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



59 



havior determine a mechanism by which 

 bigeye, but not skipjack tuna, can escape 

 through a sorting grid in a purse-seine 

 net, sorting grids are more likely to be 

 useful for overall reductions in catches 

 of small tunas than as a mechanism for 

 reducing bigeye tuna catches without re- 

 ducing skipjack tuna catches. 



Lennert-Cody and Hall (2000) used a 

 range of statistical models to determine 

 factors (e.g., area, season, characteris- 

 tics of the floating object and the purse- 

 seine net) that were associated with high- 

 er catches of bigeye and skipjack tuna. 

 Unfortunately, many factors were con- 

 founded because the fishing practices of 

 the fleet often differ in time and space, 

 making it difficult to determine which 

 gear characteristics may be important. 

 Thus, it appears unlikely that analysis of 

 fishery-collected data will lead to techni- 

 cal measures with the potential to reduce 

 catches of bigeye tuna. 



Although it may be difficult to deter- 

 mine important factors relating to bigeye 

 tuna catch rates, fisheries data can be 

 used to examine the nature of the catch- 

 es of this species. For example, we found 

 that 94% of bigeye tuna are caught in 

 sets that also caught skipjack tuna. We were interested 

 in how the bigeye tuna catches were distributed; were 

 they predominantly from a small number of sets with 

 high catches or from a large number of sets with small 

 catches? Our analysis of this question, based on data 

 for 1995-2002, is presented in Figure 6. It shows than 

 only 57c of bigeye tuna were caught in single-species 

 sets, but that about 50% of bigeye tuna came from sets 

 that contained at least 60% of this species. These sets 

 are responsible for only 7% of the skipjack tuna catch 

 from the floating-object fishery and a smaller proportion 

 of the overall skipjack catch given that about 30% of 

 skipjack tuna catch is still taken from schools unas- 

 sociated with dolphins (lATTC, 2004). 



The analysis of the catch composition of purse-seine 

 sets described above indicates that if fishing captains 

 can determine, at least roughly, the species composition 

 of an aggregation prior to setting (i.e., which species is 

 dominant), large reductions in bigeye tuna catches could 

 be achieved by not setting on bigeye-tuna-dominated 

 aggregations. Such a measure would have little impact 

 on overall skipjack tuna catches and would not require 

 the fleet to be restricted in its activity by time-area 

 closures. 



Schaefer and Fuller (2005) used a range of electronic 

 tags, supplemented with sonar images of fish aggrega- 

 tions around floating objects, to describe differences in 

 the behavior of skipjack and bigeye tunas around float- 

 ing objects. Exploitation of these differences, combined 

 with the potential ability of fisherman to identify large 

 aggregations of bigeye tuna around floating objects, 



may lead to the development of fishing practices that 

 can reduce bigeye tuna catches with minimal impact 

 on skipjack tuna catches. 



Critical to this approach will be the establishment 

 of incentives 1) to encourage both the identification of 

 the schools and 2) not to set on bigeye-tuna-dominated 

 aggregations. Similar to the dolphin mortality limits 

 currently applied by the lATTC, it could also be possible 

 to have individual vessel limits for bigeye tuna and let 

 fishermen determine how best to modify their fishing 

 operations in order to achieve a given limit. Harley et al.^ 

 used historical catch-by-vessel data and found that indi- 

 vidual vessel limits of about 350 t would be sufficient to 

 reduce purse-seine catches of bigeye tuna in the EPO by 

 50% in most years. Independent studies of fish behavior, 

 coupled with experimental work investigating modifica- 

 tions in fishing practices and gear, could be fruitful. 



Modeling potential effects of time-area closures 



We applied simple closed-area models that used his- 

 torical catch and effort data. Several assumptions are 

 implicit in these models. First, we assumed that the 

 fishing fleet has the flexibility to reallocate effort out- 



Harley, S. J., P. K. Tomlinson, and J. M. Suter. 2004. Pos- 

 sible utility of catch limits for individual purse-seine vessels 

 to reduce fishing mortality on bigeye tuna in the eastern 

 Pacific Ocean, 8 p. Inter-American Tropical Tuna Commis- 

 sion, 5'*" working group on stock assessments, 11-13 May 

 2004, Document SAR-5-05 BET A. lATTC, 8604 La Jolla 

 Shore Drive, La Jolla, California 92037. 



