Research Document - 2008/031

By N. Cadigan and J. Brattey

Reporting rates from cod tagging studies in NAFO Divisions 2J3KL and Subdivision 3Ps

Abstract

Estimates of tag reporting rates are necessary to infer fishery exploitation rates from the fraction of tags returned from tagging experiments. Changes in reporting rates can have considerable influence on estimates of exploitation rates. Some evidence was presented in the 2007 assessment of northern cod that suggested reporting rates had changed. This motivated us to examine in more detail the methods used to provide estimates of reporting rates. We estimate reporting rates using the proportion of low-reward tags returned from low and high-reward releases. We use the common Binomial logistic regression model to estimate reporting rates. This approach yielded infeasible estimates in some regions and years, wide confidence intervals, and large between-year variability in some regions. We also considered another model in which year-effects in log reporting rates were modelled as random error terms. This mixed-effects logistic regression model did not produce infeasible estimates, gave narrower confidence intervals, and little between-year variability in reporting rates for most regions. Estimates suggest a decreasing trend in single tag reporting rates from 3KL, from 70-92 % in 1997-2005 to 62 % in 2006-07, and in 3Ps from 70-81 % in 1997-2005 to 65-67 % in 2006-07.

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