mark a fish. and let him go, then recapture!!! =) this is done to many species such as birds and fish to help estimate and conserve the population and its enviroment.

by mark tag it

a

The assumptions of the mark-recapture method include: 1) The population is closed (no immigration, emigration, or deaths) during the study period, 2) Marks do not affect the probability of capture, and 3) Marks are not lost or overlooked between captures.

When there is too much of a population to count (ex 1, 2, 3, 4,...).