Analysis of Fishing Power Correction Factor Estimates from a Trawl Comparison Experiment

Abstract

Four published analytical techniques were applied to comparative trawl data to obtain fishing power correction (FPC) factors for 12 major commercial species that were caught by two resource assessment trawls used by the National Marine Fisheries Service (Alaska Fisheries Science Center). Fishing power correction techniques included ratios of catch per unit effort (CPUE), randomized block analysis of variance (ANOVA), standard least-squares regression, and a method developed by Kappenman. All four estimators generally gave similar results for species that were clearly caught in higher proportion by one trawl, but they differed in the magnitude of the adjustment. For Pacific cod Gadus macrocephalus and sablefish Anoplopoma fimbria, which were caught at very similar rates during the experiment, FPC estimates close to 1.0 (no difference between trawls) were obtained from each technique, but they differed regarding which trawl was most efficient. The Kappenman estimator gave FPC estimates of 1.0 for six species. Whenever this estimator did recognize a fishing power difference, it was always in the same direction given by the other techniques. However, the magnitude of the FPC estimate obtained from the Kappenman method was usually less than or equal to those obtained from the other methods. In addition, the Kappenman method does not require a comparative trawling experiment, as do the other methods, which are extremely costly to management agencies in terms of days at sea. Also discussed are sample size requirements and the use of 95% confidence intervals as criteria to decide if a FPC should be applied.