The potential use of time-area closures to reduce catches of bigeye tuna in the purse-seine fishery of the eastern Pacific Ocean

In this study, we investigated the potential of time-area closures to reduce bigeye tuna catches while minimizing 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 closures 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 skipjack 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 associated 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 (the fisheries have limited spatial overlap), and restrictions 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 5degree longitude areas (hereon referred to as 5 [degrees] x 5 [degrees] areas) by seasonal quarter. The FOB fishery, which is responsible for over 90% of the purse-seine catches of bigeye tuna, was in an expansion phase during 1992-94; therefore 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 (IATTC, 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.