The Mark and Recapture Technique

By far the most popular way to measure the size of a population is called the Mark and Recapture Technique. This technique is commonly used by fish and wildlife managers to estimate population sizes before fishing or hunting seasons. 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 as a basis for estimating the size of the population at the time of marking and release. This procedure was first 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 second sample was taken, the proportion of marked individuals in the second sample would be the same as was marked initially in the total population. That is,

R (marked recaptures)                        M (marked initially)

——————————— = ————————————

T (total in second sample)                 N (total pop. size)

By rearrangement we can estimate the population size, N, to be,

M*T

N = ———

R

For example, suppose you took 200 mice out of a forest having an unknown number of mice, put leg bands on them, return them to the forest and let them mix thoroughly. If you then take 250 mice from the forest and find 50 of them to be have leg bands, then M = 200, T = 250, R = 50, and the unknown total number of mice (N) could be estimated as:

N = M*T/R = (200)(250)/50 = 1000 mice