Harley and Suter; Potential use of time-area closures to reduce the catches of Thunnus obesus in eastern Pacific Ocean 



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Figure 1 



Average annual distribution of the purse-seine catches of bigeye tuna [Thunnus obesus), by set type and 5-degree 

 latitude by 5-degree longitude area, in the eastern Pacific Ocean, 1995-2002. The size of the circles is proportional 

 to the catch in each area. 



(the fisheries have limited spatial overlap), and restric- 

 tions on dolphin mortality limits. In addition, only a 

 small proportion (about 10%) of the purse-seine catches 

 of yellowfin tuna are taken in floating-object sets. Later 

 we discuss extensions to our analysis to include not only 

 yellowfin tuna, but a range of bycatch species taken in 

 the different purse-seine fisheries. 



Materials and methods 



Data 



We used set -by-set catch and effort data from purse-seine 

 vessels that operate in the EPO. The majority of the data 

 was obtained by scientific observers. In the absence of 

 observer data, we used records from the logbooks of the 

 vessels. Data were grouped by 5-degree latitude by 5- 

 degree longitude areas (hereon referred to as 5°x5°areas) 

 by seasonal quarter. The FOB fishery, which is respon- 



sible for over 90% of the purse-seine catches of bigeye 

 tuna, was in an expansion phase during 1992-94; there- 

 fore we restricted our attention to data for 1995-2002 

 (Table 1). Because very small amounts of bigeye and 

 skipjack tuna are caught in dolphin-associated (DOL) 

 sets, we excluded these from the analysis and instead 

 focused on sets of tuna associated with floating objects 

 and sets on schools not associated (UNA) with dolphins. 

 For 1995-2002, these two set types were responsible for 

 over 99% of bigeye and skipjack tuna catches from the 

 purse-seine fishery (lATTC, 2004). These two set types 

 were combined in the closed-area model because it was 

 possible to switch effort between those two types of sets. 

 The spatial distribution of catches by set type for bigeye 

 and skipjack tuna are provided in Figures 1 and 2. 



Definition of "hotspot" 



In defining the spatial and temporal extent of the bigeye 

 catches, we looked for areas where the ratio of bigeye to 



