1. Fishing changes the structure of fish communities and the relative impacts of fishing are assessed usefully against a baseline. A comparable baseline in all regions is fish community structure in the absence of fishing. 2. The structure of unexploited communities cannot always be predicted from historical data because fisheries exploitation usually precedes scientific investigation and non-fisheries impacts, such as climate change, modify ecosystems over time. 3. We propose a method, based on macroecological theory, to predict the abundance and size-structure of an unexploited fish community from a theoretical abundance-body mass relationship (size spectrum). 4. We apply the method in the intensively fished North Sea and compare the predicted structure of the unexploited fish community with contemporary community data. 5. We suggest that the current biomass of large fishes weighing 4-16 kg and 16-66 kg, respectively, is 97.4% and 99.2% lower than in the absence of fisheries exploitation. The results suggest that depletion of large fishes due to fisheries exploitation exceeds that described in many short-term studies. 6. Biomass of the contemporary North Sea fish community (defined as all fishes with body mass 64 g-66 kg) is 38% lower than predicted in the absence of exploitation, while the mean turnover time is almost twice as fast (falls from 3.5 to 1.9 years) and 70% less primary production is required to sustain it. 7. The increased turnover time of the fish community will lead to greater interannual instability in biomass and production, complicating management action and increasing the sensitivity of populations to environmental change. 8. This size-based method based on macroecological theory may provide a powerful new tool for setting ecosystem indicator reference levels, comparing fishing impacts in different ecosystems and for assessing the relative impacts of fishing and climate change.