The approach was first used by Petersen (1896) to study European plaice in the Baltic Sea and later proposed by Lincoln (1930) to estimate numbers of ducks. Petersen's and Lincoln's method is often referred to as the Lincoln-Petersen Index, even though it is not an index but a method to estimate actual population sizes.
Their method involves capturing a number of animals, marking them, releasing them back into the population, and then determining the ratio of marked to unmarked animals in the population (Wildlife Management, 2000).
The population (P) is estimated by the formula:
# of "Marked" Fish in Sample = Total # of Marked Fish
Total # in Sample Population (What We're Solving For)
We cross multiply to solve to the estimated total population.