What is capture recapture?

Capture-recapture process showing first sample of 50 fish tagged and released, mixed in a pond, then second sample of 40 fish with 5 tagged, labelled M, S and R.

Capture recapture is used when a population is too large or hidden to count directly. You take a first sample, mark it, release it, then later take a second sample and see how many marked individuals are found again.