Enter your capture session data — including Total Caught, Previously Marked, and Newly Marked animals across up to three sampling rounds — and the Capture-Recapture (Schnabel) Calculator will estimate your population size (N̂), along with Total Animals Caught, Total Animals Marked, and your Overall Recapture Rate.
Estimated Population Size (N̂)
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Total Animals Caught
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Total Animals Marked
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Overall Recapture Rate
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The Schnabel method is an extension of the Peterson method that uses multiple capture-recapture sessions to estimate population size. It provides a weighted average of Peterson estimates from each sampling event, making it more accurate than single-session methods.
While the Peterson method uses only two capture sessions, the Schnabel method incorporates data from multiple sampling sessions. This reduces sampling error and provides more reliable population estimates by accumulating marks over time.
The method assumes that the population is closed (no births, deaths, immigration, or emigration), marks don't affect catchability, all animals have equal probability of capture, and marks are not lost between sampling sessions.
While you can use as few as 2 sessions, 3-5 sessions typically provide better estimates. More sessions reduce sampling variance, but diminishing returns occur after 5-6 sessions in most cases.
If R = 0 for any sample, the population estimate becomes undefined. This suggests either a very large population, poor marking technique, or mark loss. Consider increasing sampling effort or checking marking methods.
The recapture rate indicates the proportion of previously marked animals caught in subsequent samples. Higher rates (>10%) suggest good marking retention and adequate sampling, while very low rates may indicate problems with the study design.
The Schnabel method works best for species that can be safely captured, marked, and released without affecting their behavior or survival. It's commonly used for fish, birds, mammals, and some invertebrates.