Figure 3. Stock recruitment relationships for study systems, fit with the Ricker stock-recruitment model.

The difference between recruitment and the replacement line is considered surplus production that can be sustainably harvested. This difference is maximized at E_MSY, but the lower and upper target escapements are often well below estimates of E_MSY. The escapement in the absence of the fishery, E_m, is estimated at the steady state of the Ricker model, which is best visualized at the intersection of the Ricker and replacement lines.