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

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, resulting 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 aggregations 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 avoidance 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 differential behavior of cod (Gadus morhua) and haddock (Melanogrammus aeglefinus), it was found that it was possible 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 (3); IATTC (4)). Unless studies of bigeye and skipjack tuna behavior 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 reducing skipjack tuna catches.

Lennert-Cody and Hall (2000) used a range of statistical models to determine factors (e.g., area, season, characteristics of the floating object and the purse-seine net) that were associated with higher catches of bigeye and skipjack tuna. Unfortunately, many factors were confounded 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 technical measures with the potential to reduce catches of bigeye tuna.

Although it may be difficult to determine important factors relating to bigeye tuna catch rates, fisheries data can be used to examine the nature of the catches 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 5% 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 unassociated with dolphins (IATTC, 2004).