Recent archaeological investigations indicate that coastal Fort Walton cultures in the St. Joseph Bay region of northwest Florida emphasized marine and estuarine foraging. These late prehistoric, Mississippi-period (A.D. 1000–1500) peoples collected fish, shellfish, and other aquatic resources. At the Richardson's Hammock site (8Gu10), radiocarbon-dated to about A.D. 1300, large, predatory gastropods were a major subsistence component. This adaptation is in sharp contrast with that of contemporaneous inland Fort Walton societies, who relied on maize agriculture, and raises the question whether coastal groups were separate hunter–gatherer–fisher populations or migrated seasonally from inland farming villages. We perform stable oxygen and carbon isotopesclerochronology on lightning whelks (Busycon sinistrum) to determine the seasonality of Fort Walton foraging and to compare the environment of prehistoric St. Joseph Bay with that of the modern bay. Oxygen isotope profiles suggest that shellfish collecting was relegated primarily to the summer months, producing a scheduling conflict with the primary growing season for maize in northwest Florida. Thus, coastal and inland Fort Walton sites probably represent separate culture groups. The relationship between δ18Oshell and δ13Cshellindicates similar environmental and climatic conditions between prehistoric St. Joseph Bay and today. However, modern whelks are depleted in 13C compared to Fort Walton whelks, which reflects both twentieth century CO2 emissions and years of dredging and wastewater pollution entering the bay.