Petersen capture- recapture formula

N= Mn divide m

Or

m divide n = M divide N

M= number of fish marked then released
n = size of the recapture sample
m = number of the marked fish in recapture sample

The population is closed- no migration

All members of the population are equally likely to be captured in each sample

Capture and marking do not affect catchability and markings are not lost

The population does not change due to deaths or births between sampling occasions

The sample is large enough to be representative of the population