Estimating migration rates from two tag–release/one recovery experiments

We propose a method for estimating migration (movement) rates from two tag–release/one recovery experiments, regardless of tag-shedding or incomplete tag-reporting which are major problems when tag–recovery techniques are applied to commercially exploitable populations. The entire survey area is divided into multiple strata in advance. The first release is limited to one stratum; then, the second release occurs in every stratum. Recoveries from both releases occur in every stratum. The migration rate between the time of the first and second release is estimated together with its variance. To check the applicability of our method, we applied it to tag–recovery data for skipjack tuna from various places at different times and unequal time intervals. The precision of the estimates was low and the coefficient of variation was 22.6–42.1% because of the small number of recoveries. The experimental design necessary to improve precision is discussed.