Determining the optimum interval for abundance surveys

Surveys to determine the abundance of marine mammals are expensive, and it is therefore advisable to use an objective process for planning the frequen cy and intensity of surveys. Previous authors have addressed methods for de signing surveys such that 1) a particular level of precision is achieved in single or multiple surveys or 2) a specified trend in abundance is detecte d with a given probability and number of surveys. We propose an alternative method to consider in designing a series of surveys aimed at minimizing th e probability of incorrectly classifying a stock relative to management goa ls. An example of the above is the classification of stocks as "strategic" under the U.S. Marine Mammal Protection Act. To address this question a ser ies of simulations were performed, where the underlying population level an d level of human-related mortality were specified. The effect of survey int erval on the rate of incorrectly classifying a stock was examined for a ran ge of precision levels for abundance and human-caused mortality estimates. Four case studies were used to examine the effect of survey interval in mor e detail. In general, coefficients of variation of the abundance estimates of less than 0.5 were necessary to achieve error rates of less than 0.1, un less the estimates of human-caused mortality were precise (<0.3). Recommend ed survey intervals between 1 year and 8 years (i.e., the predetermined max imum interval in the analysis) depend upon the level of precision that had been achieved in previous estimates of abundance and human-caused mortality . In addition, averaging abundance and mortality estimates over specified t ime periods substantially reduced the rate of mis-classifying a fishery.