Decoupling emigration from mortality is critical for accurately estimating mortality in recapture surveys. However, open-population models often suffer from their inability to distinguish between these two parameters. A model is presented here that separates emigration from mortality based on movements of a tagged population of 42 hatchery-reared queen conch, Strombus gigas, released into a 20 m × 20 m plot in the nearshore waters of the Florida Keys. The model was constructed in a 4-step process. Recapture sampling of uniquely tagged individuals was used to derive a frequency distribution based on the movements of these individuals over a given time period. A probability density function was then fit to the frequency distribution. A probability of emigration was assigned to each cell in the plot. This value represented the probability of an individual located in that cell emigrating from the plot. Missing tagged individuals were assigned emigration probabilities (survival) based on the distance between their last observed location and the distance to the periphery of the plot. The overall survival of the population was estimated by constructing a survivorship table to estimate survival within and external to the plot for each sampling interval. After 3 mo, 14 conch were recaptured alive and another eight survivors were estimated to be present outside the plot representing an overall survival of 52.7%.
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Vol. 24 • No. 2