Parameter estimation performance of a recapture-conditioned integrated tagging catch-at-age analysis model

Abstract

Recapture-conditioned models are infrequently used to analyze tag-recovery data, but have been proposed as an alternative to release-conditioned models for estimating movement from tagging studies when tag-loss processes (e.g., tag reporting, tag shedding) can be assumed constant and estimates of these processes are not available. Through simulations, we investigated the performance (bias and precision) of a recapture-conditioned integrated tagging catch-at-age analysis (ITCAAN) that assumes yearly natal homing under varying model complexities and intermixing rates and compared the results to those from a release-conditioned ITCAAN. We also investigated how misspecification of natural mortality, parity in population productivities, tag shedding, and spatially-varying reporting rates affected estimation of model parameters. At low intermixing rates, estimation of total abundance and spawning population abundances was accurate and precise, with precision decreasing when natural mortality was estimated for the recapture-conditioned ITCAAN. Accuracy and precision of individual population abundances declined with higher intermixing rates, with the largest bias and lowest precision occurring when estimating relative reporting rates. Assuming reporting rates were spatially constant in the ITCAAN when they varied regionally in the operating model led to biased estimation of movement rates and population abundances for both ITCAANs; attempting to estimate relative reporting when reporting varied spatially greatly improved parameter estimation compared to assuming spatially constant reporting. When tag shedding was simulated to occur, the recapture-conditioned ITCAAN yielded relatively unbiased estimation of total abundance without additional data on the tag-shedding rate, whereas the release-conditioned ITCAAN requires external data to inform this tag shedding process. Incorrect specification of tag-shedding rates by 20% resulted in only a 5% bias in estimation of total abundance from the release-conditioned ITCAAN. To avoid biased estimation from the release-conditioned model, externally derived data for all tag loss rates (e.g. tag shedding and tag-induced mortality) must be provided to the ITCAAN. Movement rates in the recapture-conditioned ITCAAN framework are theoretically unbiased by spatially uniform loss rates, but yielded biased estimates at high movement rates in this simulation study as a consequence of the model's poor ability to estimate population-specific abundances. For most scenarios investigated, estimation by the release-conditioned ITCAAN was less biased and more precise compared to the estimation by the recapture-conditioned ITCAAN presumably as a consequence of the former providing additional information on region-specific survival. However, both models performed poorly in estimating population specific abundances for scenarios with high intermixing rates and when reporting rates varied regionally but were assumed to be regionally constant in the ITCAANs.