An analysis of the possible utility of time-area closures to minimize billfish bycatch by U.S. pelagic longlines


Goodyear, C. Phillip
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This study examined the relative rates of billfish bycatch and target species catch by areas (1°, 2°, and 5° latitude and longitude) and months in the catch data reported in mandatory log books kept by U.S. pelagic-longline fishermen in order to identify potential time–area strata that could reduce billfish bycatch. The 1986–91 mean percentages identified month–area strata with high percentages of sailfish and marlin bycatch and marlin only bycatch. The analyses indicated that the elimination of effort in cells selected according to percentages of billfish in the catch could have reduced the 1986–91 billfish bycatch by 50% and the target species from 13.9 to 19.2%, depending on the spatial resolution employed. The corresponding analysis of marlin only indicated a 50% reduction in marlin bycatch could have been attained and a 16.4–20.7% reduction in the target species catch. The time–area closures identified in the 1986–91 logbook data were applied to the data for 1992–95 and provided a test of the spatial and temporal stabilities of these results. For the evaluation of sailfish and marlin combined, the reductions in both billfish bycatch and target species catches averaged less than the predicted values, but in all cases billfish were selectively protected. For the evaluation of marlin only, the reduction of sailfish bycatch was less than the predicted amount and the reduction of the target species was slightly greater than the predicted value. The agreement between the predicted level of protection for billfish or marlin and the mean value for the 1992–95 test period increased with increasing size of the grid. At the 5°cell size, the mean reduction was 22.8% for the targeted species and 48.6% for marlin (compared with predicted values of 20.7 and 50% respectively). These results suggest that time and area restrictions on fishing could significantly reduce the bycatch of billfishes in the pelagic-longline fisheries without equivalent reductions in the catch of target species.