Mark and Recapture Techniques for Determining Population Size.

Often it is not possible to directly count all of the organisms in a habitat to determine the size of a population. In these cases an estimate of the population size can be made by marking a segment of the population at one time and later recapturing the organisms (hence "Mark and recapture techniques"). The ratio of marked to unmarked individuals in the second sample can then be used to estimate the total number of organisms in the population.

The procedure for estimating population size begins with the capture of individuals at the study site. The organisms are then marked in some way so that they can later be identified by the investigator. Typical marking methods include ear tags, leg bands, dyes or paints, and clipping of fins or toes (ouch!). These animals are then returned to the habitat and allowed sufficient time to mix with the other members of the population, thus establishing an as yet unknown ratio of marked to unmarked animals in the total population. After an appropriate period of time, a second sample is collected to determine the ratio of marked to unmarked animals in the total population. Since a record is kept of the number of marked individuals in the first sample and the number of marked and unmarked animals in the second sample, this information can be used to estimate the total number of individuals in the population by the following formula:  

N =    M_t (U_c + M_r)
                   M_r       
Where:

• N = The estimated population size.
• M_t  = The number of organisms marked, then released during the first sampling interval.
• U_c = The number of unmarked individuals captured during the second sampling interval.
• M_r = The number of marked individuals in captured during the second sampling interval.

As an example, assume that 50 animals were captured, marked, and returned to a population during the first sampling period (M_t = 50). On returning to the field, 60 animals are captured. Of these 60, 55 are unmarked (U_c = 55 and M_r = 5). Plugging these values into our formula:  

N = M_t (U_c + M_r)  = 50 (55 + 5) = 3000 = 600
               M_r                      5                 5

The estimated number of animals in the entire population is 600. 

There are three assumptions of the mark and recapture technique:

1. Marked and unmarked animals are captured randomly. Field mice may become trap-happy or trap-shy. If your marks make the animals more conspicuous, you may collect too many marked animals during the recapture phase of the survey.
2. Marked animals are subject to the same mortality rate as unmarked organisms. Fish marked at sea may be weakened and suffer higher mortality or marks may make your study animal more conspicuous to predators.
3. Marks must not be lost or overlooked. Leg rings can be lost by birds or arthropods may molt and lose the mark.