Trappability estimates for mark–recapture data

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Abstract
Mark–recapture methods produce more accurate estimates when a high fraction of the population is trapped. Three estimates of trappability are used in the literature, and we compare them for four species of Microtus livetrapped in Longworth traps. The trappable population is defined by the weight at first capture and varies greatly in different species. Jolly trappability is the best estimate of trappability and is on average 4% above minimum trappability and 8% below maximum trappability. Trappability in Microtus varies seasonally, is lowest in summer, and may be reduced at high densities. Females are, on the average, 2% more trappable than males. Trappability is the same in fenced and unfenced populations and is highly consistent in different populations of the same species. Comparisons between different population studies or trapping techniques should utilize the Jolly trappability measure and recognize that it will usually have a positive bias if capture probabilities are heterogeneous. Minimum trappability is a good second choice.