Estimation of annual mortality rates for eastern oysters (Crassostreavirginica) in Chesapeake Bay based on box counts and application ofthose rates for population growth projections

摘要

In an effort to restore the ecological role of oysters in Chesapeake Bay and the economic benefits of a commercial fishery, the states of Maryland and Virginia are considering the introduction of the non-native Asian oyster, Crassostrea ariakensis. As part of the ecological risk assessment (ERA) to evaluate the proposed action and restoration alternatives, demographic modeling is applied to project the change in populations of both Asian and eastern oyster populations in the Bay in space and time. We present two approaches to parameterize the annual mortality rates for the Asian and eastern oyster for the demographic model. Mortality rates were estimated from empirical data collected by the Maryland Department of Natural Resources (DNR) in annual surveys of oyster beds in Maryland. We compared counts of recent boxes (dead oysters including gapers about two weeks old, in which tissue is still found within the shell, as well as boxes with no fouling or sedimentation on the inner valve surfaces), old boxes (dead oysters in which fouling and/or sedimentation is found on the inner valve surfaces and no tissue remains), and live oysters in both market and small size classes. Our mortality estimates from recent box counts consistently differentiated between years with high versus low disease intensity and wet and dry years, and also between salinity zones. In contrast, traditional estimates of yearly mortality based on total box counts were often out of phase with measured disease intensity levels, and type of weather year (dry or wet).