Which method is used when species move too quickly to count directly?
Mark–recapture is designed for mobile organisms that cannot be reliably counted in place. Investigators capture an initial sample, mark individuals harmlessly, release them, allow mixing, and later capture another sample. The proportion of marked animals in the second catch estimates the marked proportion in the entire population, leading in the simplest case to N ≈ MC/R. Quadrat sampling is better suited to sessile or slow-moving organisms, while line transects estimate occurrence or detectability along a path and do not by themselves solve rapid movement among count locations. Satellite tagging tracks movements of selected animals but is not the standard basic abundance estimator implied here. Mark–recapture remains valid only when marks persist, marked and unmarked individuals have comparable survival and capture probability, and population closure or an appropriate open-population model is used. Fast movement can actually help marked organisms mix, but movement across the study boundary can violate closure. More advanced capture–recapture designs estimate survival, recruitment, and detection separately when repeated sampling occasions are available.
Ref: Ecology: Concepts and Applications, Molles, 9th Ed., Ch. 9