How do we study marine mammal populations?
One way to study marine mammal populations is to identify and study individual animals. Marine mammals are identified by natural markings (such as color patterns or scars), or by applying tags and brands. By resighting identified animals from year to year, we can learn a great deal of information, such as the movements, reproductive success, and survival of individuals. This, in turn, can help us learn things about the population as a whole.
How do scientists identify individual animals?
There are many methods used to identify marine mammals. Some of these methods are long-lasting, such as using natural markings, or branding animals. Other methods are more temporary, such as marking animals with bleach (which lasts for several months), or applying tags (which may last for several years). The appropriate method of identification usually depends on the species of interest and its environment. Questions that help determine the appropriate method include:
Can the animal be handled for tagging or branding? If it is hard to handle the animal, natural markings may be more appropriate to use in identifying the animal.
Is it more practical to observe the animal from a distance? Branding may be more appropriate than tagging if the animals are observed from a distance (because brands are usually larger than tags) and if animals are to be identified for life
Are natural markings large enough to be reliably seen?
Some methods of identification also give insight into genetic relationships. DNA analysis of skin plugs not only enables identification of individuals, but also allows researchers to look at paternity and interbreeding between populations.
Two different humpack whale flukes. NMML scientists use the markings on flukes to identify individual whales.
Why study individuals?
By identifying and learning about individual animals, we gain insight into their life histories, and can determine statistics for the population as a whole. Examples of these statistics include:
Age-specific survival rate. In other words, the proportion of animals at a particular age that survive. For example, only 50% of 1-year-olds survive, while 85% of 8-year-olds survive.
Age of first reproduction. The average age (for a population) that an animal first gives birth. This age can be different for different populations, even if the species is the same. For example, elephant seals at one location have an average age of first reproduction of 5 years, but elephant seals at another location may have an average age of first reproduction of 3 years.
Age-specific reproductive rate (natality). The proportion of animals at a particular age that have young. For example, 60% of 5-year-olds have a pup, while 85% of 8-year-olds have a pup.
Longevity. How long an animal lives or maximum age.
We can also use identified animals to determine differences between the behavior of males and females of different ages. In some seal species, immature males look very similar to adult females. If we can distinguish immature males from females, then we can study differences in behavior. By identifying individuals, we can also gain more insight into social behavior and relationships within groups (for example, family relationships within killer whale pods).
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