Is Oyster Shell a Sustainable Estuarine Resource?

The decline of the eastern oyster (Crassostrea virginica) as an estuarine resource is well documented for many estuaries on the United States east coast. This decline is often associated with a decline in the shell resource and ultimately the disappearance of the shell bed. We develop a model that expressly and conjointly evaluates oyster abundance and surficial shell quantity and examine whether stability in the stock and the habitat can be simultaneously achieved. Simulations suggest that a steady-state shell content exists for any set of recruitment and naturalmortality rates and that the amount of shell present at steady state varies over a wide range as recruitment and natural mortality vary. Shell mass is maximized at a natural mortality rate near the rate observed in unfished populations unimpacted by disease. A species dependent on the maintenance of hard substrate for survival, as is the oyster, might have a life span adapted to maximize the accretion of carbonate; thereby sustaining the substrate on which it depends. Relatively small changes in the recruitment rate produce large changes in abundance and consequently shell mass and the scale of variation dwarfs that of natural mortality or fishing. Only variations in the rate of shell loss or the average size of animals at death produce equivalent excursions in shell mass. In comparison, the ambit of natural mortality imposed by the disease process fortuitously occurs in a range that restrains the change in carbonate mass, probably because increased mortality reduces abundance but also increases the death rate, thus adding more shell. Simulations covering a range of fishing rates indicate that no fishing rate exists that is likely to be sustainable of the shell resource over the long term. Fishing will always abet the taphonomic and depositional processes conspiring to debilitate the oyster bed. Successful management of the oyster shell resource is obstructed by the simple fact that no additional mortality, whether imposed by disease or through fishing, can occur that will not result in habitat loss at some rate. The shell resource is maximized when the population is at predisease natural mortality rates and unfished. Thus, if fishing is to be permitted or if disease has increased persistently the natural mortality rate, the only recourse of the manager is the perpetual addition of shell in compensation to the loss or the acceptance of the degradation of the shell bed.