INDIRECT EFFECTS OF EXCESSIVE SHELLFISH HARVESTING BY HUMANS ON BIRD PREDATORS IN SALT MARSHES OF THE LITCHFIELD BEACH AREA, SOUTH CAROLINA OVER THE LAST 1800 YEARS

Shell middens are refuse deposits of discarded materials. Salt marshes in the Litchfield Beach area of South Carolina contain sixteen archaeological shell middens (dated 1770-1590 to 300-80 cal. years B.P.) dominated by the bivalve mollusk Mercenaria mercenaria. Jones and Quitmyer (2008) found a declining trend in mean ontogenetic age of Mercenaria from 1770-320 B.P. followed by a rebound; the decline in ontogenetic age has been attributed to population growth and/or intensification of harvest. To assess possible human impacts on other salt marsh predators of Mercenaria, this study focused on diagnostic, posteriorly located, v-shaped scars found on many Mercenaria specimens from the same samples used by Jones and Quitmyer (2008) in their study. These scars represent failed predation attempts on Mercenaria by Haematopus palliates, the American Oystercatcher. Oystercatchers attack bivalve prey using a strong, elongated beak, which they pierce into the sediment to search for prey and pry open shells. Predictably, trends in body size (a proxy for ontogenetic age) of Mercenaria matched those found by Jones and Quitmyer (2008). Body size of Mercenaria specimens peaked at 1770-1590 cal years B.P. (avg. height at death = 66 mm, n=134) and was smallest from 660-320 cal years B.P. (avg. height at death=54 mm, n=232). The frequency of shell repair (the percent of shells with at least one v-shaped scar) prior to the rebound also followed this same trend (predation scar frequency = 18% (n=134) and 7% (n=232) for middens dated 1770-1590 and 660-320 cal years B.P., respectively).The declining frequency of shell repair implies that excessive shellfishing (1770-320 B.P.) by humans may have depleted Mercenaria as a prey resource for oystercatchers, forcing these predators to switch to a different prey item. However, because the accumulation of shell repairs is often size dependent, future work is needed to standardize repair scar frequency by Mercenaria size to test the robustness of this conclusion. Analysis will also be expanded to include the period of rebound (320-80 B.P.) in body size, in which oystercatcher predation is expected to intensify.