58 



Fishery Bulletin 105(1) 



In the remainder of this article we discuss our find- 

 ings in terms of the recent stock assessment recommen- 

 dations for reductions in fishing effort — more specifi- 

 cally whether reductions predicted in our study would 

 be sufficient to reach management objectives. We also 

 discuss alternative measures for reducing bigeye tuna 

 catches in the purse-seine fisheries of the EPO and de- 

 scribe potential improvements for our time-area closure 

 modeling approach that may lead to a more accurate 

 analysis of the likely performance of closures that may 

 be considered in the future. 



Predicting performance of time-area closures 



Following Hall (1996), we looked for time-area strata 

 in which there were high bigeye tuna to skipjack tuna 

 ratios. These areas were relatively confined geograph- 

 ically and did not vary greatly by quarter. For this 

 reason, the hotspot and practical closures predicted 

 similar results. 



Simulation of a practical closure (and one that able 

 to be implemented) indicated that moderate average re- 

 ductions in bigeye tuna catch (11.5%) could be achieved 

 with lesser average reductions in skipjack tuna catches 

 (4.9%). When we considered these reductions in terms 

 of total catch by weight, the annual bigeye tuna catch 

 reductions ranged up to 20,206 t (average 5722 t) and 

 up to 32,773 t (average 6,807 t) for skipjack tuna. 



Based on the current mix of fishing gears in the 

 bigeye tuna fisheries in the EPO, and the estimated 

 maximum sustainable yield (MSY) of about 77,000 t 

 (lATTC, 2004), the purse-seine share of the MSY was 

 around 40,000 t (S. J. Harley, unpubl. data). Consider- 

 ing current purse-seine catches of over 60,000 t, and 

 the 11.5% reduction predicted for the practical closure, 

 we believe that these closures alone are unlikely to 

 yield the required reductions in bigeye tuna catches 

 from the purse-seine fishery. 



The closures investigated in our study were based on 

 strata where the ratio of bigeye tuna to skipjack tuna 



catches was the greatest. For these closures, the reduc- 

 tion in catches (in metric tons) is about the same for 

 bigeye and skipjack tuna, but if a closure is larger or 

 longer, the losses in skipjack catches would quickly out- 

 weigh the reductions in bigeye tuna catches. Therefore, 

 although we did not examine larger or longer closures 

 in our study, it is unlikely that these closures could 

 lead to the necessary reductions in bigeye tuna catches 

 without unacceptable losses in skipjack tuna catches. 



The lack of effectiveness of the time-area closures is 

 related to the extent of the interaction between skip- 

 jack and bigeye tunas. For the 1995-2002 period, 94% 

 of the bigeye tuna caught by purse-seiners was taken 

 in sets that also caught skipjack tuna (Table 3). This 

 percentage is greater than the proportion of skipjack 

 tuna catch that was taken in association with bigeye 

 tuna (68%). Given this fact, it is not surprising that 

 time-area closures are insufficient. 



Management alternatives to reduce catches 

 of bigeye tuna 



We have shown that time-area closures alone are unlikely 

 to result in the necessary reductions in fishing mortality 

 for bigeye tuna; therefore alternative or supplementary 

 management actions would be appropriate. In many 

 instances, studies of fish behavior (Wardle, 1983) and 

 gear technology (Larsen and Isaksen, 1993) have led to 

 changes in gear configurations and deployment, result- 

 ing in significant reductions of catches of unwanted 

 species. A good example of this type of change is the 

 reduction of dolphin catch from tuna-dolphin aggrega- 

 tions in the EPO (NRC, 1997). 



In the 1970s, many thousands of dolphins (mostly 

 Stenella sp. and Delphinus sp.) were caught and killed 

 by purse-seine vessels that set on dolphins in order to 

 catch the yellowfin tuna that were associated with them 

 (NRC, 1997). Through the introduction of fine-mesh net 

 panels, use of a "back-down" procedure, and the avoid- 

 ance of areas where oceanographic conditions could lead 

 to net collapse, this mortality was reduced dramatically 

 by the 1990s (NRC, 1997). 



It is also possible to exploit behavioral differences 

 among fish species. Through examination of the differ- 

 ential behavior of cod (Gadus morhua) and haddock (Me- 

 lanogrammus aeglefinus], it was found that it was pos- 

 sible to configure bottom trawl nets to catch the target 

 species and allow the other species to escape through 

 larger meshes (Cotter et al., 1997). Sorting grids have 

 also been used to allow the escape of unwanted species 

 (Larsen and Isaksen, 1993; Misund and Beltestad^; 

 lATTC^). Unless studies of bigeye and skipjack tuna be- 



3 Misund, O. A., and A. K. Beltestad. 1994. Size-selection of 

 mackerel and saithe in purse seine. International Council for 

 the Exploration of the Sea Council Meeting, 1994/B:28. 



^ lATTC (Inter-American Tropical Tuna Commission). 

 1999. Report of the bycatch working group, 25 p. 63rd 

 Meeting of the lATTC; June 8-10, 1999. lATTC, 8604 La 

 JoUa Shore Drive, La Jolla, California 92037. 



