Abstract:Due to the patchy spatial distribution of marine biota, data collected from bottom trawlers exhibit a skewed distribution with many zero catches and some very large catches. A common problem in the analyses and interpretation of skewed survey data is that, a single immense catch may account for 50% or more of the total catch during the survey. These extreme values not only greatly affect the estimate of the means but also of the variance. Based on the data collected from the bottom trawl survey conducted in the East China Sea, we estimated the average stock density of Todarodes pacificus in summer 2005, Engraulis japonicus in spring 2007, and Trachurus japonicus in autumn 2008 by employing two methods, namely the Δ-distribution method and the design-based method. The results revealed that the estimators of the means and the standard errors of stock density for the three populations based on the Δ-distribution were obviously lower than those based on the design based method. It was concluded that the mean and the standard error would be overestimated using the design-based method. The Δ-distribution was more reasonable when the survey data follow lognormal distribution. Therefore,Δ-distribution is a robust estimating method and is suggested to be widely usable in further studies.
