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Fishery Bulletin 105(1) 



caught in the longhne fishery are larger (110-160 cm) and 

 considerably more valuable than the smaller bigeye tuna 

 (50-80 cm) caught mostly by the purse-seine fishery. 



Improving the long-term sustainability of the bigeye 

 tuna fisheries could be achieved by reducing the fish- 

 ing mortality of the smaller individuals that are caught 

 predominantly in the FOB fishery. Annual catches of 

 skipjack tuna from the purse-seine fishery in the EPO 

 are larger and more economically important than those 

 of bigeye tuna (Table 1). Furthermore, there are no 

 concerns regarding sustainability of the skipjack tuna 

 population in the EPO (Maunder, 2002a). Thus, bigeye 

 tuna caught by the FOB fishery are essentially bycatch 

 of the targeted skipjack tuna fishery; thus determining 

 a mechanism by which the catches of bigeye tuna are 

 reduced while minimizing losses in the catches of skip- 

 jack tuna is an important management issue. 



Hall (1996) argued that to understand and solve by- 

 catch problems it is important to classify the problem 

 by a number of factors (e.g., time, space, and the level 

 of control that fishermen have). With this information, 

 there are many potential tools that can be used by 

 fisheries managers to reduce fishing mortality, e.g., 

 gear regulations, catch limits, closed seasons, and 

 closed areas (Beverton and Holt, 1957). Time and area 

 closures (time-area closures) are recommended as a 

 means to reduce catches of sharks (Baum et al., 2003), 

 protect billfishes from exploitation by the longline fish- 

 ery (Goodyear, 1999), and protect biodiversity hotspots 

 (Worm et al., 2003). Although time-area closures are 

 not particularly appropriate for fisheries managed un- 

 der quota systems, they may be beneficial for effort- 

 managed fisheries (Horwood et al., 1998) or fisheries 



targeting multispecies (Hilborn et al., 2004), such as 

 those for tunas in the EPO. 



In this study, we investigated the potential of time- 

 area closures to reduce bigeye tuna catches while min- 

 imizing impacts on the catches of skipjack tuna. In 

 contrast to common closure-strategy studies, i.e., those 

 studies devoted to fisheries targeting a single species, 

 we investigated the potential impacts of time-area clo- 

 sures on two species: a large and highly productive 

 skipjack tuna stock, and a considerably smaller and less 

 productive bigeye tuna stock. 



We used catch and effort data from the purse-seine 

 fishery to search for potential time-area hotspots for 

 bigeye catches and then applied simple "in-sample" 

 closed-area models to predict the potential impact of 

 closures of these areas. We discuss the likely use of 

 such closures in the light of our findings, alternative 

 management actions that could possibly reduce bigeye 

 tuna catches, and finally, the strengths and weaknesses 

 of the approach used for the closed-area models. 



Although yellowfin tuna form an important part 

 of the purse-seine fishery in the EPO, where annual 

 catches are greater than those for bigeye and skip- 

 jack tuna combined, we did not consider them in our 

 analysis. Within the EPO purse-seine fishery there 

 are essentially two fleets: one targets yellowfin tuna 

 schools associated with dolphins or schools not associ- 

 ated dolphins and the other targets mainly skipjack 

 tuna associated with floating objects. In our study, we 

 focused on the second fleet and there are many reasons 

 to believe that effort could not be transferred from one 

 fleet to the other, e.g. markets, technological differences 

 (the vessels require different equipment), geographical 



