Ancient Clam Gardens Increased Shellfish Production: Adaptive Strategies from the Past Can Inform Food Security Today

Maintaining food production while sustaining productive ecosystems is among the central challenges of our time, yet, it has been for millennia. Ancient clam gardens, intertidal rock-walled terraces constructed by humans during the late Holocene, are thought to have improved the growing conditions for clams. We tested this hypothesis by comparing the beach slope, intertidal height, and biomass and density of bivalves at replicate clam garden and non-walled clam beaches in British Columbia, Canada. We also quantified the variation in growth and survival rates of littleneck clams (Leukoma staminea) we experimentally transplanted across these two beach types. We found that clam gardens had significantly shallower slopes than non-walled beaches and greater densities of L. staminea and Saxidomus giganteus, particularly at smaller size classes. Overall, clam gardens contained 4 times as many butter clams and over twice as many littleneck clams relative to non-walled beaches. As predicted, this relationship varied as a function of intertidal height, whereby clam density and biomass tended to be greater in clam gardens compared to non-walled beaches at relatively higher intertidal heights. Transplanted juvenile L. staminea grew 1.7 times faster and smaller size classes were more likely to survive in clam gardens than non-walled beaches, specifically at the top and bottom of beaches. Consequently, we provide strong evidence that ancient clam gardens likely increased clam productivity by altering the slope of soft-sediment beaches, expanding optimal intertidal clam habitat, thereby enhancing growing conditions for clams. These results reveal how ancient shellfish aquaculture practices may have supported food security strategies in the past and provide insight into tools for the conservation, management, and governance of intertidal seascapes today

Gaussian models: effects of clam garden treatment and intertidal height.

<p>Parameters for the modeled responses of biomass (kg/0.018 m³ +/− SE), density (count/0.018 m³ +/− SE), and growth (mean +/−SE) of surveyed and transplanted <i>L. staminea</i> (<i>L.s.</i>) as a function of intertidal height. Each response was predicted by modeling a Gaussian curve to the data, <i>y</i>  =  <i>a</i>*exp(−0.5*((<i>x</i>−µ)/<i>σ</i>)²) (Eq. 1), where <i>y</i> = response, <i>x</i> = intertidal height, <i>a</i> = height, µ = mean, and <i>σ</i> = standard deviation.</p

Survey: Bivalve Biomass and Density.

<p>B) Density (count/0.270 m3 +/–SE) and C) Biomass (kg/0.270 m3 +/–SE) of four most abundant bivalve species; A) <i>L. staminea</i> density (count/0.270 m3 +/–SE) of 5 size classes in Clam Gardens and Non-Walled Beaches.</p

Experimental effects of clam garden treatment and tidal station.

<p>The effects of clam gardens (Beach Type) and tidal station on the growth and survivorship of transplanted <i>L. staminea</i>. * designates significant p-values (p≤0.05).</p

Effects of clam garden treatment and oceanographic context.

<p>The effects of clam gardens (Beach Type), oceanographic context (Waiatt Bay vs. Kanish Bay), and Tidal Station on the biomass and density of surveyed <i>L. staminea</i>, <i>S. giganteus</i>, <i>V. philippinarum</i>, <i>Macoma spp</i>, and total clams (per survey transect, 0.027 m³). * designates significant p-values (p≤0.05).</p

Study Design and Predictions.

<p>A) We surveyed clams across a vertical transect (black line) from the top of clam habitat (H) to ∼1.0 m in non-walled beaches (left) and to the edge of the rock wall (W) in clam gardens (right). We then transplanted clams in mesh bags at 5 evenly stratified tidal stations (blue and green colored squares) across a vertical transect (purple line) from the top of clam habitat (H) to ∼0 m in 5 Non-Walled Beaches (left) and to the edge of the rock wall (W) in 6 clam gardens (right). B) Hypothesis 1: predicted clam productivity as a function of tidal station. Tidal station 1  =  top of clam habitat, tidal station 15  =  top of clam garden wall in clam gardens or ∼1.0 m tidal height in non-walled beaches. C) Hypothesis 2: predicted clam productivity as a function of intertidal height.</p

Clam garden effect on density and biomass.

<p>The effect of clam gardens (Beach Type) on density and biomass (per survey transect, 0.027 m³) of <i>L. staminea</i> (littleneck clam), <i>S. giganteus</i> (butter clam), <i>V. philippinarum</i> (Japanese littleneck clam), <i>Macoma</i> spp (macoma clams) and total clams. * designates significant p-values (p≤0.05).</p

Mollusc harvesting in the Pre - European contact Pacific Islands: investigating resilience and sustainability

Mollusc shells, which are often quite numerous in archaeological sites throughout the Pacific Islands, offer good proxies for assessing environmental change as well as human impact. Documented changes in species size, richness, and abundance have often been interpreted as evidence of resource abuse by shellfish gatherers. While this may be valid in some cases, archaeologists need to consider other variables to explain change (or stability) in shell distribution. A better understanding of ecological and biological (life history) characteristics associated with shell midden deposits, as well as greater awareness of ethnographic and ethnoarchaeological data on the interaction between people and varieties of marine resources, may result in a reinterpretation of past human behavior. A growing interest in indigenous resource management among contemporary Pacific Island communities has led some archaeologists to seek tangible evidence of community resilience and sustainable use of resources in the past. This chapter draws primarily from direct observations and semi-structured interviews among mollusc gatherers in Kiribati, eastern Micronesia, and examines selected case studies of archaeological shell deposits from the Pacific Islands that could shed new light on marine resource management to complement the more widespread research conclusions that depict human impact in largely negative terms