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Marine zoning in the Pacific Ocean, in combination with other measures, could significantly improve numbers of heavily overfished bigeye tuna and improve local economies, a fish modelling study has found. Scientists have found that a network of marine zones in the Pacific Ocean could be a more effective conservation measure than simply closing relatively small areas to some types of fishing. These marine zones, where different fishing activities are allowed in different areas, may have significant and widespread benefits for bigeye tuna numbers. Dr John Sibert of the university of Hawaii Joint Institute of Marine and Atmospheric Research is one of four scientists leading the study. After testing the effectiveness of a range of conservation measures with an ecosystem and fish population model.
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Longer-lived than the closely related yellowfin tuna, the bigeye has a lifespan of up to 12 years, with sexual maturity at age four. Spawning takes place in June and July in the northwestern tropical Atlantic, and in January and February in the Gulf of Guinea, which is the only known Atlantic nursery area.
Bigeye tuna are amongst the tuna species most threatened by overfishing. Juvenile bigeye tuna associate closely with floating objects such as logs, buoys and other flotsam, which makes them susceptible to purse seine fishing in conjunction with man-made fish aggregation devices. The removal of large numbers of juvenile bigeye, before they reach breeding age, is a major concern to fisheries managers, scientists and sport fishermen. Most seafood sustainability guides encourage consumption of other types of tuna.
Dr Sibert says the team found that the most effective measures were to restrict longline fishing in tuna-spawning areas and to manage the use of fish-aggregating devices (e.g. moored or drifting buoys which attract fish) in purse-seine areas.
“We found that simply closing areas off to fishing doesn’t work, because the boats just move their operations to neighbouring zones and fish even harder. It’s going to need a combination of approaches,” he says.
“The model will help people evaluate alternative policies to manage tropical tuna fisheries. Our predictions can help countries estimate how effective conservation measures might be, relative to any economic effects, and tailor measures to suit their goals. The advantage of this approach is that effects can be estimated locally, as well as for the stock as a whole.”
Half the current bigeye tuna catch is by longline, which targets high-value tuna sold as fresh fish. These fish command a market premium and sell for over $10 per kilogram.
The other half is caught in purse-seine nets as incidental bycatch when aiming to catch skipjack tuna. These juvenile bigeye tuna are sold to the canning industry for $1.70 per kilogram.
Dr. Silbert says the most effective conservation measures are those “which protect fish throughout their lifetime.”
For further information see Zones.
Tuna image via Wikipedia.
