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Low-cost estimates of mortality rate from single tag recoveries: addressing short-term trap-happy and trap-shy bias

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Conventional single tag-recovery data are widely available for stock assessments, notably of invertebrate fisheries, worldwide. Though not commonly used for this purpose, the times-at-large in single tag-recovery data provide (relatively) direct information about average mortality rate as a sample of survival times. Mortality rate is estimated using simple formulas given as functions of the mean time-at-large of tagged and recaptured animals. Here we extend an earlier time-at-large mortality estimator to address a potentially common source of bias: trap-happy or trap-shy behavior shortly following tag release. A maximum likelihood solution is derived, yielding an unbiased estimate of instantaneous mortality rate where the interval of usable times-at-large for observed recaptures may be truncated on both sides to any biologist-chosen experimental (recapture) time frame. In tests of the new doubly-truncated mortality estimator using simulated tag-recovery times-at-large, omitting the first 8 weeks of recaptures from the mortality estimate largely eliminated the bias introduced by simulated short-term trap-happy and trap-shy behavior. Bias in the mortality estimate declined by an order of magnitude more than the observed increase in standard error.

Document Type: Research Article


Publication date: March 17, 2012

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  • Published continuously since 1901 (under various titles), this monthly journal is the primary publishing vehicle for the multidisciplinary field of aquatic sciences. It publishes perspectives (syntheses, critiques, and re-evaluations), discussions (comments and replies), articles, and rapid communications, relating to current research on cells, organisms, populations, ecosystems, or processes that affect aquatic systems. The journal seeks to amplify, modify, question, or redirect accumulated knowledge in the field of fisheries and aquatic science. Occasional supplements are dedicated to single topics or to proceedings of international symposia.
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