Oyster reef broodstock enhancement in the Great Wicomico River, Virginia

摘要

The Great Wicomico River is a small, trap-type estuary on the western shore of the Chesapeake Bay that once supported substantial oyster populations. These populations were essentially eliminated by the combined effects of Tropical Storm Agnes in 1972, and subsequent disease mortalities related to Perkinsus marinus and Haplosporidium nelsoni. Oyster broodstock enhancement was initiated in June 1996 by the construction of a three-dimensional intertidal reef with oyster shell, followed by the "seeding," in December 1996, of that reef with high densities of large oysters from disease-challenged populations in Pocomoke and Tangier Sound. Calculations of estimated fecundity of the reef population suggest that oyster egg production from this source is within an order of magnitude of total egg production in the Great Wicomico River prior to Tropical Storm Agnes. Field studies in 1997 indicate spawning by reef oysters from July through September. P. marinus prevalence increased from 32% in June to 100% in July, whereas intensity increased from June to September; H. nelsoni was absent. Plankton rows recorded oyster larval concentrations as high of 37,362 +/- 4,380 m(-3) on June 23. Such values are orders of magnitude higher than those typically recorded in Virginia subestuaries of the Chesapeake Bay in the past three decades, and lend support to a premise that aggregating large oysters may increase fertilization efficiency. Drifter studies suggest strong local retention of larvae, a suggestion reinforced by marked increases in local oyster spatfall on both shellstring collectors and bottom substrate compared with years prior to 1997. In locations where local circulation promotes larval retention, the combination of reef construction with broodstock enhancement may provide an accelerated method for oyster population restoration.