CCAMLR currently establishes catch limits each year for Patagonian toothfish, Dissostichus eleginoides, that can be sustained over the generation time of the species, maintaining the population at a productive level with only a small chance of become depleted. The increasing levels of illegal, unregulated and unreported (IUU) fishing for toothfish in the CCAMLR area threatens the success of CCAMLR achieving ecologically sustainable fisheries as well as threatening the status of populations of this species. As a result of IUU fishing, the catch limits set by CCAMLR are being reduced each year to compensate for the effects of historical levels of IUU catches. To date, the rate of reduction of the legal catch limits does not seem to be commensurate with the rising levels of IUU catches. This paper explores the consequences of different rates of IUU fishing to the legally-binding TACs set by CCAMLR. The scenarios explored are with IUU annual catch at 0.33x, 1x, 2x and 4x a legal catch limit derived from an assessment based on the 2001 WG-FSA assessments, which is approximately 3 000 tonnes. The IUU catches are applied with the legal catch limit being adjusted each year after the IUU catch and the legal catch were combined in the catch history and a new long-term sustainable annual catch was estimated using the GYM and applied as the legal catch limit for the following year, assuming no future IUU fishing. If there was no IUU fishing, the long-term sustainable annual yield is set according to the escapement part of the decision rule. With fishing at levels greater than the long-term sustainable yield because of IUU fishing then the threat of depletion is increased and the legal catch limit needs to be reduced following each year that the total catch is above the sustainable levels. The closure of the legal fishery occurs when the spawning stock has been almost depleted to 20% of the pre-exploitation median biomass with a high probability of being depleted during the future projection period. Thus, low levels of IUU fishing may be tolerated, but only in the short term, while the spawning stock is above the target status of abundance. A longterm sustainable fishery requires that IUU fishing be at least reduced to levels that are much less than the longterm sustainable annual yield estimated using the CCAMLR assessment process. Continued IUU fishing at the rates reported by CCAMLR in 2001 will result in a closure of the legal fisheries in the Indian Ocean within the next 12 years because it is in excess of the estimated long-term sustainable annual catch level. The rate of reduction of the legal TAC is low because the effects of IUU fishing are averaged out in the assessment process over 35 years, assuming no more IUU fishing takes place following the assessment. The consequence of IUU fishing is a precipitous decline in the legal TAC once IUU fishing has mined the stock to a point that there is a high probability of the stock becoming depleted during the future projection period. Consequently, action to control IUU fishing would be too late if it was delayed until there was a significant reduction in the long-term annual yield obtained in the assessment process.