The average biomass of a stock that has yet not been fished (in an equilibrium sense). Biomass of an unexploited (or quasi unexploited) stock. Rarely measured. Most often inferred from stock modeling. Used as a reference value to assist the relative health of a stock, monitoring changes in the ratio between current and virgin biomass (B/B0). It is usually assumed that, in absence of better data, B = 0. 30 B0 is a limit below which a stock should not be driven.