Since 1968, the breeding population of Weddell seals that inhabits Erebus Bay in Antarctica's Ross Sea has been under the watchful eye of scientists. Every year, several thousand adult seals haul themselves out onto the "fast ice," or sea ice that is anchored (or "fastened") to the coastline, so that females can give birth and nurse their pups. The study of this colony is one of the longest continuous scientific investigations of a long-lived mammal species in its natural habitat. To date, more than 17,000 individuals have been tagged for tracking, including all pups born within the study area since 1973.
One of the key data sets being compiled concerns body mass, or the measure of an animal's weight. Ecologist Jen Mannas is in charge of collecting and recording body mass data for mother–pup seal pairs. A seal mother nurses its pup over a 40-day period, transferring as much as half of its body mass to the pup in the process. The more weight the mother can transfer, the better the pup's odds are for survival. Pups are weighed at birth, at 20 days, and again just before they wean and begin finding food on their own. By analyzing this and other collected data, researchers can better understand how body mass relates to the overall health of the seal population.
Food availability is just one of many environmental factors that can influence a mother seal's body mass. Changes in the availability of food may affect her pup's survival rate and thereby affect the entire seal population. For example, Weddell seals mainly eat fish, including the Antarctic toothfish. In the last few years, the fishing industry has targeted the toothfish, which is commonly sold in markets and restaurants as Chilean sea bass. It is not yet known precisely what effect this may have on the Weddell seal population, but overfishing of toothfish would likely cause disruption in the food web as organisms make adjustments to their diets.
Another influential factor is the sea ice itself. In 2004, the Weddell study observed unusual conditions. Giant icebergs that calved from a nearby ice shelf blocked the exit route that annual sea ice uses when it blows out to sea each summer. When breeding season came, researchers observed far fewer numbers of adults returning to the study area and only 150 pups being born—compared with 450 to 650 pups in a typical year. The researchers believe that the trapped sea ice, being thicker and more tightly packed than normal, prevented many seals from accessing the surface. This demonstrates the impact that changing environmental conditions can have on a population.
Ecologists like Jen Mannas study animal populations in order to better understand what is called population dynamics. Population dynamics is a branch of life sciences that studies short- and long-term changes in animal populations, and the biological and environmental processes that drive these changes. What is learned through the Weddell seal study can add to a broader body of knowledge about the population dynamics of other long-lived organisms. This knowledge can, in turn, be helpful in developing wildlife conservation strategies.