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

Fishery Bulletin, Jan, 2007 by Shelton J. Harley, Jenny M. Suter

Abstract--Skipjack (Katsuwonus pelamis), yellowfin (Thunnus albacares), and bigeye (Thunnus obesus) tunas are caught by purse-seine vessels in the eastern Pacific Ocean (EPO). Although there is no evidence to indicate that current levels of fishing-induced mortality will affect the sustainability of skipjack or yellowfin tunas, fishing mortality on juvenile (younger than 5 years of age) bigeye tuna has increased, and overall fishing mortality is greater than that necessary to produce the maximum sustainable yield of this species. We investigated whether time-area closures have the potential to reduce purse-seine bigeye catches without significantly reducing skipjack catches. Using catch and effort data for 1995-2002, we identified regions where the ratio of bigeye to skipjack tuna catches was high and applied simple closed-area models to investigate the possible benefits of time-area closures. We estimated that the most optimistic and operationally feasible 3-month closures, covering the equatorial region of the EPO during the third quarter of the year, could reduce bigeye catches by 11.5%, while reducing skipjack tuna catches by 4.3%. Because this level of bigeye tuna catch reduction is insufficient to address sustainability concerns, and larger and longer closures would reduce catches of this species signficantly, we recommend that future research be directed toward gear technology solutions because these have been successful in many other fisheries. In particular, because over 50% of purse-seine catches of bigeye tuna are taken in sets in which bigeye tuna are the dominant species, methods to allow the determination of the species composition of aggregations around floating objects may be important.

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The Inter-American Tropical Tuna Commission (IATTC) was established by an international convention in 1950 and is responsible for the conservation of tunas and management of fisheries for tunas and other species taken by tuna-fishing vessels in the eastern Pacific Ocean (EPO). Such conservation and management is accomplished by measures imposed by the nations participating in the fishery in response to recommendations by the scientific staff of the IATTC. Currently, the IATTC has adopted two measures to ensure the conservation of bigeye tuna in the EPO (IATTC (1)): catch limits for each longline fleet (based on their 2001 catch levels) and a series of closures for the purse-seine fleet. In this article, we examine the use of the temporary closure of a given area, referred to as a "time-area closure," for management of the purse-seine fishery.

Since the early 1990s, considerable purse-seine fishing effort in the EPO has been directed at tunas associated with floating objects, including manmade fish-aggregating devices (Lennert-Cody and Hall, 2000). The predominant species captured are skipjack (Katsuwonus pelamis), bigeye (Thunnus obesus), and yellowfin (Thunnus albacares) tunas. The floating-object (FOB) fishery has had no noticeable affect on skipjack tuna abundance (Maunder, 2002a) and little effect on yellowfin tuna because the catches of yellowfin tuna from the floating object fishery are small compared to the catches from other purse-seine fisheries (Maunder, 2002b). However, the FOB has led to a considerable increase in fishing mortality on juvenile bigeye tuna (Maunder and Harley, 2002; Harley et al., 2005).

The most recent bigeye tuna stock assessment (IATTC, 2004) has indicated that overall fishing effort should be reduced by at least 38% to allow the stock to produce the maximum sustainable yield (MSY). This assessment is based on a single EPO stock with no net migration between the eastern and western Pacific; however, a "Pacific-wide" assessment has provided a very similar picture of low movement rates for bigeye tuna in the EPO (Hampton et al. (2)).

Since the expansion of the FOB fishery, catches of bigeye tuna from the purse-seine fishery have exceeded those from the longline fishery in some years (Table 1). The bigeye tuna caught in the longline 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 fishing 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 by-catch 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 skipjack 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 fishery (Goodyear, 1999), and protect biodiversity hotspots (Worm et al., 2003). Although time-area closures are not particularly appropriate for fisheries managed under 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.

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

Fishery Bulletin, Jan, 2007 by Shelton J. Harley, Jenny M. Suter

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