A Simple Test for Nonmixing in Multiyear Tagging Studies: Application to Striped Bass Tagged in the Rappahannock River, Virginia

The Brownie‐type models for multiyear tagging studies allow the estimation of age‐specific and year‐specific total survival. An important assumption of these models is that the tagged cohorts are thoroughly mixed, or more specifically, that they have identical spatial distributions. We propose a chi‐square test to assess the validity of this assumption and apply the method to striped bass tagging data from the Rappahannock River, Virginia. The current protocol for estimating striped bass survival involves fitting a suite of Brownie‐type models to tag recovery data. Because moderate levels of nonmixing can induce significant bias, we examined tagging data for two size ranges of fish to determine if the well‐mixed assumption was violated. We suggest that examining spatial patterns of recaptures should be a routine part of analyzing tagging data from multiyear studies. For the striped bass data, the analysis showed little evidence of assumption violation, but in some cases the power of the test was probably low because the number of recaptures was small.