How does mark-recapture work?

Mark-recapture is a method of estimating a population size. A sample from the population is captured using an appropriate method, marked, and released. A second sample is then captured after an interval and the total number of organisms in the sample, and the number within the sample who are marked, are recorded. The mark-recapture method works by assuming that the proportion of marked organisms recaptured in the second sample is the same as the initial marked in the total population. This is represented by the equation R(recaptured)/C(captured in 2^nd sample)=M(marked initially)/N(total number in population). In this equation the number marked, captured, and recaptured are known, so the equation can be rearranged to give N=(MxC)/R. This method will not work as an estimate of population size if the population is fluctuating so several key assumptions are made, all relating to population size. It is assumed that birth rate is the same as death rate, that there is not emigration or immigration, that there is no population depletion via predation, and that marking does not influence survival of the marked individuals. As can be seen, if these assumptions are not true then the proportion of marked individuals within the total population will be changing preventing an accurate estimate of population size.