PATTERNS OF CHANGE IN OYSTER POPULATION STRUCTURE THROUGH THE LATE HOLOCENE: DISTINGUISHING THE EFFECTS OF HUMAN-MEDIATED OVERHARVESTING IN SOUTHWEST FLORIDA’S ESTUARIES

Southwest Florida’s oyster reefs have a deep temporal record and have been integral to its estuarine ecology since the mid Holocene. Though Crassostrea virginica has not been commercially harvested since development began in the mid-1950s, pre-Columbian, Calusa Native Americans were significant users of the oyster, building middens between 1240 BC to AD 1220, while existing in large populations beginning in early AD. A conservation paleobiological study of oysters from archaeological deposits and modern reefs within Estero Bay and Pine Island Sound was undertaken to determine if oyster productivity has changed due to aboriginal overharvesting. Archaeological samples include those from: (1) the Late Archaic of Useppa and Calusa Islands (Pine Island Sound), early in Calusa history; and (2) Caloosahatchee I and II of Mound Key (Estero Bay), peak middle Calusa history. Modern oyster death assemblages were collected from reefs neighboring Mound Key and Useppa Island. Oyster convex (left) and cap (right) valve lengths and widths were measured; a subsample of convex valves from each sample was sectioned to count ligament pit growth lines and served as a proxy for oyster age and growth rate. Finally, the biologic taphonomic grade (a measure of biocorrosion and encrustation) was scored for the interior valve surface for all samples and its variability compared. Biologic taphonomic grade should be near pristine for oysters collected live for consumption. All archaeological samples exhibit significantly better and near pristine taphonomic grades when compared to samples from modern reef death assemblages, confirming oysters were harvested for consumption, rather than as just mound-building cultch. Convex valve length decreased significantly between early and middle Calusa time, while modern assemblages were statistically indistinguishable from early Calusa collections. This supports the hypothesis of overharvesting during middle Calusa time, and suggests modern oyster populations retain the capacity for large growth. When the slopes of linear regression curves of valve length versus growth lines are compared, all samples show similar growth rates, indicating that aboriginal activity and more recent effects of watershed management have not resulted in a permanent microevolutionary population shift.