SMRU > Research > Demographics
The nuts and bolts of all population processes include such factors as the mortality rates, ages at sexual maturity, fecundity and longevity of the individuals within a population. These factors all need to be estimated if we are to fully understand how populations might change through time, and are crucial for any population modelling. Each of these factors on its own may also provide some insight into the conservation status of the population concerned, especially if long term data are available
Fundamental to all demographic studies is our ability to determine the age of any individual marine mammal. The most frequently used method involves taking a section a tooth and after suitable preparation, counting annual growth layers, in much the same way as trees are aged. Teeth are routinely collected from any dead animals that are sampled by SMRU, and stored for later analysis. After cleaning they are decalcified – and then sectioned and stained so that incremental growth layers within the structure of the teeth can be counted. If a large enough and unbiased sample of the population can be obtained, then the resulting age frequency distribution can tell us about the longevity and the age-related survival or mortality of the population being studied. Furthermore, if we can determine the sexual maturity status of each animal from which a tooth has been taken, we can also estimate the age at sexual maturity.
Fecundity is usually estimated by examining the uterus and ovaries of dead females and looking for signs of pregnancy or recent ovulation. Where individual animals can be identified in long term studies it is also possible to obtain minimum estimates of fecundity for individuals. Mortality rates can also be estimated be capture-recapture methods, from photo-id studies or by tagging or marking individuals of known age and tracking their persistence in the population through the time