Size Bias in Line Transect Sampling

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Abstract
Line transect sampling is used to estimate the number of individuals in a study area. It is usually assumed that the detections of individuals constitute independent events, and that the probability of detecting an item is a function of its perpendicular distance from the transect. However, certain species of plants/animals tend to aggregate, or cluster, thus possibly violating the assumption of independence. Also, larger clusters may tend to have a higher probability of detection, thus distorting the observed probability distribution of cluster sizes. This can cause overestimation of the true population size. A similar problem occurs if the experimenter is using line transect data to estimate the mean and total amount of some attribute of the items, and this attribute influences the item''s probability of detection. Bivariate detection functions that incorporate the cluster size, or another attribute, as a covariate are proposed. Estimators of total population size and of the moments of the additional variate are derived using the theory of weighted distributions. An application of the method to a shipboard survey of minke whales is provided.

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