A mark-recapture method is not valid when the captured animal has a negative experience during its capture. Due to these bad memories, the animal might lern to avoid the traps because it would not like to experience the events; Thus, they will less likely to be recaptured. Consequently, the number of recaptured will be lower and the population size will be overestimated.
Work Step by Step
Population size = sn/x s - Number of individuals marked and released in the first sampling n - Total number of individuals recapted x - Total number of captured individuals in the second sampling