Using poststratification to improve abundance estimates from multispecies surveys: a study of juvenile flatfishes

Population assessments seldom incorporate habitat information or use previously observed distributions of fish density. Because habitat affects the spatial distribution of fish density and overall abundance, the use of habitat information and previous estimates of fish density can produce more precise and less biasedpopulation estimates. In this study, we describe how poststratification can be applied as an unbiased estimatorto data sets that were collected under a probability sampling design, typical of many multispecies trawl surveys.With data from a multispecies survey of juvenile flatfish, we show how poststratification can be applied to a data set that was not collected under a probability sampling design,where both the precision and the bias are unknown. For each of four species, three estimates of total abundance were compared: 1) unstratified; 2) poststratified by habitat; and 3) poststratified by habitat and fish density (high fish density and low fish density) in nearby years. Poststratification by habitat gave more precise and (or) less design-biased estimates than an unstratified estimator for all species in all years. Poststratification by habitat and fish density produced the most precise and representative estimates when the sample size in the high fish-density and low fish-density strata were sufficient (in this study, n≥20 in the high fish-density stratum,n≥9 in the low fish-density stratum). Because of the complexities of statistically testing the annual stratifieddata, we compared three indices of abundance for determining statistically significant changes in annualabundance. Each of the indices closely approximated the annual differences of the poststratified estimates. Selection of the most appropriate index was dependent upon the species’ density distribution within habitat and thesample size in the different habitat areas. The methods used in this study are particularly useful for estimatingindividual species abundance from multispecies surveys and for retrospective st