The rise and fall of Crassostrea virginica oyster reefs: The role of disease and fishing in their demise and a vignette on their management

摘要

We describe a model designed to simulate the shell carbonate budget of an oyster reef.We identify five parameters descriptive of basic characteristics of the shell carbonate budget of a reef that limit simulation accuracy. Two describe the TAZ (taphonomically-active zone) and the distribution of shell carbonate within it. One is the taphonomic rate in the TAZ. Two determine the volume contribution of shell carbonate and the taphonomic loss rate within the reef framework. For Mid-Atlantic estuaries, model simulations suggest that reef accretion only occurs if oyster abundance is near carrying capacity. Simulations further suggest that reef accretion is infeasible for any estuarine reach where dermo is a controlling influence on population dynamics. We forecast that the oyster disease dermo is a principal antagonist of reef persistence through its ability to limit shell addition. Model simulations suggest that reefs with inadequate shell addition 'protect themselves' by limiting the volumetric content of shell carbonate in the TAZ. Thus, a dominant process is the transient expansion and contraction of the shell resource, otherwise termed cultch, within the TAZ, rarely expanding enough to generate reef accretion, yet rarely contracting enough to foster erosion of the reef framework. The loss of framework carbonate thusly is curtailed during periods when the surficial shell layer deteriorates. Stasis, a reef neither accreting nor eroding, is a preferred state. Reef recession requires an inordinately unbalanced shell carbonate budget. Results strongly argue for expanded focus on the dynamics of the shell resource within the TAZ, as this likely fosters a feedback loop with abundance through recruitment, serves as the protective layer for the reef during periods of reef stasis, and establishes the threshold conditions for reef accretion and recession. Model simulations suggest that attaining maximum sustainable yield and maintaining a biomass capable of supporting sufficient shell production for reef accretion are irreconcilable goals over a large component of the oyster's range. Reef stasis would appear to be the only achievable restoration goal in Mid-Atlantic estuarine reaches where dermo holds sway. Exploitation rates much above 5% of the fishable stock per year restrict availability of surficial shell and foster reef erosion. In contrast, in the Gulf of Mexico at the high-productivity end of the oyster's range, an enhanced fishery and reef accretion may be compatible goals.