Tracking pygmy blue whale diving behaviour and validation of foraging areas defined from horizontal movement data

Foraging in large vertebrates is often inferred from horizontal movement behaviour from satellite tracking data, but the strength of this inference has not been adequately verified. Here, nine pygmy blue whales ( Balaenoptera musculus brevicauda ) were tagged with satellite tracking tags and pop-up archival tags (PATs) providing depth and accelerometry time series to determine where actual foraging occurs during migration. Four PATs were recovered, providing depth and accelerometry data at 1 Hz, and the remaining tags transmitted 75 s sample-rate data for up to 40 days. The depth time series allowed us to distinguish foraging dives, with accelerometry data key to distinguishing lunge-feeding. Despite a weak temporal relationship between putative (inferred from a movement model) and actual foraging, there was generally good spatial overlap detected, predominantly in high use areas, while more opportunistic foraging areas with lower use were less likely to be detected by the model. More opportunistic foraging occurred off north-west Australia where foraging dives were shallower, horizontal travel rates faster, and there was an absence of a diurnal pattern in diving. This suggests a reliance on more ephemeral prey than off south-west Australia where whales have high residency. Foraging/feeding occurred every 1–10 days at a rate of 4 hours (median) per day (range 0.1 – 12) and occurred during migration, providing support for use of a mixed breeding strategy (income and capital) in this sub-species. Our test of movement models to define foraging is extremely useful given its common usage in ecology and our spatial delineation of foraging areas assists with conservation management.