• Capture-recapture;
  • Mark-recapture;
  • Petersen estimator;
  • Salmon escapement;
  • Salmon spawning

Summary. The Petersen estimator estimator of abundance can be biased when the assumption of homogeneous capture probability or homogeneous recapture probability is violated. Often this heterogeneity is related to the time or place of capture or recapture, and if these can be stratified, the stratified Petersen estimator reduces the bias caused by this heterogeneity. In some experiments, not all the recovered tagged animals can be examined, and only a subsample has its stratum of release and recovery determined. We develop methods for this modified experiment and apply them to estimate the number of salmon returning to spawn in a river in British Columbia, Canada.