Comparison of Two Methods of Combining Catch-Per-Unit-Effort Data from Geographic Regions

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Abstract
We examined two methods of combining catch-and-effort data from small geographic regions into large regulatory areas to estimate catch-per-unit-effort (CPUE): (1) weighting regional CPUE by bottom area occupied by the population; (2) weighting by fishing effort. Variance estimates and other statistical properties are presented for CPUE estimates by region and for the two methods, for the type of fishery where total catch is known. The CPUE estimator from area-weighting is approximately unbiased as an index of population density; the CPUE estimator from effort-weighting is biased but less variable than the former. Conditions of similarity are derived for estimators from the two methods to be identical, or, less restrictively, to show the same trend over time. In general, the area-weighted method is preferred, if sufficient samples are taken from all pertinent regions in the regulatory area. However, the effort-weighted method may be used with a gain in precision and no substantial bias, if there are no substantial changes in relative CPUE and effort among regions, or if relative CPUE and effort are inversely related. The two methods applied to catch and effort data for Pacific halibut (Hippoglossus stenolepis) and to produce highly similar CPUE estimates over time, despite large relative changes in both CPUE and effort among regions. The reasons for the similarity are traced back to conditions in the fishery.Key words: catch-per-unit-effort, combining data, sampling commercial fisheries, variance estimation, comparison of methods

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