Overexploitation and subsequent collapses of major worldwide fisheries has made it clear that marine stocks are no inexhaustible. Unfortunately, the perception remains that marine fished are resilient to large population reductions, as even a commercially 'collapsed' stock will still consist of millions of individuals. Coupled with this notion is the idea that fisheries can, therefore, have little effect on the genetic diversity of stocks. We used DNA from archived otoliths collected between 1924 and 1972 together with 2002 juvenile;s tissue to estimate effective population size (Ne) in plaice (Pleuronrctes platessa). Ne was estimated at 20,000 in the North Sea and 2000 in Iceland. These values are five orders of magnitude smaller than the estimated census size foe the two locations. Populations examined between 1924 and 1960 were in Hardy-Weinberg equilibrium, whereas populations examined after 1970 were not. Extensive testing was performed to rule out genotyping artefacts and Wahlund effects. The significant heterozygote deficiencies found from 1970 onward were attributed to inbreeding. The emergence of inbreeding between 1905 and 19070 coincides with the increase in fishing mortality after World War II. Although the biological mechanisms remain speculative, our demonstration of inbreeding signals the need for understanding the social and mating behaviour in commercially important fishes.