[ASC-media] Media release: Scientists discover marlin breeding grounds

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Mon Mar 12 23:12:25 CET 2012

:: Marine Conservation Science Institute
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Scientists discover marlin breeding grounds

March 13, 2012 – for immediate release

Every year, giant black marlin head from all corners of the Coral Sea to spawn in a small area adjacent to Australia’s Great Barrier Reef (GBR).

In a remarkable piece of field science, researchers from the US and Australia have identified the area as a critical breeding ground, essential to the survival of billfish from as far afield as the waters off Papua New Guinea, the Solomon Islands, Micronesia, New Caledonia, Kiribati, Vanuatu, Fiji, Tuvalu and Nauru.

A paper recently published in the open access journal, PLoS One, presents conclusive evidence that black marlin from all corners of the Coral Sea are aggregating once a year close to the GBR to spawn.

This annual gathering of black marlin has long been known to recreational anglers, who converge on the Great Barrier Reef (GBR) from around the globe for a shot at catching one of the largest fish in the sea. 

Between October and December, game boats criss-cross the reef front in hope of hooking a giant black marlin. These can reach 700 kg in weight and achieve speeds near 130 km/h, making them one of the ocean's most prized game fish. Although the game boats have killed many black marlin in the past, today’s recreational fishing ethics result in the release of the vast majority of the fish they catch. In fact, the scientists took advantage of this catch-and-release practice to assist their research.

Dr. Michael Domeier, from the Marine Conservation Science Institute and Australian colleague Peter Speare collected eggs and larvae (baby marlin) to confirm that black marlin are indeed spawning during their annual visit to the reef. Then they used satellite pop-up tags to determine where the marlin go after they leave the spawning grounds.  

They concluded that the fish that spawn off Australia are primarily residents of the Coral Sea, with no individuals coming from other known black marlin hotspots, like the Indian Ocean or South China Sea. 

Why the giant fish travel so far to spawn in this one location was the big puzzle. “The habitat inside the GBR may hold the key. Juvenile black marlin can be found inside the GBR lagoon year round, indicating that it is a prime nursery area,” the researchers say. 

“The lagoon likely has just the right combination of water temperature and prey availability to optimize the survival of the young marlin. Marlin are voracious predators right from hatching, and the ample supply of spawning coral reef fishes provides a rich diet of tiny fish for the marlin to eat,” Dr Domeier adds.

After spawning, the adult black marlin were tracked with satellite tags throughout the Coral Sea, ending up as far away as Micronesia, New Caledonia, Fiji and Vanuatu. 

Interestingly, it appears that black marlin return to the same breeding grounds during the spawning season (October to December) each year, as evidenced by conventional tag data provided courtesy of New South Wales Department of Primary Industries.

Black marlin have been the target of industrial Japanese commercial  fisheries in the Coral Sea in the past, but protective measures put in place by the Australian government have likely prevented the decimation of this stock. The exact impact of the remaining fishing pressure, from game boats and the domestic long line fleet that targets tuna, is unknown.

“These latest findings regarding the regional population and breeding of black marlin should be taken into account in developing any new management measures for the Coral Sea Region,” the researchers conclude.

Their article “Dispersal of Adult Black Marlin (Istiompax indica) from a Great Barrier Reef Spawning Aggregation” by Michael L Domeier and Peter Speare is published in PloS One and can be found at http://dx.plos.org/10.1371/journal.pone.0031629

More information:
Dr. Michael Domeier, MCSI, ph +1 760 271 6609, ml.domeier at gmail.com
Mr. Peter Speare, AIMS, pspeare at aims.gov.au, ph +61 (0)747534222, a/h +61 (0)747726730

The proposed Coral Sea Commonwealth Marine Reserve released by the Australian Federal Minister for Sustainability, Environment, Water, Population and Communities Tony Burke on November 25, 2012, for public comment proposes a zoning scheme that removes commercial tuna and billfish fishing throughout three quarters of the reserve, with the southernmost area still open to long line fishing. The eastern half of the Coral Sea is zoned to remove game fishing, while the western half, next to the Great Barrier Reef, remains open to this type of fishing.

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