The muscle that keeps its bivalve shells tightly closed relaxes, opens, and the American oysters feeds, filtering algae and plankton out of huge quantities of brackish water. Oysters start spawning in the Gulf of Mexico when the water temperature reaches sixty-eight degrees and increase their production at seventy-seven degrees. The males release their sperm into the water. When the sperm enter the females water transport system, they spawn, producing between twenty-three million and eighty-six million eggs depending at their size and age. And, they spawn several times in a season, which in Louisiana goes from March to November and peaks in late May, June, and September.
The eggs hatch six hours after fertilization and the larvae drift in the water column for two or three weeks, feed on plankton, and evolve through several stages into juveniles. Juveniles crawl in a circular pattern using a single foot, searching out a solid surface on which to attach themselves and live out their lives. Sand will do, but something harder, like other oyster shells, is better. Once settled they squirt a droplet of liquid cement from a pose in the foot, lose the foot, become spat, and develop their shells. They become sexually mature in four weeks.
Temperature, salinity, tides, turbidity, and food influence their rate of growth. Adults tolerate a salinity range of two to forty parts per thousand. In the Gulf of Mexico they do best at twelve to thirty parts per thousand. Too much salt, they grow too fast, and fail to reproduce. Freshwater can kill them.
The nooks and crannies of the hard surface of an oyster reef are the ideal habitat for small marine animals–grass shrimp, mud crabs, red beard sponge, hooked mussels, oyster drills, and barnacles–food for red drum, speckled trout, croaker, and blue crab. The anglers follow.[i]
In order to push back on the encroaching oil for the BP’s Deepwater Horizon gusher of oil heading into Breton Sound and the Barataria Basin, Louisiana opened wide the freshwater diversions into the basins, at Caernarvon into Breton Sound and at Davis Pond into the Barataria Basin. The oysters died from these massive inputs of freshwater.
In the old days, before the Corps of Engineers ran the Mississippi levees clear down the west bank to Venice and down the east bank to Bohemia, the Mississippi overflowed its banks almost yearly and poured freshwater into the Barataria Basin on the east and Breton Sound on the west. The freshwater pushed out salt water that had crept in on gulf tides during the fall and winter, when the river stayed within its banks. This is the principle of tidal exchange.
Really big floods killed the oysters the first year, but the bi-valved critters bounced back in the following years. When you understand their reproduction cycle, you understand how that is possible.
In 1814 a century after the French settled New Orleans, the brackish marshes, where oysters thrive began creeping inland in Breton Sound. Little bit by little bit, it was a slow creep inland, even after the big levees were constructed after the Flood of 1927. With the brackish marshes creeping inland, Louisiana built the first small freshwater diversions into Breton Sound in 1926 to push back salt water and increase oyster production.
After 1960, the pace picked up. Gas and oil companies, dredged access canals through the marshes, tossed their dredge into spoil banks that lined the canals, and introduced salt water into intermediate and fresh water marshes. Canals that ran north and south conveyed salt water into the marshes on the tides. Canals that ran east and west stopped the flow of water south. Marshes to the north of the spoil banks drowned; those to the south starved for water. The big tall levees prevented the Mississippi from streaming freshwater into the basin to counter the salt water.
By 1988 the brackish marshes almost reached Caernarvon at the head of the sound. First, the oysters followed the brackish marshes north. Then, they died. Too much salt in the water. By 1990 salt water had destroyed 50,000 acres of private oyster beds.
Louisiana designed the Caernarvon Freshwater Diversion Structure to push brackish marshes south and revive the oyster industry in Breton Sound. Louisiana built the Davis Pond Diversion Structure in Barataria Basin with a similar intent, push salt water out.
So the question we must ask ourselves, which is worse: freshwater killing the oysters, which will produce well next year or the year after next when the salt water creeps in on the tides and reestablishes the salinity, or oil fouling the oyster beds and who knows when they will come back?
Update, July 21: The New York Times weighed in on the plight of the oysters.
Copyright, Quinta Scott, 2010, Words and images, All Rights Reserved
[i] Stanley, Jon G., Sellers, Mark A., Species Profile: Life Histories and environmental requirements of coast fishes and invertebrates (Gulf of Mexico)–American Oyster, U.S. Fish and Wildlife Service Biol. Rep 82(11.64) U.S. Army Corps of Engineers, TR EL-82-4, 1-13, http://www.nwrc.usgs.gov/wdb/pub/species_profiles/82_11-064.pdf