WHICH ARE BETTER, RANDOM OR SYSTEMATIC ACOUSTIC SURVEYS - A SIMULATION USING NORTH-SEA HERRING AS AN EXAMPLE

This paper considers the design of acoustic surveys for estimating the mean abundance of spatially correlated populations. We examined how t he choice of survey design affects the bias and precision of the sampl e mean as an estimator of mean abundance. Further, we investigated thr ee different ways of estimating the error variance of the sample mean: the pooled within strata variance and two geostatistical variance est imators based on spherical and exponential models. First, we analysed four surveys to determine the spatial structure of the North Sea herri ng population. We developed forty different population models based on the observed amplitude and spatial distribution. We generated 1000 re alizations of each model, each comprising 4000 potential transect mean s. Each realization was sub-sampled using eight different sampling str ategies. From each realization and sampling strategy, we calculated th e sample mean and three estimates of the variance of the sample mean. The simulations show that, for surfaces with local positive correlatio n, more precise estimates of the surface mean can be obtained using st ratified random or systematic sampling than simple random sampling. Th e best strategy considered here was (a) a systematic survey with a geo statistical variance estimator, when the main objective is to obtain t he most precise estimator of abundance, (b) a stratified random survey , with two transects per strata, and a pooled variance estimator, when an important objective of the survey is to obtain a good estimate of the variance of the abundance estimator. (C) 1995 International Counci l for the Exploration of the Sea