanui Inlet relies on a network of routers (left) positioned around the inlet to send GPS locations to a central computer. Photo: RNZ / Alison Ballance
Warrick’s research builds on previous work which has investigated the importance of Pauatahanui Inlet as a rig breeding ground. As part of this research NIWA’s Malcolm Francis used acoustic tags to study the broad-scale movement of juvenile rig in the estuary.
Rig is also known as spotted dogfish or spotted smoothhound. It is called gummy shark in Australia, and is commonly used in fish and chips under the name lemonfish.
Our Changing World has previously joined Malcolm Francis as he took genetic samples from rig in Pauatahanui Inlet to find out how the species is related to Australian gummy sharks.
You can follow Warrick's research on his blog, Facebook page or on Twitter.
