Millennial-scale sustainability of the Chesapeake Bay Native American oyster fishery

Pleistocene-to-Anthropocene Oysters.

This study of a long-term record of oyster size changes, comparing archaeological, Pleistocene fossil, and modern oysters, provides to our knowledge the first bay-wide, millennial-scale window into human harvest of Chesapeake oysters, serving as a model for future research elsewhere around the world. These data do not fully support our predictions about the effects of Native American harvest on oysters. Prehistoric archaeological oyster sizes do vary through time but are generally smaller than Pleistocene oysters, and there is no evidence for a systematic size reduction during prehistoric human occupation (∼3,500–400 y ago). At the bay-wide scale, oysters actually demonstrate an increase in size through time. No single environmental or cultural variable explains this increase, it does not occur within individual watersheds or at single sites, and we caution that our Early Woodland sample comes primarily from the lower salinity waters of Rhode River, and oyster sizes may be smaller as a result.

The size data from the Pleistocene reefs compared with later archaeological and modern samples demonstrate differences in population structure between cultural and natural accumulations of Chesapeake oysters, with more oysters in smaller size classes in the natural accumulations. These differences are not a result of our sampling for this study, because similar bulk sampling methods were used for nonmodern materials and similar random samples were used for the modern samples. We believe this likely reflects the human-selected (e.g., hand-collected) nature of the prehistoric archaeological oysters, which could have resulted in more consistent average sizes and fewer very small individuals. People were likely removing oysters from the reefs in a way that was biased toward medium-sized oysters, but there were no major decreases in mean oyster height during the prehistoric Native American occupation (3,500–400 y ago). It is unclear why the largest size classes present in the unfished Pleistocene samples were not documented in the archaeological samples. It is unlikely that these are solely a result of differing paleoenvironmental conditions, because salinity and temperature ranges for the three Pleistocene (or fossil) sites fall within ranges observed from Delaware to North Carolina today (SI Appendix, SI Text). It is possible that these largest oysters were overharvested by Native Americans before 3,500 y ago, but earlier human populations were small and only a handful of shell middens pre-dating 3,500 y ago are known to exist. Alternatively, many of the largest oysters could have lived in deeper waters that were not generally harvested by Native Americans. Another possibility is that people were choosing not to harvest the largest oysters.

Comparison of modern oyster populations with either the human-selected archaeological and historical samples or the Pleistocene oyster deposits is challenging, because nutrient pollution, disease, and intensive harvest pressure have combined to reduce recruitment in modern Maryland waters and to differentially kill off larger oysters both in Maryland and Virginia (13, 15). There are similarities between size distributions in Pleistocene Chesapeake Bay oyster populations and those along the modern Virginia coastal bays and lower Chesapeake Bay, but very large oysters (>150 mm) were more common in the Pleistocene. This could be due to the fact that modern diseases kill oysters before they reach those large sizes and that fishing regulations permit harvest of larger individuals (>76 mm) within the populations. The modern data from the upper Chesapeake Bay (Maryland) are an outlier among the modern assemblages, containing larger average sizes driven by a general dearth of small oyster size classes (35–50 mm), because the population experiences infrequent and irregular recruitment. Although dramatically reduced in number and recruitment [e.g., <1% of historical abundance (13)], Maryland oysters are still achieving average sizes comparable to those harvested in prehistoric times during the past 3,000 y. We caution that this may be due primarily to the absence of individuals in small size classes rather than growth rates comparable to those in the prehistoric period. The similarities between these two datasets may be coincidental, and require additional analysis.

Environmental Variables and Oyster Size.

Oyster size and abundance are products of the environmental conditions in which they live, and human harvest cannot be understood without also considering the potential influence of environmental change. Salinity is a driving factor for oyster size in both ancient and modern times, and influenced the size of oysters harvested by Native Americans in local watersheds (Fig. 3 and SI Appendix, Table S16). However, when viewed from the perspective of the entire bay, the correlation between salinity and oyster size is relatively weak (Fig. 2). This could result from the high geographic and temporal variability in Chesapeake salinity, which is difficult to sample at a single archaeological site with broad temporal and spatial resolution that could obscure expected variability. Despite the limited correlation between salinity and oyster size bay-wide, there is a size–salinity correlation in two individual watersheds.

Longer-term climate patterns influenced regional paleoenvironmental conditions but do not appear to have significantly influenced Native American oyster harvest. The Chesapeake region responded to the Medieval Climatic Anomaly, with warm, dry atmospheric conditions and higher salinity and higher water temperatures between A.D. 400 and 900 and cool, wet atmospheric conditions and cool, less saline water conditions between A.D. 1050 and 1750 (31, 32). Relative sea level in the Chesapeake Bay has continued to rise throughout the past 3,500 y due to the collapse of the Laurentide ice sheet forebulge, with ∼7 m of relative sea-level rise during that period (33). Our data suggest that these major shifts had little effect on prehistoric harvest patterns, at least on archaeological timescales, but they warrant more detailed study.

Nutrients and sedimentation are also drivers of oyster productivity, and likely influenced oyster populations during the historic period. A 2,000-y study of Chesapeake nutrients and eutrophication demonstrates important increases during the historic and modern periods, likely associated with landscape clearing for agriculture and increased sedimentation in the historic and modern periods, but suggests little change throughout Native American occupation (34). These historic nutrient increases likely influenced the size increase we noted in historic-period Rhode River oysters, which was similar to a pattern identified in the St. Mary’s and Patuxent rivers (21). Together, these data suggest that human activities, biotic conditions, and climatic variability likely worked in concert to influence long-term oyster productivity, size, and abundance.

Sustainability of the Native American Fishery.

With limited variability in oyster size and abundance and no systematic evidence for a size decline through time during the period from 3,500 to 400 y ago, the Native American Chesapeake Bay oyster fishery appears to have been largely sustainable. A variety of factors may help explain this, including technological limitations that resulted in primarily nearshore harvest, seasonal rounds where people only exploited oysters during a particular time of year, low human population density, and broad-spectrum human diets that incorporated a mix of marine and terrestrial resources and cultigens. Little is known about the precise methods that prehistoric Native Americans used to obtain oysters, but hand collection—perhaps using simple tools in relatively shallow water—was likely the most common method (20). Hand collection of oysters in nearshore, shallow-water/fringing reefs may have promoted consistent size classes in middens, and may have left significant oyster populations in deeper water free from human harvest. This would have limited the long-term impacts on regional oyster populations by preserving a source population to supply recruits and by leaving substantial portions of the hard substrate and/or overall 3D reef structure intact. Native Americans in the Chesapeake are known to have harvested a wide variety of marine and terrestrial foods, with evidence for maize cultivation, especially on the major western-shore drainages, beginning ∼1,000 y ago (10). These diverse subsistence strategies would have reduced pressure on single-resource classes (i.e., oysters), and may have further helped oyster recruitment following significant Native American harvest. Although there are many details of Native American harvest of oysters that remain unknown, we do know that, in contrast to historic and modern times, Native American population densities were much lower, there was no significant trade of oysters outside of the Chesapeake region, oyster shell was generally not mined for construction, and harvest methods were far less destructive than historic and modern techniques [e.g., dredging with sail and steam boats (8)].

Although our study is the first, to our knowledge, to analyze oyster size throughout the Chesapeake Bay, a few archaeological studies have investigated past oyster size at single sites or localized areas (9, 26, 27). Data from the White Oak Point site in Virginia provide the longest sequence from a single site in the region. Evidence at White Oak Point from ∼4,500 y ago through the early historic period supports our findings, suggesting continuity in oyster size (Fig. 4) (26). The St. Mary’s City site in Maryland provides evidence for a localized size decline from the 16th to 17th centuries that was associated with human population growth during the colonial period (9, 20). Additional research has suggested that this decline may have been followed by 19th-century size increases, perhaps associated with eutrophication and higher nutrient load that increased oyster growth rates (21). When considered alongside the prehistoric dataset presented here, the historic period shows collection of larger oyster size in the Chesapeake. We attribute this to technological shifts, including the introduction of tonging in the 18th century followed by dredging in the 19th century (9, 10, 12, 20), which may have increased access to larger oyster size classes from deeper waters that were previously not harvested by Native Americans. Increased nutrient load likely also played a role, but this needs additional testing (21).

Other aspects of human behavior may have contributed to the apparent sustainability observed in the Native American size data. One possibility is that Native Americans preferred medium-sized oysters or lacked the technology to open larger oysters. Although food preferences may have played a role in the sizes present in our data, the presence of both small and larger individuals suggests a wide range of oysters were harvested. We reject the notion that Native Americans lacked the technology to process larger oysters. These could be broken with a hammerstone or opened with heat, like other sizes. Similarly, salinity, nutrients, and other variables influenced oyster size, particularly in local catchments, but none show a clear correlation with the patterns we observe bay-wide. Collectively, the archaeological size and abundance data suggest that the most parsimonious explanation is that the Native American fishery was sustainable at century and millennial timescales. This does not mean that Native Americans did not have distinct temporal or localized impacts on local reefs. However, harvest patterns reveal no systematic decline in size or abundance like those observed by archaeologists in other parts of the world (28⇓–30).

Elsewhere in North America, analyses of late Holocene Crassostrea virginica from Florida (35) and New York’s Hudson River estuary (36) also document continuity in oyster size and no prehistoric size declines. Analysis of shell rings in the southeastern United States suggests a mix of human influence, with some arguing for significant human impact (37) and others for more limited effects (38). Beyond North America, analysis of Ostrea edulis in Denmark demonstrates human-induced size decreases from the Mesolithic to the Neolithic (39). Together, these data demonstrate a range of human influence on oyster size in the past but general resilience to Native American harvest pressure in several North American C. virginica populations. These data add to a growing body of research around the world that documents a continuum of ancient human influence on shellfish populations, ranging from human-induced reduction of shellfish size from overexploitation to continuity and size increases that may have resulted from changing environmental variables, especially nutrient increases (26⇓⇓⇓–30, 35⇓⇓⇓⇓–40).

Implications for the Anthropocene.

The modern Chesapeake Bay, like other estuaries around the world, is a complicated system where recovery is hindered by an array of factors. Oysters are currently combating the sustained effects of overfishing, introduced and native disease [multinucleated sphere unknown (MSX) and Perkinsus marinus (dermo)], climate change, eutrophication, and sedimentation, some of which were not major drivers of change in past ecosystems (11, 41, 42). Although many of these did not play a major role in the prehistoric oyster fishery, our research demonstrates the resilience of oyster populations through rising sea levels, changing climate, and extensive Native American harvest over several millennia.

This model of a sustainable prehistoric Native American harvest of oysters, primarily by hand collecting from fringing reefs that left deeper-water reefs largely intact, provides insight into modern restoration. Although the effectiveness of the modern restoration strategy of Chesapeake oysters is debated (5, 15), our Pleistocene-to-Anthropocene size and archaeological relative abundance data provide some support for recent Chesapeake Bay oyster restoration efforts, including reduction of modern harvest levels and creation of increased no-take zones (13, 43) that would mimic the more mobile and flexible Native American fishery. Current restoration plans (44) include enhancement of oyster density using hatchery seed, addition of new hard substrate where needed, and no-take reserves that are conceptually similar to deep-water areas where harvest was unlikely before the introduction of oyster tongs and dredges. In addition, our Pleistocene data provide a baseline against which the size distribution of oysters in no-take reserves could be evaluated. These data do not provide all of the answers to a very challenging and complicated restoration and conservation effort, but they do provide an example of an apparently sustainable millennial-scale fishery, elements of which may help inform restoration and harvest in today’s ecosystem.

Finally, our data illustrate the importance of conserving oysters beyond the ecological services or food they provide. Eastern oysters, like other oysters around the world, are deeply intertwined with the people who lived around the Chesapeake Bay and beyond (7, 10). Consequently, oysters warrant conservation for their significant role in North American cultural history, including that of Native Americans, Euro-American colonists, modern watermen, and the general public.