JIMAR Researchers Develop New Method to Track Marine Creatures

Researchers with the Pelagic Fisheries Research Program at the University of Hawaii – Manoa’s Joint Institute for Marine and Atmospheric Research (JIMAR) have developed a new method to estimate the most probable track of geographic positions (geolocations) of marine creatures directly from a series of light measurements recorded by archival tags. The new method does this without either making any light-level threshold assumptions or constraining the movement of the tag between dawn and dusk. In addition, the new method generates two geolocations per day. The covariance structure of the model is designed to handle high correlations between light measurements, such as might be caused by local weather conditions. The yearly pattern in latitude precision is estimated by propagating the data uncertainties through the geolocation process. The model has been applied to simulated data, mooring studies, and real deployments on swimming and diving fish. Tracks can be reliably estimated, even in cases where the other methods have completely failed and have produced misleading position estimates.

Background: Archival tags are electronic devices designed to be mounted on a live animal to record measurements of the animal's environment. For marine animals measurements of light, temperature, and pressure are typically stored in the tags, in the hope that reliable geolocations can be estimated from these data. Light-based geolocation algorithms have at least a 20-year history. The algorithms are best described as “threshold algorithms.” From the time series of light measurements, they determine the time when a certain light threshold is crossed. This threshold is assumed to correspond to a certain solar altitude, and from that it is possible to calculate a geolocation. The problematic part is associating a threshold with a certain solar altitude, as the time when the daily light measurements cross a certain threshold is influenced by multiple factors, including local weather conditions.

Significance: Fisheries research organizations, universities, and governments have spent millions of dollars on development, procurement, and deployment of archival tags. Archival tags can stay on for years, and potentially provide valuable information about post release survival, spawning areas, habitat preferences, and migration corridors. For the first time in the 20-year history of this technology it is possible to estimate geographic positions from the data collected by these tags. This research supports NOAA Mission Goal 1 - Protect, Restore, and Manage the Use of Coastal and Ocean Resources through an Ecosystem Approach to Management.