Movement Ecology of Humpback Whales in the South Pacific

Abstract

Determining the distribution, movements and habitat use of animals is fundamental to understanding their ecology and the development of effective conservation measures. However, studying animals that migrate thousands of kilometres and inhabit remote and inaccessible areas is extremely difficult, especially in the marine environment. The Oceania humpback whale (Megaptera novaeangliae) population has been slow to recover from the effects of commercial whaling, and our understanding of what may influence this slow recovery has been limited by a lack of knowledge about the whales outside of their tropical breeding grounds. The aim of this thesis was to investigate the migratory movements, patterns of Antarctic feeding ground habitat use and energetics of Oceania humpback whales as they migrated from the South Pacific to the Southern Ocean. Satellite tags were deployed on 25 humpback whales on their southern migration past the Kermadec Islands, New Zealand, in September and October 2015. Photo-identification and genetic data were collected to assign breeding ground origins and to determine the pregnancy rate and age-profile of the population. The whales’ migratory paths and behavioural states were investigated by applying a hierarchical state-space model to the satellite telemetry data. These data were used in linear mixed-effect models to elucidate ecological relationships between whale behaviour and the environment within the remote Southern Ocean feeding grounds. The tag data, and data from two whales tagged in east Australia, were also used to inform a bioenergetic model to estimate the relative energetic cost of different migratory routes and distances. The Kermadec Islands were an important aggregation point for Oceania’s humpback whales from a range of breeding grounds spanning ~3,500 km of ocean, almost the entire breeding ground range, as they migrated south to their Southern Ocean summer feeding grounds. The age profile of the whales (mean = 14 years) and a high (57%) pregnancy rate indicated a recovering population. The whales migrated to two key feeding areas, the Ross Sea and the Amundsen and Bellingshausen Seas (~2,000 km apart), the choice of which was influenced by the presence of a calf, as mothers with calves migrated a shorter distance to the Ross Sea region. There were marked differences in the environmental features between the two areas (e.g. oceanic vs near continental shelf) and consequently the whales used these areas differently. Overall, time lagged ice-edge dynamics were identified as a key environmental feature influencing the whales’ foraging behaviour, along with season and sea surface height. Whales with the longest migration distance migrated the fastest (97 km/day vs71 km/day) and had the highest cost of transport, but this was offset by energy savings in terms of daily maintenance costs by using 11% fewer days to complete the migration, suggesting that migration distance alone is not a limiting factor to population recovery. Whilst there is no doubt that a complex set of variables is affecting the Oceania humpback whale population recovery, the findings of this thesis have improved our understanding of the whales and their movement ecology once they depart from their breeding grounds. Furthermore, understanding the movement patterns and habitat use of this large predator also improves our knowledge of the remote Southern Ocean ecosystem and the changes occurring within it as a result of climate change. With such a wide geographic longitudinal range of feeding grounds (~4,000 km span) behavioural plasticity may play a critical role in the whales’ ability to adapt to the environmental changes, therefore affecting the species’ future recovery. This work further highlights the value of bio-logging in providing opportunities to advance ecological research.