An At-Sea Assessment of Argos Location Accuracy for Three Species of Large Whales: Deep-Diving Behavior Increases Location Error

Argos satellite telemetry is routinely used to track terrestrial and aquatic animals, including large whale species. The error associated with Argos locations has been described empirically for a variety of marine megafauna but never for whales. We used Argos-linked archival tags deployed on free-ranging blue (Balaenoptera musculus), fin (B. physalus), and sperm whales (Physeter macrocephalus) to characterize the empirical distribution of Argos location errors relative to Fastloc GPS locations also acquired by the tags for these species and examine possible behavior-related differences in error estimates. Location error magnitudes were significantly larger for sperm whale tags across most Argos location quality classes (LC) compared to blue and fin whale tags (LC B = 30.04 km vs. 20.40 km, LC 3 = 2.30 km vs. 1.42 km; p-value < 0.01). However, error magnitudes were not significantly different among species while the tags were floating at the surface after release and prior to recovery. The median difference between internal and ambient temperature measured by the tags when surfacing from a dive was significantly larger for sperm whales than for blue and fin whales (-10.3°C vs. -1.7°C), resulting in larger changes in tag temperature during post-dive surface intervals. Calculated transmission radio frequencies were higher while sperm whale tags were attached compared to when they were floating, while frequencies were more similar for blue and fin whale tags, either attached or floating. Thermal inertia from deep, long-duration dives by sperm whales caused the tags’ internal temperature to be colder than ambient waters when they surfaced, likely resulting in radio frequency drift as tags warmed at the surface. This was the likely cause of greater Argos location error compared to shallower-diving blue and fin whales. Thus, sperm whale tracks (and those of other deep-diving species like beaked whales), may be less reliable for study of fine-scale movements despite them having a higher proportion of high-quality Argos locations arising from their longer post-dive surface intervals.