Appendix B2: Replacement Yield Model Assessments of Gulf of Maine-Georges Bank Witch Flounder

The “replacement yield” (RY) assessment model is the simplest possible form of an age-aggregated dynamic surplus production model approach. The underlying rationale is that if a resource has been without trend in abundance over a period of time, the (average) annual sustainable yield from the resource over that period is given by the average annual catch. The term “replacement yield” is often used in this context rather than “sustainable yield”, with the two being equivalent in an “average over time” sense, as technically RY is that catch which will leave (a specified component of) the resource biomass at the end of a year unchanged from the level at which it started that year. The approach extends naturally to a situation where the resource is trending up or down, in which case the RY will be either greater or less than the past average catch, and to an extent that depends on the size of the trend and the size of the underlying biomass.