Modelling mortality and migration as functions of age using PIT tagging data

In fi shery applications of tag-recovery models, recovery probabilities are typically parameterised in terms of instantaneous rates of mortality, where the mortality rate can be a function of the size or age of a fi sh. For age-dependent tag-recovery models, past work on Pacifi c halibut used a fi xedage approach, in which the age of each fi sh was fi xed at the same value for each halibut of a given length. In this work, we examined age-dependent tag-recovery models that allow for variability in ages. The age of tagged halibut can only be determined for recovered tagged animals, and when modelling data from the International Pacifi c Halibut Commission’s passive-integrated transponder (PIT) tagging study on Pacifi c halibut, we used auxiliary information from the annual setline surveys to model the age of un-aged fi sh. Both Bayesian and multiple imputation methods were examined. The former appeared to suffer from problems with parameter identifi ability, and ultimately, only the latter was applied to the PIT tag data. The results showed large differences in both mortality and migration rate estimates between models that allowed for variability in age and those that fi xed age at a constant value for a given length.