I thought that it is possible to directly measure the number of individuals by determining the number of haplotypes in the region of fast evolution speed as a method to more accurately and easily measure the amount of resources. Then we focused on the regulatory region (D-Loop) on mitochondrial DNA (mtDNA), which is known as a region with fast evolution rate. It is known that the D - Loop region has an evolution rate that is 100 times faster than that of the nuclear genome, and it is a region with many mutations within the same species. We investigated haplotype diversity in eel and measured haplotype number of D - Loop sequence using DNA contained in exfoliated cells of eel present in eel breeding water, and for each eel contained in breeding water We proposed a new method to calculate the number of individuals more accurately.