Relationship of crab density distribution with environment in the Oujiang estuary during summer and autunm

A study was conducted to define summer and autumn distribution patterns of crabs in the Oujiang Estuary and to relate these patterns to environmental variation. The result indicated that the weight densities in June and September were 112.99 and 73.48 kg/km², respectively. Yet, the number densities were 16.84×10³ ind/km² and 3.80×10³ ind/km², respectively. In June, both of weight and number densities in the south-east studied area were higher than those in the west, and that of the south area was higher than that of the north area, in September, however, those in the east area was lower than the west. The crabs continually stayed at outer waters of investigated areas in June, especially in south-east where the water is affected by the Taiwan Warm Current, but higher density in south coastal waters in September. The low crab density in north waters of the Oujiang Estuary was ascribed to the existence of the Oujiang dilute water. Numerically Charybdis japonica was the most important crab species, but Portunus trituberculatus was a critic economical species. The hydrological factors were significantly related with weight density but not significantly with individual density. The total standing stock of crabs decreased from 50 845.5 kg in June to 33 066.0 kg in September in the whole fishing ground.