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New GPS tracker opens doors for marine research

14 October 2009, by Sara Coelho

For the first time scientists have managed to track a large oceanic fish using a GPS device able to relay highly accurate positioning data right to the researcher's laptop. This breakthrough will open many research possibilities with implications for conservation and fishery management.

Ocean sunfish

The ocean sunfish (Mola mola).

The device records the precise location of Global Positioning System (GPS) satellites in the sky every time the antenna surfaces. The raw data is sent by satellite to the scientists, who can then calculate the exact location of each recording. Unlike previous methods, the scientists don't need to retrieve the original tag.

'This is a most exciting technological development,' says Professor David Sims, from the Marine Biological Association, who works on the ecology and migration behaviour of marine predators. 'GPS tracking will open a huge research field and it will allow us to learn many new things about how fishes move in the oceans.'

Understanding how fishes travel around, their migratory routes and how much time they spend in different places is important for conservation efforts. 'We can't manage fisheries properly if we don't know where commercial species like tuna or sharks are spending their time,' Sims says.

Sims and colleagues tested the device on ocean sunfish, large pancake-shaped creatures that can reach 2.5 metres in length and feeds mostly on jellyfish. Ocean sunfish spend a lot of time near the surface, although they can dive up to 600 metres deep.

The team attached tracking devices to three ocean sunfish caught in the Gulf of Cadiz. Two of them stopped transmitting data shortly afterwards, but the third device remained active for nearly three months, providing a wealth of information about the sunfish's comings and goings.

The data revealed that the fish travelled south towards warmer waters as winter progressed, just as expected based on what is known about ocean sunfish seasonal migration. But the GPS data highlighted a previously unknown behaviour. The route south is not straightforward - the sunfish stops frequently along the way, presumably to rest and feed on patches of jellyfish.

Sims compares this 'stopover' behaviour to what is seen in long-distance migratory birds, which stop a few days from time to time to recover their strength. 'We would never have learned about this pattern of movement with old tracking methods for fish,' he says.

Still, there is room for improvement. The experiment reported last week in the journal PLoS ONE used a 14 centimetre-long device, attached to the fish by a 1.5 metre long line. Sims predicts that the technology will get smaller and better and that it will be possible to attach small devices directly to fish's fins.

With time 'we'll be able to track smaller fish species, or even juveniles,' he says. For instance, 'we still don't know where juvenile sharks are for most of the time and understanding their behaviour is important for their conservation given the global declines in many pelagic species that are occurring.'

Sims DW, Queiroz N, Humphries NE, Lima FP, Hays GC (2009) Long-Term GPS Tracking of Ocean Sunfish Mola mola Offers a New Direction in Fish
Monitoring. PLoS ONE 4(10): e7351. doi:10.1371/journal.pone.0007351

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