The Aboriginal fish traps appeared again recently. They've been underwater for two years as the rain threaded through the karris and into the inlet.
According to an archaeologist the stone traps were built around seven thousand years ago, when sea levels rose and inundated what was previously a forest. He carried out radiocarbon dating on some tree rings in the middle of the inlet to work this out.
Back then, the inlet was tidal all year around and the sand bar blocking the inlet from the sea didn't even exist. The fish traps worked this way: fish swam over the rocks at high tide and then were stuck behind the stones at low tide. Easy pickings. What I love about Indigenous hunting techniques is that they tend to require little energy and not involve carting stuff around. Lizard traps are based on the same principle. The calorific exchange - the amount of calories garnered versus the amount expended to get those calories - is always a pretty productive ratio.
Eventually the sea level rise settled down and the sand bar began to form. At this particular inlet it is guestimated at about three thousand years ago. I guess the traps became less viable then, or perhaps they were used more as holding pens for fish after a mullet-herding session. (As a mullet fiend and fisherwoman I can testify that yes, it is possible to herd mullet.)
The sides of the trap would have been built up with brushwood and spars to make fish yards.
Above is a similar fish trap at Lligwy beach in Scotland. Built by ancestors with the same ingenuity and simplicity as the Australians.
Image by Richerman - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=33720685