Using electronic tag data to improve mortality and movement estimates in a tag-based spatial fisheries assessment model

Despite increased deployment of archival tags on exploited fish species, analytical methods for including archival tag data in fishery assessment models are lacking. We present a method for integrating archival tag data into a spatial tag–recapture model for estimating natural mortality, fishing mortality, abundance, and movement. Archival tags provide important information on fish movement not available from conventional tags that facilitates separation of movement from mortality estimates. Using simulations, we evaluate the benefit of including archival tag data in the model using two model formulations: one with a general spatial structure and one with movement and fishery dynamics based on juvenile southern bluefin tuna (SBT; Thunnus maccoyii ). If fish are not tagged in all regions and time periods, then including archival tag data can substantially improve the precision of the fishing mortality and movement estimates. For example, with the general spatial structure and specific scenario presented, standard errors of the fishing mortality estimates decreased by an average of 34% (21%) when 25 archival tags were released in addition to 500 (2500) conventional tags in each region and period of tagging, respectively. Furthermore, with the SBT spatial structure, archival tag data were necessary for all parameters to be estimable.