The feeding behaviour of humpback whales while on migration: methods, driving factors and its importance to whale ecology

Abstract

Many migratory species use stopover sites to reduce the cost of transport or increase energy intake. In terrestrial taxa, the quality of prey at these sites is linked to future survival and reproductive success with animals spending more time feeding when prey quality is high. The humpback whale (Megaptera novaeangliae) makes annual migrations between high latitude feeding grounds and low latitude breeding grounds, with historical data indicating that whales fast while migrating. However, there is growing evidence of whales feeding while on migration, yet little research describing or understanding this behaviour. The aim of this thesis was to develop appropriate methods to study the feeding behaviour of whales while on migration, determine factors that elicit feeding, and, from these results, infer the potential importance of feeding at migratory stopovers to whale ecology. Data were collected off the coast of Eden, NSW, Australia for three weeks in 2011 and five weeks in 2012, during the southward migration of the whales towards feeding grounds. Behavioural data were collected using group surveys (n = 82), digital tag (DTAG) deployments (n = 9) and focal follow sampling (n = 18). Data on the prey type and density available to the whales was also collected concurrently. First, a method was developed to detect both surface-feeding lunges and lunges at depth using accelerometer data from the DTAG. The lunges detected from the DTAGs were then used to determine the amount of time spent feeding and the rate of energy intake in relation to the prey species available. Given that whales in the east Australian population feed predominantly on krill on their main feeding grounds, it was hypothesised that the availability of krill along the migratory route would provide an opportunity for whales to consume a familiar prey item and begin to accumulate energy reserves prior to reaching feeding grounds. It was assumed that if the feeding behaviour observed was important to whale ecology, the behaviour of whales would be more similar to the behaviour described on main feeding grounds and different from the behaviour of whales during migration. When fish were available, whales had relatively straight track lines as well as group sizes and compositions that were similar to those of whales on migration. In contrast, when krill were available, whales regularly looped back through the same area and had group sizes and compositions that were more similar to those on the main feeding grounds. Additionally, individual whales spent a significantly greater amount of time feeding and a larger proportion of the groups observed were determined to be feeding when krill, rather than fish, were available. Therefore, feeding behaviour on migration may be more important to whale ecology than previously thought and is likely influenced by the type of prey available. Given this observation, the influence of prey type, and prey depth on the rate of energy intake was investigated. Whales lunged at a significantly higher rate when feeding on krill compared to when feeding on fish, consuming prey at a rate of up to 3.7 times their energetic requirements. Therefore, feeding on migration may contribute more to annual energy budgets than previously thought and allow whales to begin to restore their energy supplies prior to reaching the Antarctic.As changes in the prey species available influenced the behaviour of whales while on migration, a technique to determine the diet of migrating whales is required. Whales often feed underwater and migrate through remote areas, which can make the observation of feeding during migration difficult. This thesis tested whether stable isotope analysis of the skin of the whales would reflect the short-term dietary changes that occur on migration. Stable isotope analysis of skin was not a suitable technique as the skin still displayed historical dietary signatures of Antarctic prey. Therefore, further development of a technique to determine the most recent diet of whales is required in order to determine the diet of whales while on migration. This study is the first to investigate the factors that influence the fine-scale feeding behaviour of whales while migrating. The results suggest that a change in the type of prey that is available at migratory stopovers influences behavioural characteristics of whales such as the amount of time spent feeding and the rate at which energy is consumed. Future research should focus on determining how often feeding occurs over larger spatial and temporal scales, and whether the rate of energy intake observed is sustained for longer periods. This will assist in determining the contribution of feeding behaviour while on migration to the annual energy budgets of humpback whales.