Confidence interval estimation of CPUE year trend in delta-type two-step model


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Abstract:  A procedure is suggested for estimation of the approximate confidence intervals of the extracted catch per unit effort (CPUE) year trend in the delta-type two-step model used for CPUE standardization with a lot of zero-catch data. This method is a simple way to combine the Taylor expansion and delta method and is suitable for practical use. This model was applied to the catch and effort data with more than 80% zero catch for silky shark in the North Pacific Ocean caught by Japanese training vessels. As a result, realistic values of the 95% confidence interval of CPUE year trend are obtained. A method for left–right unsymmetrical interval estimation based on the asymptotic normality of the natural logarithm of CPUE is also suggested. In the example of silky shark, both CPUE year trends obtained from these two methods are similar.