Depth-based geolocation processing of multi-year striped marlin archival tag data reveals residency in the Eastern Pacific Ocean

Background: The first successful application of implanted archival tags on striped marlin showed great potential in obtaining long-term tracks for an improved understanding of movement ecology, which is important for informing fisheries management decisions. Ten tags were physically recovered from fish tagged in the Eastern Pacific between 2008 and 2016, but unfortunately due to the failure of the external sensor stalk which houses the light sensor, full records of daily fish position could not be attained. Results: Depth-based geolocation, which exploits the diel swimming patterns of striped marlin, was applied to derive position estimates for archival tagged fish up to 7.7 years. Reconstructed tracks revealed tagged striped marlin remained in the Eastern Pacific throughout the tracked duration. Trans-equatorial movements were also documented for the first time for striped marlin in this region, as were extended occupancy of > 1 year in pelagic waters. Conclusions: Striped marlin connected both coastal and offshore habitats with seasonal runs, likely in fulfillment of their life history requirements from foraging to reproduction. Circadian rhythms in billfish and other pelagic fishes are well-established, and could provide a viable, alternative means to position an individual in a low or no light environment, and situations with sub-optimal or limited bio-logging capabilities. Depth-based geolocation, however rudimentary, has revealed variability in striped marlin horizontal movements over the multi-year observation period, and offered a unique spatiotemporal perspective that was unavailable to scientists and fisheries managers until now.