Migration has the potential to violate the assumptions of tag recapture models used in tag-based assessments. Evaluating the potential bias introduced into stock assessments and tag-based biomass estimates when the distributions of tagged fish, fishing effort and the underlying stock distribution are spatially heterogeneous is an important area of fisheries stock assessment research.

This study used a fisheries simulation model to evaluate the impacts of migration on a biomass estimates and sustainable catch limits from a closed population tag-based CASAL stock assessment. Our simulations demonstrated that emigration of tagged fish out of the assessed area results in initial and current spawning biomass, and stock status being over-estimated.

We then estimated migration of rates of Kerguelen Plateau Patagonian toothfish between the French and Australian EEZs using a catch-conditioned modification of the method used by Hilborn (1990). We demonstrated that the bias can be corrected by increasing the tag-shedding parameter in the existing CASAL assessment by value of the migration rate, providing a simple yet effective approach to correct for the effects of emigration. The key benefit of making such an ad-hoc increase to the tag-shedding parameter is that it avoids the need to develop new assessment frameworks to account for emigration.