The yield from a fishery which provides the greatest overall benefit to the nation with particular reference to food production and recreational opportunities; it is based on MSY as modified by economic, social or ecological factors. Precision and Accuracy Precision is the closeness to each other of repeated measurements of the same quantity or object, while accuracy is closeness of a measured or computed value to its true value.

The amount of fish that will provide the greatest overall benefit to the Nation, particularly with respect to food production and recreational opportunities and taking into account the protection of marine ecosystems. MSY constitutes a "ceiling" for OY. OY may be lower than MSY, depending on relevant economic, social, or ecological factors. In the case of an overfished fishery, OY should provide for rebuilding to BMSY.

In addition to hydrologic considerations (see Safe Yield and Practical Sustained Yield), optimum yield accounts for social, economic, and legal factors. Optimum yield may allow short-term, intensive extractions (conscious overdevelopment) or, conversely, no development at all. “Optimum yield is in the realm of the planner; a hydrologist can assist, but the decisions regarding what constitutes optimum yield may be largely nonhydrologic.” (U.S. Water Resources Council. 1980. Essentials of Ground-Water Hydrology Pertinent to Water-Resources Planning, Bulletin 16, revised). The ISWS will no longer use this term. Instead, the ISWS elects to use the term Yield and explicity state all conditions and assumptions that stand behind the determination of it.