An assessment of light-based geoposition estimates from archival tags

Archival tags record information about the environment of tagged animals over long periods of time (months to years). In theory, position can be estimated from a record of changes in light intensity with time. We describe two approaches to estimating geoposition based on estimating either the time of maximal rate of change in light intensity or the time that a reference light intensity is reached. Digital signal processing is investigated as a method of increasing the signal-to-noise ratio of the light record. Our test data suggest that the daily position of a tagged animal can potentially be estimated within an average error of about 140 km (SD's of 0.9° of longitude and 1.2° of latitude), approaching the resolution of the best eddy-resolving physical oceanographic models of ocean currents. The source of the remaining large-scale errors in geoposition appears to be extrinsic to the tags and may be related to large-scale weather systems. The accuracy of current archival tags is sufficient to permit an assessment of the open-ocean migration pathways of animals such as maturing salmon and may be sufficient for use in some parts of the continental shelf as well.