P>This research examines the current water quality status of Izmir Bay, using the trophic index (TRIX) as a tool for the regulation of Turkish marine finfish aquaculture. In 2007, new legislation was enacted in Turkey related to the protection of coastal waters, especially those of enclosed bays and gulfs, from pollution by fish farming. However, the legislation does not apply to any other coastal zone stakeholders; for this reason the entire Izmir Bay was examined. Use of the composite trophic status index (TRIX) produced mean TRIX values of 3.6 for the aquaculture area (AA) and 2.5 for areas of the outer bay where no aquaculture takes place; this indicates 'no risk of eutrophication' as defined by Turkish law. In the inner Izmir Bay, there is a mean TRIX value of 4.3, which is above the threshold of four and typical for 'high eutrophication risk' areas, as expected because of heavy urbanization. The study then applied the UNTRIX indices adjusted to local conditions, revealing that both the inner bay and the aquaculture area (AA) can be classified as of 'poor' status whereas the outer bay can be defined as 'good'. The UNTRIX-based trophic classification is in good agreement with TRIX for both the outer and inner parts of the bay; however, there is no agreement regarding the classification of the aquaculture area.