A – Abundance Estimation

Publisher Summary This chapter covers the abundance estimation, the range of techniques by which the size of a population of marine mammals can be estimated. When it is difficult to estimate absolute abundance with an acceptably low bias, relative abundance indices are often used instead. These are indices that are believed to be proportional to population size, apart from stochastic variation, allowing trends in the population in space and/or time to be assessed. The main techniques for abundance estimation (relative or absolute) are distance sampling, mark–recapture, migration counts, and colony counts. Many populations of large whales conveniently file past coastal watch points on migration, allowing observers to count a large proportion of the population. This count can then be corrected for animals passing outside watch periods to estimate population size. The methods usually used for modeling migration counts were developed for the analysis of surveys of the California gray whale (Eschrichtius robustus). In colony count abundance estimation researchers often make counts of populations when they gather for breeding and pupping at certain times of the year. Often photographs are taken of the colonies. These are brought back to the laboratory for analysis and form a permanent record of the population.