The government of the People′s Republic of China implemented a seasonal fishing moratorium annually from 16 June to 15 September in the East China and Yellow Sea from 26°00′ to 35°00′N since 1995. The fisheries resources could be restored during the moratorium. The major environmental factors affecting the distribution and productivity of exploited marine fishes include: temperature, salinity, food availability, and larval supply. These factors are dependent on the dynamics of water masses. As fishing is prohibited during the moratorium, the restoration of fish resources can reflect the effect of water masses on the exploited fishes. Using cluster analysis, this study analyzes the distribution of the water masses in the East China Sea region from June 2003 to June 2005. Their relationships with the effects of the seasonal fishing moratorium are discussed. The results show that the areas of the East China Sea Surface Water Mass are 3.86×10⁴km², 664×10⁴km², and 8.33×10⁴km², while the Yellow Sea Surface Water Mass are 3.86×10⁴km², 5.71×10⁴km², and 2.32×10⁴km². The areas of the Yellow Sea-East China Sea Mixing Water Mass are 5.87×10⁴km², 7.41×10⁴km², and 648×10⁴km². The areas of the Coastal Diluted Water Mass are 2.78×10⁴km², 1.39×10⁴km², and 1.85×10⁴km². For the Subsurface Water Mass, the areas are 9.57×10⁴km², 8.64×10⁴km², and 10.80×10⁴km². The areas of the Yellow Sea Cold Water Mass are 10.03×10⁴km², 7.41×10⁴km², and 8.03×10⁴km². The areas of the Yellow Sea Bottom Water Mass are 1.85×10⁴km², 3.40×10⁴km², and 1.54×10⁴km². During the summer fishing moratorium, abundance of exploited pelagic fish stocks increased by 6.48 and 22.24 times in 2003 and 2005, respectively, which represent the slowest and fastest rates in 2003 to 2005. The major fishing ground for pelagic fishes is from 28°00′ to 32°30′N, west of 125°30′E, where the East China Sea Surface Water Mass dominates in September. For the demersal fishes, the largest growth in abundance is 10.53 times recorded in 2005, while the lowest growth was 7.5 times recorded in 2003. The major fishing ground for the demersal fishes is from 29°30′ to 33°00′N, west of 127°00′E, where the East China Sea Subsurface Water Mass dominates in September. It is concluded that the rate of restoration of the demersal fish resources in June is positively related to the strength of the East China Sea Surface Water Mass, but negatively related to the strength of the Yellow Sea Cold Water Mass. As such, we can predict the effects of the summer fishing moratorium on the fish resources from the strength and distribution of water masses in the East China Sea and Yellow Sea.