Document Type

Report

Department/Program

Virginia Institute of Marine Science

Publication Date

1982

Series

Special scientific report No. 111

Abstract

The James River is the primary source of seed oysters for planting private beds in the Chesapeake Bay. A sharp decline in setting rates after 1960 accompanied cessation of oyster culture on private beds in the lower sector of the river. These broodstocks were eliminated and the beds were barren after 1960. High salinities permitted a new oyster disease caused by Minchinia nelsoni (MSX) to make planting in the lower river hazardous because of high mortalities (Andrews, 1964 and in press).

Studies of larval transport mechanisms were begun in the James River in 1950, and extensive sampling was done from 1963-1965. Scarcity of larvae, even during moderate setting of early years, was a problem, especially lack of advanced larvae.

During the years 1963-1965, thousands of plankton samples were collected with submerged pumps ·at various depths for larval counts. Advanced larvae remained scarce, but early-stage larvae were found at all depths and river-wide in channel and shoal waters. A pattern of high counts around high slack water and low counts around low slack water was found. Shallow waters and boundary zones exhibited fewer larvae than mid-depth zones in the channel.

A conceptual theory of larval transport is based on belief that healthy larvae swim continuously. Dispersal is extensive and distribution river-wide before setting occurs after 10 to 14 days of pelagic life. Although larvae were patchy in distribution, setting exhibited a declining gradient of intensity from river mouth to upper end of the oyster-growing sector. Setting records were collected in the 1940's and 1950's when spatfall was heaviest (Andrews, 1982a).

Data show that all species of bivalve larvae exhibit these fluctuations in abundance with tidal stage. Because low freshwater discharge in late summer results in weak salt-wedge regimes in the channel, transport of larvae from channel to oyster bed flats is believed to result from net non-tidal flow and trapping of larvae in shallow waters by boundary effects on currents.

Numerous broods of larvae occurred in the James River over a continuous setting period of 90 to 100 days prior to 1960. Losses of larvae due to dispersion and predation are much higher than in trap-type estuaries which are typical of most seed areas in Chesapeake Bay. A large stock of brood oysters appears to be necessary to insure adequate reproduction in these open-type estuaries with high flushing rates. -

Keywords

Oysters -- Larvae -- Virginia -- James River

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