UC Santa Cruz

Southern hemisphere humpbacks whales are seasonally fasting mammals who concentrate foraging in summer when Southern Ocean waters are most productive, coincident when their primary prey, the Antarctic krill, are most accessible. To accumulate blubber energy stores necessary to fuel energetically costly migrations and breeding events during the winter fasting period, humpbacks optimize foraging behaviors to exploit the ephemeral distribution and behavior of their prey. While humpback foraging in fall, prior to a northward migration, is well described, foraging upon arrival to the foraging grounds in summer is poorly understood. Between 2010-2019 we deployed suction cup attached biologgers onto 83 adult humpbacks along the Western Antarctic Peninsula throughout the austral summer and early fall. Tags remained attached for 18 hours on average, recording high resolution motion, depth and audio of the animal. Using idiosyncratic motion signals, we manually detected feeding lunges for each deployment, yielding 33,246 total detected lunge events between depths of 0 – 461 meters. Our results did not support previous hypotheses of increased humpback foraging from summer to fall along the WAP, conversely, we found a 51% reduction in daily feeding rates over the same period. There was nearly continuous daylight during the early summer period where whales fed during all hours of the day, this changed such that foraging occurred mostly during nighttime hours in the fall. Changes in lunge depths observed over the season indicate that humpbacks track the diel vertical migrations of krill that appear to change over the foraging season. Our results provide novel information on seasonal changes in foraging behavior of humpback whales and demonstrate that these animals, contrary to nearly all other animals that seasonally fast, likely maximize their food intake immediately upon exiting the fasting period. More direct information on krill densities concurrent to whale foraging would allow quantification of food intake to better test this hypothesis.