Transport of bivalve larvae in James River, Virginia

For nearly 100 years, the James River has been the primary source of seed oysters for Virginia. A disease caused by Minchinia nelsoni (MSX) killed most oysters in high-salinity waters in the lower river in 1959 and 1960, and planting has not been resumed in these areas (Andrews 1983). Large populations of oysters on Hampton Bar and near the mouth of the river which served as broodstocks were destroyed. After 1960, setting declined drastically in regularity and intensity to about one tenth of that which occurred in the 1950\u27s. Setting patterns suggest two types of seed areas in Chesapeake Bay: (1) high freshwater discharge, open or flushing estuaries with light spatfalls that decrease in intensity with distance from the river mouth; the James River is a typical example; and (2) low discharge, trap-type estuaries where intensive sets are heaviest near the head of the saline sector; examples are the Piankatank and Great Wicomico rivers in Virginia. Larval transport systems in the two estuarine types differ in quantity of larvae retained and regularity of spatfalls. Hourly plankton samples in the James River during 10 days in 1964 and 1965 revealed regular cyclic abundance of larvae with tidal stages. Larvae were 5 to 10 times more numerous during high-tide periods than at low-tide periods. Mostly early-stage larvae were distributed randomly throughout vertical columns of water. Larvae of other bivalve species exhibited similar distributions and fluctuations in abundance with tidal stages. Patterns of larval distribution were similar for all depths at five stations, both in the channel and over oyster beds, during 16 tidal cycles in 1965. Frequent recruitment of new larval broods and disappearance of most oyster larvae before ages of 3 to 5 days suggest losses due to physical dispersion and predation. Only when larvae reached advanced umbo stages did they actively select deeper water strata in the channel which provided a transport system to carry them upriver. In the 1950\u27s, spatfall occurred every week in the James River from 1 July to 1 October each year; since 1960, light, erratic setting has prevailed every year. If one assumes that predation, larval ecology, and physical transport systems have not changed, it appears that broodstocks have become inadequate, or that larvae were killed by toxic substances