The mark-release-recapture formula only gives a valid estimate if certain assumptions are true. If any of them is false, the estimate will be inaccurate.1. Marked individuals mix randomly with the rest of the population, so that the proportion marked in the second sample matches the proportion in the whole population. Enough time must be left between samples.
2. Marking does not affect survival. For example, bright paint must not make animals more visible to predators, and marking must not harm them.
3. Marks are not lost, for example by wearing off or when an arthropod moults.
4. There are no significant births, deaths, immigration or emigration between the two samples, so the population size stays the same.
5. Marked animals are no more or less likely to be recaptured. For example, they must not learn to avoid traps or be attracted back to the bait.Breaking an assumption changes the number of marked animals in the second sample, which is the divisor in the formula:• If marks are lost, or marked animals are eaten, fewer marked animals are recaptured, the divisor is too small and the population is overestimated.
• If marked animals become trap-happy and return to the traps, too many are recaptured and the population is underestimated.Worked example
40 animals are marked, and a second sample of 50 contains 10 marked, giving 200. If 2 marks had been lost, the true number marked would be 12, giving 40 × 50 ÷ 12 = 167. The lost marks made the estimate 33 too high.
2. Marking does not affect survival. For example, bright paint must not make animals more visible to predators, and marking must not harm them.
3. Marks are not lost, for example by wearing off or when an arthropod moults.
4. There are no significant births, deaths, immigration or emigration between the two samples, so the population size stays the same.
5. Marked animals are no more or less likely to be recaptured. For example, they must not learn to avoid traps or be attracted back to the bait.Breaking an assumption changes the number of marked animals in the second sample, which is the divisor in the formula:• If marks are lost, or marked animals are eaten, fewer marked animals are recaptured, the divisor is too small and the population is overestimated.
• If marked animals become trap-happy and return to the traps, too many are recaptured and the population is underestimated.Worked example
40 animals are marked, and a second sample of 50 contains 10 marked, giving 200. If 2 marks had been lost, the true number marked would be 12, giving 40 × 50 ÷ 12 = 167. The lost marks made the estimate 33 too high.