why cant mark and recapture methods be used for stationary organisms

Answer 1

From the Internet:

Mark and recapture is a method commonly used in ecology to estimate an animal population's size. A portion of the population is captured, marked, and released. Later, another portion is captured and the number of marked individuals within the sample is counted.

Sessile organisms don't move. Therefore, their numbers can't be estimated by a catch and release method.