Satellite tracking works for mysterious black marlin
The black marlin, a fish that's notoriously difficult to study, has been successfully tracked using a new kind of pop-up satellite tagging technology.
A prototype Pop-up Satellite Tag, which is designed to detach itself from the fish after a pre-programmed period, popped up to the ocean surface off Cairns, northern Australia, last week -- just 40 nautical miles from where it was attached one year ago.
Its carrier was a female black marlin weighing more than 600 pounds, which was caught by an Australian tuna longliner, tagged, and released.
"This is great news because we now know that this cutting-edge technology works, and that the tags can stay on the fish for a long time, in this case a year," said CSIRO Marine Research scientist Mr John Gunn.
What scientists don't know is exactly where the marlin would have travelled during that time. The migration patterns of black marlin are a mystery. Some travel down the coast of Queensland after spawning in the Coral Sea between September and January. But is not known whether adults return to the same spawning ground each year, or whether there is any exchange of fish between the Indian and the Pacific Oceans.
Efforts to learn more using conventional tagging techniques have been unsuccessful. Conventional tagging relies on assistance from the fishing industry, and involves attaching identification tags to any black marlin captured before it is released, and then noting the numbers and location of any tagged fish re-caught.
According to Mr Gunn, less than one in a hundred black marlin tagged are ever re-caught -- a rate much lower than for other fish. The advantage of the new technology is that tagged fish can be tracked whether or not they are re-caught.
The new tag is attached just below the second dorsal fin with a small surgical steel anchor, and "looks like a big cigar with a float on one end and a little aerial," said Mr Gunn. It has a microprocessor that records time and temperature data, and after the pre-determined period the battery sends a signal to a corrodible pin. Within an hour the tag detaches and rises to the surface, where it signals its location via the Argos satellite system.
"There are a number of questions that only this technology can answer," said Mr Gunn. "At the moment, our population assessments are based on a number of assumptions. We don't even know how long black marlin live. If we can reduce some of the uncertainties, our population estimates will be much more accurate."
Black marlin are highly sought after by the game fishing industry. "They're unbelievable animals to see in the water," said Mr Gunn. "They're so beautiful -- they have this pale iridescent blue colour as they move." Commercial tuna boats also catch black marlin as bycatch, but in Australian waters they are required to return them to the water. In game fishing they may be landed for weighing.
Mr Gunn said the new pop-up tags could also be used on other fish species which travel long distances, such as the commercially important blue-fin tuna.
The research was a collaboration between CSIRO Marine Research, Julian Pepperall of Pepperell Research, and game fishing organisations.