Things that do not change over time. In fishery science, refers usually to a parameter or life history character-istic that does not change over time. The concept is closely related to that of equilibrium, although it is possible to have one without the other. For example, yield-per-recruit can be stationary (constant) if the fishing and natural mortality, exploitation pattern, and growth remain stationary, but the overall yield can vary over time if the number of recruits varies.