15. In the 1989 reassessment by DFO, a relatively crude "bulk-biomass" model used for northern cod stock assessment since 1977 was replaced with a more sophisticated age-structured model. This, in concert with a change in the relative weighting of the research vessel survey and offshore catch-per-unit-of-effort indices of abundance used to "tune" the model, led to a drastically revised estimate of stock size and trajectory. The 1989 assessment concluded that the exploitable biomass (fish aged four years and older) had not grown five-fold since 1978 as previously believed but only about three-fold and was now static (DFO/4396 1990) or possibly in decline (Harris 1990). Further, if the stock size had been seriously over-estimated, then the quotas, set to achieve a target fishing mortality (expressed numerically as some value of "F" such as Fo 1 or roughiv twenty per cent of the exploitable biomass), had, in fact, resulted in annual removals by fishing of one third or more of the available population (Alverson 1987). If this was true, then not only was the stock much smaller than had been thought but its ability to reproduce itself may have been weakened, perhaps dangerously so (Harris 1990).