Aging & diet

Diet

The food consumed by any animal is a key aspect of its ecology. The food of whales is diverse, with most whales feeding on multiple species as a result of their migrations between different oceanic areas. Baleen whales feed on a range of small animals that can be filtered from the ocean by their baleen plates which act like nets to separate the food from seawater. The most important groups that baleen whales regularly feed on are krill, copepods, mysids, small fish and amphipods. All of these groups of animals have multiple species in different oceans and seas. Toothed whales target larger prey such as squid and large fish, which they hunt by catching individual prey items. Squid and large fish are also species-rich groups, with different parts of the ocean containing different ranges of species.

Whale diet has in the past generally been studied by observation of feeding behaviour and sampling of the probable food from the water they are feeding in, or from analysing the stomach contents of whales killed as part of fishery operations. However, both of these methods are limited in the information they can provide. Observational research on whale diet is limited by the ability to effectively sample the water that the whale has been feeding in, which may be very deep, or in the case of toothed whale feeding may contain food items that are themselves very large and difficult to catch (such as giant squid, which are eaten by sperm whales). Stomach content analysis is now not possible in most areas because there are no longer commercial whale fisheries and in many countries, including Australia, lethal research on cetaceans is not legally allowed, or considered ethical. As a result of this, the AMMC has pioneered the use of DNA-based analysis of whale faeces to identify food items consumed by whales. We have used prey DNA identification to study diet of blue whales, fin whales and Bryde's whales.

Aging

Non-lethal methods for estimating the age of whales are a key goal of our current research. The age structure of a whale population is the relative proportions of whales of different ages. An age structure for a rapidly growing whale population has a relatively large proportion of young whales, whereas the population age structure for a population that has not been reproducing well, or in which whale calves fail to survive to maturity, has more older whales. The age structure of a given whale population therefore provides very useful information on the reproductive activity and survivorship of different age classes within a population.

Visual estimates of whale age are practically limited to identification of calves, sub adults and adults. This limits our ability to estimate population age structure because whales may reach adulthood in the first fifth of their lifetime, leaving the last four fifths of their lifespan indistinguishable visually. As a result of this most obvious method being uninformative, a wide range of other methods have been experimented with, including analysis of tooth growth rings from stranded toothed whales such as pilot whales and sperm whales; measurements of growth rings in ear plugs from baleen whales; and chemical changes in the eyeball. All of these methods require a dead whale to work with, however, so development of non-lethal methods to estimate whale age is imperative. The two groups of methods that have been been researched most extensively are the analyses of lipids and of nucleic acids (DNA and RNA) in either skin or blubber sampled by biopsy darts. We are researching the measurement of different relative concentrations of RNA molecules in skin and blubber of whales as a means of estimating their age.

Simon Jarman leads the diet and aging research within the Australian Marine Mammal Centre.