Insights from historical human-clam coexistence

Coastal ecosystems are not only threatened by habitat loss and climate change; a breakdown of traditional aquaculture practices could also have contributed significantly to their deterioration.

This has been illustrated by an 11,500-year analysis of human coexistence with clams in British Columbia, Canada, published in the Proceedings of the National Academy of Sciences.

Ginevra Toniello, from Simon Fraser University, Canada, and colleagues gathered paleoecological, archaeological and modern records of butter clams (Saxidomus gigantea) in the northern Salish Sea to understand their relationship with humans throughout the Holocene.

The cultural significance of these bivalves is revealed by stories, rituals, language and the deep piles of ancient shell middens that line kilometres of coastlines. Archaeological and ethnographic analyses suggest clams were popular food sources harvested seasonally and all year round and enjoyed both fresh and preserved. 

191015 clam

Butter clams from 11,500-11,000 years ago (left) and 10,900-9,500 years ago (right) showing the sometimes dramatic differences in size through time.

Credit: Ginevra Toniello

Archaeological records can also provide insights into the ecological impacts of interactions between humans and fauna, while palaeoecological information can reveal the ecology of species without human interference.

“Taken together,” Toniello and coauthors write, “these two records can offer a powerful lens through which to assess coupled social-ecological systems over broad spatial and temporal scales and can help establish ecological baselines for modern management.”

To peek into the past, the researchers gathered clam shells from middens at five coastal sites and measured the size and width of growth rings in the mollusc’s shells. They put these into context according to their historical location, before, during and after evidence of management by indigenous people and were able to group the samples into seven time periods.

Together, these data allowed them to analyse predictors of clam size throughout the Holocene.

They found the mollusc shells’ size and growth was limited in early postglacial times, but then flourished over the next few millennia until the early-Late Holocene, likely reflecting more favourable habitat conditions.

Middens showed evidence that humans then harvested them around 9000 years ago, and about 5500 years later started constructing clam gardens – “intertidal rock-walled terraces” – as a form of aquaculture management.

The gardens made the bivalves more accessible to harvesters by reducing the beach slope, exposing more beach during low tide, and bringing them closer to human settlements.

The researchers believe the gardens’ construction reflected population growth and increased complexity of social structures, necessitating measures to preserve the clams for food and trade.

By around 2700 years ago, harvesting intensified, yet evidence suggests the clam populations flourished throughout the Late Holocene.

The clam habitats were likely preserved by the gardens built by generations of Indigenous peoples, the team suggests. Along with cultivation methods such as tilling, removing non-human predators, removing rocks, modifying the substrate and monitoring access, the Indigenous people were able to maintain a sustainable harvest.

Toniello and colleagues speculate that the course sediment garden terrace and rock wall also facilitated abundant growth and access to other marine foods like crabs, sea cucumbers and seaweeds.

Sadly, modern records indicate that growing conditions declined since European settlement replaced traditional management practices with industrial activities, with an impact comparable to the ice age.

“It is striking that the growth patterns of clams living in the beach today are most similar to the clams that lived and died in the unstable and relatively unproductive habitats of the Early Holocene,” the group writes.

“As in the Early Historic Period, we propose that the current low productivity is due to the decline in traditional management, including ongoing tilling through harvesting.”

They note it could also be attributed to deposition of fine silts on the clam beaches – less favourable for clam growth than the coarse grains used as garden substrates – as a result of logging, along with warmer ocean temperatures and associated declines in productivity.

Nonetheless, they suggest modern humans could learn much from traditional practices for aquaculture management, which also has broader ecological and ethical implications.

“Examining the deep and specific history of human-species relationships, such as that between people and clams, is requisite for understanding and better managing our resources and ecosystems today,” they write.

“Documenting these interactions between humans and coastal ecosystems, such as we have done here, also counteracts the erasure of the long-term connections of Indigenous peoples to their lands and seas.”

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