Assessing the effect of intra-haul correlation and variable density on population estimates from marine surveys
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In a previous paper (Pennington and Vølstad, Biometrics 47, 1991) it was suggested that reducing the size of the sampling unit generally used in marine surveys could increase the precision of abundance estimates. But if unit size is reduced, fewer animals would be caught during a survey. Concern has been expressed that this reduction in total catch would lower the precision of estimates of population characteristics, such as mean fish length, of importance for stock management. In this paper we examine the effect of sampling unit size, intra-cluster correlation and variable density on the precision of population estimates. Based on an examination of some survey data, it appears that reducing the size of the sampling unit generally employed and using the time saved to take samples at more locations could also yield more precise population estimates.