Estimating Effective Sample Size for Monitoring Length Distributions: A Comparative Study of Georges Bank Groundfish

Because of the aggregative behavior of fish, effective sample size is more meaningful than total sample size for estimating the length distribution of a fish stock. We estimated the effective sample sizes for mean length distributions of six groundfish species from Georges Bank using data collected by a Study Fleet Program implemented by the School for Marine Science and Technology (SMAST) at the University of Massachusetts-Dartmouth. Both statistical and simulation methods were applied. Skewness and kurtosis were also used as proxies for length distributions. The statistical result suggests that the estimated effective sample size is much less than the number of fish measured in the initial SMAST Study Fleet Program, while the simulation method suggests that for most groundfish species reducing the total number of fish to 20% will not affect the variance, skewness, and kurtosis of the population length. For the SMAST Study Fleet Program, increasing the number of sampling stations can improve sampling precision better than increasing the number of fish measured within each station. This study helps to develop an optimal sampling design that balances the quality of the data collected and costs of the sampling effort. Received July 31, 2011; accepted August 11, 2012