Magazine article Ecos

Tagging along with Sharks

Magazine article Ecos

Tagging along with Sharks

Article excerpt

Katherine Johnson describes the sophisticated surveillance devices used to expose the secret lives of sharks.

In blue waters off Western Australia's Ningaloo Reef, divers approach the broad head and speckled body of a whale shark, the world's largest living fish.

A large eye the size of a human fist follows the divers' movements as they pause a few metres from the shark's back. Minutes later, the job is done. The divers move away and the submarine-like mass glides past. Trailing above and behind its dorsal fin is a satellite tag.

As the 10-metre shark surfaces, the tag begins transmitting its position via satellite to a computer. For 13 days and 420 kilometres, the movements of the whale shark are no longer secret.

The shark travels south along the reef before turning north and moving offshore. Then the signal changes. The transmission becomes more regular and frequent, indicating the tag has come off the shark and is now drifting in currents. It is eventually recovered 800 km to the south.

Dr John Stevens of CSIRO Marine Research is not surprised by the outcome. `These are early days,' he says. `We are tracking the largest fish in the world with one of the most sophisticated tagging methods available. There are bound to be glitches.'

With further refinements to the tagging system, the researchers hope to learn about the behaviour and migratory patterns of whale sharks.

`As with other species we are studying, this will help us understand the extent of their population, and help determine their vulnerability to fishing and other pressures,' Stevens says. `Ultimately the goal is better conservation and management.'

Flaked out

Another species of shark being tracked by CSIRO marine researchers is the school shark, the ubiquitous flake sold in fish and chip shops of southern Australia.

School sharks are fished commercially in Australia's south eastern waters and make up about 30% of the value of the southern shark fishery, worth $15 million each year. But the species is considered to be overfished and management plans are in place to rebuild the population.

To learn about the travels of this species, researchers are using a data-logging tag that records depth, water temperature and light levels every four minutes for up to two years. The sharks are tagged and released by researchers aboard commercial fishing boats. When the shark is recaptured by fishers, the tags are sent back to CSIRO for analysis.

The research aims to understand the detailed movement patterns of school sharks. It is funded jointly by the Fisheries Research and Development Corporation and CSIRO, and is assisted by the fishing industry.

Barely the size of a cigarette lighter, these tags are testing the limits of electronics creativity. They must operate to depths of at least 800 m, detect low levels of light, and store large amounts of data for up to several years.

So far, 30 school sharks have been tagged with data-logging (or archival) tags in South Australian waters. Researchers can calculate the shark's daily position from patterns of light levels recorded and stored in the tags. The record extends from when the shark is tagged to its recapture, providing it remains at depths where the light signal is sufficiently strong.

`Without this technology, we simply cannot get the information we need on depth, behaviour and movement,' Stevens says.

`While we have known that school sharks can travel between Australia and New Zealand from the earlier tagging work using simple tags with an identification number and return address, we can now find out what happens in between the points of release and recapture.'

Last year, two school sharks fitted with archival tags were released within 10 minutes of each other in South Australia and were recaptured more than 100 nautical miles away.

The first shark, a female, was recaptured only seven days after its release in Victoria having travelled 150 nautical miles west. …

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