The stratified Petersen estimator with a known number of unread tags

The Petersen estimator estimator of abundance can be biased when the assump tion of homogeneous capture probability or homogeneous recapture probabilit y is violated. Often this heterogeneity is related to the time or place of capture or recapture, and if these can be stratified, the stratified Peters en estimator reduces the bias caused by this heterogeneity. In some experim ents, not all the recovered tagged animals can be examined, and only a subs ample has its stratum of release and recovery determined. We develop method s for this modified experiment and apply them to estimate the number of sal mon returning to spawn in a river in British Columbia, Canada.