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Capture and recapture

Populations that are hard to count directly can be estimated by capture and recapture. You mark a number of individuals, release them and let them mix. Later you catch again and see what share is marked. If the marked ones have spread evenly, the marked share in the second catch is about equal to the marked share in the whole population. The method assumes that marking does not change survival or catchability, and that the population does not change much between the two catches.

N = \frac{n_1 \cdot n_2}{m}

estimated population

Symbols

$n_1$	marked the first time
$n_2$	caught the second time
$m$	of these that are marked
$N$	the population

Example

You mark 50 fish. Later you catch 40, and 5 are marked.

$N = 50 \cdot 40 / 5 = 400$ fish.

If the marked fish are easier to catch again, the population is underestimated.

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