A new method for estimating tag shedding rates with application to data for Australian salmon, Arripis trutta esper Whitley

Abstract

A new procedure is described for estimation of tag shedding rates from a double-tagging experiment, when exact times of recapture are known. The method requires identical assumptions to existing procedures using recapture data pooled over time, but it avoids approximations inherent in those procedures. Estimates are obtained independently of the history of fishing and natural mortality throughout the experiment. The procedure was developed principally to handle those situations in which the pooled data methods are unsuitable, due to sensitivity of the estimates to the pooling interval adopted. This commonly occurs when recaptures are few and irregularly spaced over time.

Use of the procedure is illustrated with data for adult Australian salmon, A. trutta esper, which were tagged with single and double internal tags of two sizes in March 1978. No difference was found between the recovery rates of the two tag types. Of 125 originally double-tagged fish, 47 were recovered and 41 of these retained both tags on recovery. An annual tag retention of 74 5% was estimated, with 95% confidence limits 52 1-89.6%.