L'aventure de marque de thons de Giulia

Nouméa

Giulia Anderson & the crew of the Gutsy Lady 4 returned to Honolulu on Friday 02 October after a 7-week research expedition to monitor the health of the world's largest tuna fishery. Giulia had only just landed her dream job as a fisheries molecular geneticist with the Pacific Community (SPC) in New Caledonia, when she suddenly found herself on board a tagging expedition across the Western & Central Pacific. 

After 50 days at sea our expedition has been a success by any standard, having tagged 6387 tuna, a record for number of tags deployed on an SPC cruise in the area visited since the tagging programme began in 2006. We also collected biosamples from over 500 fish and genetic samples from 800 specimens.

Our expedition was an important one for the Pacific region. With most international tourism shut down due to Covid-19, many Pacific island countries and territories are more dependent than ever on revenue from the US$6bn fishing industry. For many, the revenue from tuna fishing in their waters of the western and central Pacific accounts for over half of their government income.

Since 2006 SPC has tagged almost 500,000 tuna, generating the most comprehensive data set for tuna science and management in the world. The tagging information is vital to show how fishing activities and climate change affect the health of fisheries. For example, recent studies warn climate change could induce an eastward redistribution of skipjack and yellowfin tuna away from established fishing areas towards cooler water in the high seas, potentially leading to a loss of revenue for Pacific island countries and territories in the horizon of 2050.

During our 7-week voyage across the Kiribati EEZ and the Phoenix Islands Marine Protected Area, our exact path zigzagged based on real-time information shared by the fishing sector about the locations where tuna numbers were expected to be high.

We employed two types of tagging, as opportunities arose. Conventional tagging involves attaching a thin plastic tag that enables us to relate the size and location of a fish when it is recaptured to that where it was released. Each month, tags are returned from tuna processing plants all over the world. 

Our electronic tags generate a much richer dataset, providing data that tracks a fish’s diving behaviour, lateral movements, and surrounding water temperature. Deploying an electronic tag requires surgically implanting a 7cm capsule of electronics in tuna that are at least 65 cm in length. These electronic tags have identified fish swimming thousands of miles, sometimes following a similar track repeatedly over seasons or years, only to be captured in roughly the same area as it was released. The data provided by archival tags can radically adjust our understanding of a species’ life history.

Throughout the expedition, we used two different fishing techniques known as dangling and jigging. Danglers are a great way to get a lot of conventional tags in the water quickly. The fishing style consists of coaxing a school of fish to the surface with chum and water sprayers that simulate the presence of prey. The vessel then dangles multiple lures with barbless hooks just at the surface of the water. As fish get caught on the lures, crew members haul them onboard and ferry them to an available tagging cradle.

Most of the time, this method was deployed at daybreak and attracts smaller fish that are naturally near the surface. Once or twice we have dangled schools of much larger fish. It is a glorious kind of chaos when 12 fish at 80+ cm length are hooked at the same time.

With jigging we dropped heavily weighted lures down between 20 and 60 fathoms (120 meters) or more using rod and reel. Jigging brings up one fish at a time and invests far more effort per tag than dangling. It was usually undertaken during darker hours to catch larger fish that rarely venture near the surface. Despite the reduced efficiency, jigging accounted for almost half of the fish we tagged - a testament to the hard work and enthusiasm of the crew. 

At the end of the voyage, having broken the tagging record for any one SPC expedition in the central Pacific, the final fishing session ended in whoops, hollers, and hugs. The captain blasted the boat’s horn and people variously jumped, backflipped, and were thrown in the water—the first swim of the entire cruise.

As we tallied up the number of fish tagged, there were some other telling numbers that told their own tale of our 7 weeks at sea that were gruelling, productive and fun in equal measure:

  • Distance travelled: 6061 nautical miles
  • Fuel burned: 80,000 L
  • Person hours fished: 935
  • Jigs bitten off by sharks and wahoo: 395
  • Fishing rods broken or lost: 3 (although one got a second shot at life as “custom-shortened”)
  • Number of other ships seen: 1
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Division
Pêches, Aquaculture et écosystèmes marins

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