So what should a natural mortality curve look like for oysters?

Natural mortality rate (M) of a population describes the interaction of recruitment, growth and loss to environmental factors (both physical and biological). It reflects species life history traits, a product of selection over evolutionary time scales. Fishing mortality (F) describes loss to exploitation. Estimates of natural mortality in extant oyster populations arguably diverge fromthese evolved norms because of cumulative impacts of environmental degradation, age truncation by disease, and fishing. Pre-1900 literature describes very large oysters that, extrapolating from truncated modern growth curves, are suggested to have terminal ages in the 15–20 year ranges. The lengths of these oysters can be used with Hoenig plots to estimate natural mortality in preexploitation, pre-disease situations. A Hoenig plot inherently suggests a constant mortality rate with increasing age, but is this correct for oysters? We describe a length frequency distribution for an unexploited population of Crassostrea gigas, currently invading the Oosterschelde in the Netherlands, that includes representation of all size classes up to 200mm in length – an analog of a preexploitation, pre-disease population. From this demographic we suggest a probabilistic age structure and estimate age specific mortality for a long-lived, undisturbed oyster population.