49 



Abstract — Skipjack (Katsuwonus 

 pelamis), yellowfin (Thunnus alba- 

 cares), and bigeye iThunnus 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 fish- 

 ing-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 fish- 

 ing mortality is greater than that 

 necessary to produce the maximum 

 sustainable yield of this species. We 

 investigated whether time-area clo- 

 sures have the potential to reduce 

 purse-seine bigeye catches with- 

 out 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 investi- 

 gate the possible benefits of time-area 

 closures. We estimated that the most 

 optimistic and operationally feasible 

 3-month closures, covering the equa- 

 torial region of the EPO during the 

 third quarter of the year, could reduce 

 bigeye catches by ll.S^r, while reduc- 

 ing 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. 



The potential use of time-area closures 



to reduce catches of bigeye tuna (Thunnus obesus) 



in the purse-seine fishery 



of the eastern Pacific Ocean 



Shelton J. Harley (contact author) 

 Jenny M. Suter 



Inter-Amencan Tropical Tuna Commission 



8604 La Jolla Shores Drive 



La Jolla, California, 92037-1508 



Present address lor S. J. Harley; Ministry of Fisheries 



PC Box 1020 

 Wellington, New Zealand 



Email address for S. I. Harley: harleysSfish govt.nz 



Manuscript submitted 2 April 2004 to 

 the Scientific Editor's Office. 



Manuscript approved for publication 

 5 April 2006 by the Scientific Editor. 



Fish. Bull. 105:49-61 (2007). 



The Inter-American Tropical Tuna 

 Commission (lATTC) 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 conser- 

 vation and management is accom- 

 plished by measures imposed by the 

 nations participating in the fishery 

 in response to recommendations by 

 the scientific staff of the lATTC. Cur- 

 rently, the lATTC has adopted two 

 measures to ensure the conservation 

 of bigeye tuna in the EPO (lATTCM: 

 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 clo- 

 sure," 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 man- 

 made fish-aggregating devices (Lenne- 

 rt-Cody and Hall, 2000). The predom- 

 inant species captured are skipjack 

 {Katsuwonus pelamis), bigeye {Thun- 

 nus 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 in- 

 crease in fishing mortality on juvenile 

 bigeye tuna (Maunder and Harley, 

 2002; Harley et al., 2005). 



The most recent bigeye tuna stock 

 assessment (lATTC, 2004) has in- 

 dicated that overall fishing effort 

 should be reduced by at least 38% to 

 allow the stock to produce the maxi- 

 mum sustainable yield (MSY). This 

 assessment is based on a single EPO 

 stock with no net migration between 

 the eastern and western Pacific; how- 

 ever, a "Pacific-wide" assessment has 

 provided a very similar picture of low 

 movement rates for bigeye tuna in the 

 EPO (Hampton et al.^). 



Since the expansion of the FOB 

 fishery, catches of bigeye tuna from 

 the purse-seine fishery have exceed- 

 ed those from the longline fishery in 

 some years (Table 1). The bigeye tuna 



' lATTC (Inter-American Tropical Tuna 

 Commission). 2003. Resolution on 

 the conservation of tuna in the eastern 

 Pacific Ocean, 3 p. Resolution C-03-12, 

 lATTC, 8604 La Jolla Shores Drive, La 

 Jolla, California 92037. 



- Hampton, J., P. Kleiber, Y. Takeuchi, H. 

 Kurota, and M. Maunder. 2003. Stock 

 assessment of bigeye tuna in the western 

 and central Pacific Ocean, with compari- 

 sons to the entire Pacific Ocean, 81 p. 

 SCTB16 BET-1. Sixteenth meeting of the 

 standing committee on tuna and billfish, 

 Mooloolaba, Queensland, Australia; 9-16 

 July 2003. 



