'Seafood Island' Surprises Scientists

Imagine you were one of the first groups of people to arrive by boat on an island in the middle of the tropical Pacific Ocean in ancient times.

You had been at sea for weeks, your concern growing as your supplies were exhausted and you had to depend on the fish and the birds you could catch…and the rain that fell from time to time.

Then the high islands you had been searching for appeared faintly over the eastern horizon, so you steered a course towards them.

Quenching your thirst from the rivers you found there, you looked for a ready source of food and found abundant shellfish, clinging to rocks or buried in the sand of the shallow sea floor and the mud of the river estuaries.

You gorged yourself on the soft parts of these shellfish, discarding their hard shells in piles along the shoreline.

And these piles grew. And grew. Until…

Scientists in the 21^st century start exploring these remarkable shell middens, revealing history, geography - and the dietary preferences of the people who lived in such places long, long ago.

In January 2017, we were undertaking reconnaissance fieldwork along the lightly populated north coast of Vanua Levu, the second largest island in the Fiji group.

As part of a project with the Fiji Museum focused on identifying and mapping ancient hill forts in this part of the archipelago, we had been talking to local residents about any memories, passed down orally, that might illuminate the ancient and unwritten history of this part of the world.

Following conversations with people in Bua Lomanikoro and Dreketi Rice, we surveyed a remote, uninhabited area of the coast named Culasawani.

At the mouth of a small creek, just off the edge of the mainland, we stumbled across an area of low ground that seemed to be made almost entirely from the remains of edible shellfish, mostly small clams (Anadara spp.).

The area covered by these shellfish remains was enormous; it extended about 150m in length and 20-30m wide in places.

Burrowing crabs had obligingly excavated great piles of shell material from beneath the ground surface so it was clear this was not simply a surface deposit. Fragments of pottery were also found.

Several years later, with funding from the European Union's Horizon 2020 programme, we returned to Culasawani with groups of local researchers and excavated several pits, taking samples of shellfish remains for radiocarbon dating.

The results were astonishing.

All 10 radiocarbon ages clustered around the year 760 CE (Common Era, ±310 years), which suggested the creation of this enormous shell deposit occurred within about 500-600 years, more than 1,200 years ago.

At first we thought the Culasawani sand deposit might have been created by a giant wave, perhaps a tsunami generated by a sea-floor earthquake along the Fiji Fracture Zone, 40km to the north, where one giant crustal plate grinds slowly past another.

But the geography of the deposit and the largely undisturbed nature of its contents suggested a more likely explanation.

That the Culasawani deposit was an authentic 'shell island', habitable land created wholly from the remains of edible shellfish discarded by the people living there more than 1,200 years ago, is incredible.

Here's how the scenario is likely to have unfolded.

Twelve centuries ago, a group of people was sailing along the north coast of Vanua Levu Island in search of somewhere they could live.

Somewhere no-one else lived. Somewhere with abundant fresh water, ample supplies of shallow-water marine foods (like shellfish), and lots of trees for building shelters.

Culasawani beckoned. All around the mouth of a river were beds of shellfish, mouth-wateringly large from lack of sustained predation.

The people built their settlement on the land but daily - for 500 years or so - they harvested these shellfish beds, cracking the shells in place to extract the flesh which they transported in pots back to their settlement on the mainland.

So numerous were the cracked shells that over time they built up the ground surface, raising it above the surface of both the river and the ocean to create an island.

This then is a true 'shell island', built unintentionally by people from the remains of edible shellfish.

It is a remarkable find.

People colonised the islands of the South Pacific Ocean from west to east, beginning in the outer islands of Papua New Guinea about 1350BCE, then passing through Solomon Islands and Vanuatu to reach Fiji about 1100BCE, just over 3,000 years ago.

There are shell islands like Culasawani known off some of the islands in Papua New Guinea but Culasawani is the first to be discovered anywhere else in the western Pacific.

Studying the people of Culasawani and their interactions with local ecosystems also gives us a lesson in sustainable living, for people lived here for 500 years, feasting on edible shellfish, until their consumption built an island that altered the geography and inevitably impacted the ecology of the area.

Probably the shellfish resource declined and the people moved elsewhere. A consequence of unsustainable living…not unlike many more recent situations elsewhere in the world.

This research was published in the scientific journal Geoarchaeology and is free to read here . It was co-authored by Professor Patrick Nunn and Roselyn Kumar from the School of Law and Society at UniSC with collaborators from the Fiji Museum, the University of the South Pacific and the University of Cork.