Measures of effort in tuna longline fisheries: changes at the operational level

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Changes at the operational level of a fishery can affect the efficiency of gear and need to be taken into account when catch per unit effort indices are used to assess the status of fish stocks. Catch per unit of effort indices form the basis for the assessment of most tuna longline fisheries. Effort in these fisheries is measured in terms of the total number of hooks set and differences at the operational level in the number of hooks per longline set are generally not considered. It is shown that the number of hooks per set in the Pacific Japanese longline fishery has increased steadily from 1962 to 1986 by about 40%. The increase does not appear to be related to vessel size. Changes in the number of hooks per set can potentially affect the average efficiency of individual hooks. Analyses of catch rates (number per hook), based on data from 1979 to 1986 and categorized according to the number of hooks per set, suggest no consistent trend, although the category with the largest number of hooks per set tended to have higher catch rates for yellowfin and slightly lower rates for bigeye. These results suggest that the number of hooks per set does not interact with the efficiency or catchability of an individual hook. Comparison of catch rates, based on the number of hooks and the number of sets, suggests that the choice of an effort measure can substantially affect either observed trends in catch rates over time or the relative changes in their magnitude. In recent years, decreases in the catch per hook have been offset at the set level by increases in the number of hooks per set. Overall, the results suggest that the number of hooks should continue to be used as the best measure of effort for stock assessment purposes, but is not appropriate for economic analyses.