The population identity and foraging ecology of humpback whales in the Western South Atlantic

Abstract

Introduction: Migratory marine species face complex conservation challenges due to their dependence on habitats spanning vast scales. Humpback whales in the Western South Atlantic illustrate this complexity, undertaking long-distance migrations between tropical breeding grounds and productive foraging areas, where environmental conditions and anthropogenic pressures, such as climate change and commercial krill fisheries, can influence survival and reproduction. Whilst humpback whales in the Western South Atlantic have shown substantial recovery since the end of commercial whaling, certain key aspects of population structure, reproductive dynamics, and foraging behaviour remain poorly understood. This thesis addresses these knowledge gaps by integrating molecular and biochemical approaches to provide a comprehensive view of population identity, demography, and foraging ecology in Western South Atlantic humpback whales. Materials and Methods: I combined genetic, hormonal, isotopic and fatty acid analysis on humpback whale skin-blubber biopsy samples collected at their high-latitude foraging grounds of South Georgia, the Falkland Islands, and a single opportunistic sample from the South Orkney Islands. Genetic analyses were conducted to determine the sex of whales, calculate genetic diversity and assess population structure and connectivity to other foraging grounds and low-latitude breeding grounds. Hormonal analyses measured progesterone to infer female pregnancy rates and testosterone to make inference on male reproductive cycles. Fatty acids and epigenetic markers explored potential tools for assessing population age structure. Foraging ecology was investigated using stable isotope and fatty acid analyses to infer diet composition, trophic position and foraging location Results: Genetic analyses showed a sex ratio which did not significantly deviate from 1:1 and confirmed that whales foraging in the Southwestern Atlantic are connected to breeding stocks on both sides of the Atlantic Ocean. Genetic diversity estimates were comparable to estimates from other Southern Hemisphere humpback whale populations. Hormonal analyses revealed substantial interannual variability in pregnancy rates, while male testosterone concentrations reflected expected seasonal patterns. Stable isotope and fatty acid analysis indicated that the majority of individuals feed on Antarctic krill, primarily in the regions around South Georgia and the Sub-Antarctic Front, but some whales exhibited alternative foraging on the Patagonian shelf. Conclusions and Implications: By combining multiple lines of evidence on a representative (if small) sample size, this thesis provides a comprehensive understanding of population identity, reproductive demography, and foraging ecology in Western South Atlantic humpback whales. The results highlight the importance of ecosystem-based management for conservation, particularly in the context of climate-driven changes to prey distribution and ongoing anthropogenic pressures such as krill fisheries. Improved knowledge of population structure, reproductive patterns, and foraging behaviour provides a foundation for informed management and supports the continued recovery of humpback whales in the region.