SCIENTIFIC COMMITTEE SEVENTH REGULAR SESSION 9-17 August 2011 Pohnpei, Federated States of Micronesia Prospects for effective conservation of bigeye tuna stocks in the Western Central Pacific Ocean WCPFC-SC7-2011/MI-WP-05

The Western and Central Pacific Fisheries Commission (WCPFC) agreed to close certain high-seas areas in the Western and Central Pacific Ocean to purse seine tuna fishing starting in 2010. These measures have potential economic benefits to the countries surrounding the closed areas and may also have potential stock conservation benefits for tunas. We used a spatially explicit ecosystem model of tuna population dynamics, SEAPODYM, to simulate the effects of closures on stock biomass and catch of bigeye tuna (Thunnus obesus) 1980 through 2003. The fate of the fishing effort displaced by these closures was not considered in the WCPFC conservation measures. Therefore we examine two different effort displacement scenarios: (1) complete loss of the displaced fishing effort; and (2) redistribution of effort proportional to the historical (average) distribution of catch per unit effort (CPUE). When fishing effort is redeployed, the benefits to the stock are not detectable. The beneficial effect on stock biomass is greatest when the displaced fishing effort was completely lost. However, even in this latter case, the effects of the closures on stock size are quite small (less than 4 % averaged over the simulation period). In view of the limited stock conservation benefits of the closures, we also considered other potential bigeye conservation measures. If spatial closures are extended to longline fisheries, the biomass increase becomes greater (approximately 7%). Prohibition of the use of fish aggregating devices by the purse seine fleet produces a similar biomass increase. We conclude that: 1.Closing areas to purse seine fishing without consideration of the fate of displaced fishing effort will not be effective for bigeye conservation. 2. Conservation measures that combine closing areas to purse seine fishing and proportional reduction of fishing effort may yield a small bigeye conservation benefit. 3. Restricting longline fishing in known bigeye spawning areas in combination with purse seine area closures and effort reduction offers the best option for achieving effective bigeye conservation. 4. Limitation of FADs use would have a strong positive impact on bigeye stock conservation. 5. Benefits from any bigeye conservation measure will only be detectable after 10 years and be fully realized after two decades, i.e. in the 2030s assuming timely implementation. Recovery will be modulated by both natural and anthropogenic climate-related ecosystem variability. Environmental changes induced by anthropogenic release of greenhouse gases should be clearly visible by the end of the 2030's (Lehodey et al 2010b). The status of bigeye …