The method also assumes that no individuals are born or die during the period over which sampling is done. The mark and recapture method involves marking a number of individuals in a natural population, returning them to that population, and subsequently recapturing some of them. This provides a basis for estimating the size of the population at the time of the marking and release. This procedure was used by C.J.G. Petersen in studies of marine fishes and F.C. Lincoln in studies of waterfowl populations, and is often referred to as the Lincoln Index or the Petersen Index. It is based on the principle that if a proportion of the population was marked in some way, returned to the original population and then, after complete mixing, a 2 nd sample was taken, the number of marked individuals in the 2 nd sample would have the same ratio to the total number in the 2 nd sample as the total of marked individuals originally released would have to the total population.
All individuals are equally likely to be caught within each capture period.
That is, marked individuals must not become either easier or more difficult to catch during the second capture period compared to unmarked individuals. However, the time period must not be so long that assumption 1 breaks down. An alternative to the previous two methods is the depletion method of estimating population size. Depletion is based on the assumption that with repeated sampling of a population over a short period of time, a constant fraction of the remaining population will be taken in each sample. The slope of the regression line is the estimate of the fraction of the remaining population taken with each sample.