Humpback whale migration : new insights into the energetics of a long-distance migrant

Knowledge of animal energetics is fundamental to the understanding of long distance migration and the strategies required for survival and reproduction during extensive journeys away from foraging areas. Baleen whales are ideal for such an investigation as their large body size allows huge energy stores and facilitates one of the longest migrations of all mammals. However, this large body size also prohibits their capture for measurement, making data collection extremely difficult. In this study I used a range of techniques to examine cetacean energy stores in order to improve our understanding of the energetics of the annual long-distance migration of humpback whales between high latitude feeding grounds and low latitude breeding grounds. Specifically, I aimed to (1) ascertain the different energy store requirements of a capital breeding cetacean compared to those of an income breeding cetacean; (2) investigate how lactating female humpback whales manage their energy stores to maximise the growth of their dependent calves; (3) identify the extent of the Breeding Stock D (BSD) humpback whale calving grounds and discuss the energetic implications of an extended calving range. I used a unique set of historical whaling records that detailed total body lipid to quantify the energy stores of two large cetacean species, the income breeding sperm whale (Physeter macrocephalus) and the capital breeding humpback whale (Megaptera novaeangliae). The relationships I quantified between body length and total body lipid demonstrated that cetacean energy stores vary according to body size, life history strategy, and reproductive class. More specifically, the capital breeding humpback whale stored 31.9 - 74.9% more energy to fuel the costs of their long-distance migration than the income breeding sperm whale; and pregnant female humpback whales stored 26.2% and 37.4% more energy than non-pregnant females and males, respectively, to satisfy the high energy demands of lactation. The pregnant females that arrived in the breeding grounds later, presumably those that remained in the foraging grounds for longer durations, had larger energy stores than those that departed earlier. The findings highlighted that smaller (shorter) individuals, with their relatively small energy stores, and relatively high metabolic rates, experience higher levels of energetic stress during the migratory fast. This is particularly important for small juveniles and maternal females as they have high energy costs and relatively small endogenous energy reserves. In addition to providing information about cetacean energy stores that can be used in ecosystem and bioenergetics models, this study has provided a baseline of humpback whale body condition from the last century that can be used for comparison in the current era. To extend the findings from the historical whaling era, I used aerial photogrammetric techniques on free-swimming humpback whales to quantify maternal energy stores and offspring growth. By determining relative width measurements (width : length ratio) at multiple locations along the body, at two different stages of the migration, I illustrated that the location mid-way along the body (at 50% of the body length from tip of rostrum to tail notch) is a dominant energy storage site that can be used as an index for estimating body condition. By comparing this metric within and between seasons I determined that the body condition of lactating cows declined at different rates each year, decreasing at a mean rate of 0.36 cm/day in 2013 and 0.65 cm/day in 2015. Calf growth rate also differed each year, with calf body length increasing at a rate of 3.1 and 2.4 cm/day during early development in 2013 and 2015, respectively. These findings illustrate annual variations in maternal energy reserves and calf growth and highlight the influence of maternal investment on the growth and development of young baleen whale calves. Furthermore, this study provides a reliable technique of estimating the body condition of live whales that can be used to build a time series of humpback whale health and its variability between years. Such fluctuations can provide valuable information about prey availability in the feeding grounds that can be used to indicate ecosystem health. This information can be compared with historical values to investigate biological responses to a changing climate. During the photogrammetric study above, I made opportunistic observations of small calves at an area considered to be outside the recognised calving grounds of the BSD population. From these observations I classified the developmental stage of the calves according to their colour and size. Combined with behavioural data and survey details, I estimated that a minimum of ~20% of the expected number of calves in the population were born near, or south of, North West Cape on the Western Australian coast. As this location is situated 1000 km southwest of the recognised calving grounds (on the Kimberley coast of Western Australia), these findings illustrate geographical expansions that can take place as populations increase. For the BSD humpback whale population, the expansion of calving areas represents the re-colonisation of areas that were documented as being used previously, i.e. during the commercial whaling era. These expansions may begin to overlap with areas of high anthropogenic activity, particularly in areas of high human use such as coastlines or coral reefs. This highlights the need for monitoring the location and extent of calving and breeding areas as baleen whales recover from near extinction during the commercial whaling era last century. By using two different methods of quantifying energy stores, I have created a unique time series of cetacean body condition that spans multiple eras. Historical whaling records have provided a measure to quantify cetacean energy stores and their variation within and between species, demonstrating the substantial energy stores required by capital breeders to complete their annual migration. Photogrammetric techniques have provided a method to quantify energy stores of free-swimming whales during the annual migration, demonstrating that maternal energy stores vary between years and influence offspring growth and development. Information on energy stores can be used to assess individual or population health in a recovering population or expanded over time to monitor biological response to environmental change. Identifying the expanded extent of the BSD humpback whale population has highlighted the need for monitoring the location and extent of calving areas to allow appropriate management of important habitat, particularly where high levels of anthropogenic activity may occur.