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.

Despite exhibiting one of the longest migrations in the world, half of the humpback whale migratory cycle has remained unexamined; until this point, no study has provided a continuous description of humpback whale migratory behavior from a feeding ground to a breeding ground. We present new information on the satellite derived offshore migratory movements of 16 humpback whales from Antarctic feeding grounds to South American breeding grounds. Satellite locations were used to demonstrate migratory corridors, while the impact of departure date on migration speed was assessed using a linear regression, and a Bayesian hierarchical state-space animal movement model was utilized to investigate the presence of feeding behavior en route. 35,642 Argos locations from 16 tagged whales from 2012-2017 were collected. The 16 whales were tracked for an average of 38.5 days of migration (range 10-151 days). The length of individually derived tracks ranged from 645–6,381 km. Southern hemisphere humpback whale populations are recovering quickly from intense commercial whaling and, around the Antarctic Peninsula, are doing so in the face of a rapidly changing environment. The current lack of scientific knowledge on marine mammal migration is a major barrier to cetacean conservation. This multi-year study sets a baseline against which the effects of climate change on humpback whales can be studied across years and conditions and provides an excellent starting point for the investigation into humpback whale migration.

Despite its importance within marine habitats, most of what we know about auditory masking is based on terrestrial species and theoretical assumptions about signal processing in animals. To fill data gaps and improve models that predict active listening space for marine mammals, I have measured hearing thresholds for tonal sounds with highly trained sea lions, walruses, and seals in the presence of precisely and experimentally varied background noise conditions. My aim is to provide empirical measurements of frequency-dependent masking parameters to inform a quantitative understanding of the acoustic scenarios encountered by free-ranging individuals. Three frequency-dependent aspects of masking are considered: critical ratios, critical bandwidths, and masker level effects. Critical ratios (CR), or the signal-to-noise ratios required for auditory detection of pure tones embedded in controlled, spectrally-flat noise, were measured for sea lion and walrus subjects across a frequency span from 0.2 to 16 kHz. Despite differences in hearing sensitivity, these masking metrics were similar for the subjects and followed expected frequency-dependent trends observed in terrestrial carnivores. When compared to published data for seals, sea lion and walrus CRs were generally higher, indicating that, among these marine Carnivores, seals are especially adapted for hearing in noise. To evaluate how the spectral content of noise contributes to masking, I determined the frequency bandwidth of noise that interferes with the detection of a given tonal signal, the ‘critical bandwidth’ (CBW). I conducted hearing measurements with three subjects–a sea lion, walrus, and seal–while varying the frequency content of surrounding noise. The study subjects showed an expected increase in absolute CBW with increasing frequency. While data for the sea lion and walrus were similar, the seal exhibited narrower CBWs that increased as a constant percentage of center frequency, further suggesting additional specialization for hearing in noise for this group. Finally, to explore how noise level contributes to masking, I conducted a series of tone detection measurements with one California sea lion in a highly controlled acoustic environment. Across experimental trials, I gradually increased the amplitude of surrounding noise from a level of no effect to capture masking onset. The data revealed a frequency-and bandwidth-dependent transition zone that occurs before complete masking is evident. The reported masking parameters provide insight into how some marine mammals hear within noisy conditions. These data, obtained using behavioral, psychoacoustic methods, can be applied to estimate masking effects for amphibious marine carnivores listening in air or water. Further, because they extend to lower frequencies where noise tends to be high and few hearing data are available, these results have clear and actionable outcomes and implications for real-world scenarios and conservation. The findings identify the frequencies where these species are most vulnerable to noise, highlight differences in auditory biology among pinniped lineages, and enable improved predictions of the extent of masking in marine environments dominated by natural and anthropogenic noise.

Baleen whale populations in the Southern Ocean are recovering after intense commercial whaling in the 20th century. Along the Western Antarctic Peninsula (WAP), this recovery is occurring in one of the planet's most rapidly changing marine ecosystems. Understanding how climate-driven changes influence the population dynamics of whales in this region is critical for understanding what conservation and management actions must be prioritized to maintain the structure and function of this marine ecosystem. This is even more important as this region has seen extraordinary increases in industrial krill fishery pressure, which overlaps in both time and space with whales foraging in this region, as well as increased human presence in the form of ecotourism. Thus, to begin understanding the dynamics of whale recovery under continued environmental change, we need to study these whales' demography and population dynamics. My dissertation aimed to examine and describe the demographics and population dynamics of two species of Southern Hemisphere baleen whales (Antarctic minke whales and Southern Hemisphere humpback whales) in the context of a rapidly changing ecosystem. To do this, I used one of the most extensive, non-lethal tissue archives of these two species, collected as part of the National Science Foundations (NSF) Palmer Station long-term ecological research (LTER) project. I found that on average, Antarctic minke whales reproduced each year and estimated that two-thirds of females along the WAP were sexually mature. More importantly, these data represent the first non-lethal approach to studying this species. Furthermore, I found that broad-scale environmental variation affecting krill abundance and availability along the WAP adversely impacted humpback whale pregnancy rates. This indicates that continued warming along the WAP that results in subsequent changes in the distribution and abundance of prey may adversely affect the recovery of this humpback whale population. Lastly, I found that blubber cortisol levels were not significantly different between male and female humpback whales but were significantly different across different demographic groups of females and across months. Blubber cortisol levels also significantly decreased in 2021, a year when human presence along the WAP was greatly reduced due to the COVID-19 pandemic. These findings provide a critical baseline of cortisol levels for whales in a rapidly changing region and show direct relationships between cortisol levels and human presence. These are some of the first non-lethal quantitative observations of the demography and population dynamics of recovering whale populations in the Antarctic and provide a critical reference point for future work as the Antarctic climate continues to change and populations continue to recover from whaling.