When I started searching for information on fish traps I was originally thing of small traps that could be made from flexible branches or maybe stainless steel mesh.
Certainly a small fish trap would be good if you are traveling to the local river or lake.
Then I saw this:
I was really blown away by the simplicity, permanence, age and scale of this thing.
It’s estimated to be 1000 years old, but no longer able to be used as the boulders have sunken into the sand.
The latest Google earth pictures are taken at high tide and this structure can’t be seen because of that (the original pictures were taken in 2006 at low tide).
But feel fee to search for ‘Poppit sands’ on Google maps
Its worth pointing out that the trap is constructed close to a major estuary and estuaries attract fish.
This type of structure might not work as well if not placed near an estuary. Other important feature are the shallow sloping sandy bottom of the sea bed and the local abundance of rock.
The natural rock reefs attract fish as the tide is lower and a fish swim out of the estuary.
Fish traps were so efficient they were banned from fresh water by the Magna Carta (1215) because they could simply empty the entire river of fish, they were also a hazard to shipping.
So how does it work?
As the tide flows out the water level drops and some fish will be trapped in the artificially constricted reef. Some modern observers believe that the fish were only netted at the opening apex of the trap.
I suspect that because there are two large opening in the sides of the trap (seeming to be too large for coincidence) the fish may have also been netted there.
Likely people were sent into the trap at low tide to scare the fish into one of the 3 openings, possibly using a moveable drag net in the process.
The other possible reasons for the gaps in the sides of the trap are:
To allow storm surges to move through he trap better and make it less likely to be destroyed in a storm.
To make the wall seem like less of a trap and to make the fish accustomed to swimming through the gap.
Prevent buildup of sand in the trap.
Damage from storm surges that was not intentional.
I have a feeling the construction of this structure would have been a community effort and likely the use of it would have been a community activity, certainly the number of fish caught could have easily fed a village.
If you consider at each of the three openings, two people holding the net and one collecting the fish in baskets, you are looking at a minimum of nine people not including the fish scaring and the transporting of the baskets to shore.
How to make one?
I’m really not sure and I’ll have to research this some more.
my speculation so far:
As even light rock weight about 2700 Kg per cubic meter, It’s not going to be easy to move these boulders very far easily, so a the local source of rock is key. Perhaps the local river system was used to transport the rock from upstream.
Of course moving things heavy on water is easier because of boast and rafts, and submerged rock looses some weight due to buoyancy. so perhaps the rocks were suspended in the water and moved with boats and rafts or possibly rolled into position under water using logs and smaller rocks as pivots and levers. I suspect the proximity of the river would have meant there would have been boats available. Nothing attracts boats like a river mouth or estuary.
Which part to build first?
Apex stones first? place furthermost stones in water first and work your way back to the shore?
Start at the shore and use the out going current to assist dragging the rest of the boulders into place?
Utilization of leeward scour? A solid rock place on sand will create a scoured out depression in the turbulent current leeward (down current) from the rock, eventually the depression will become so large the rock will roll in this depression.
If the slope of the floor is correct and the rock if roundish, it will continue to roll until it falls into the depression created by another rock. If the walls closet to the shore were created first the rest of the rock might have just been dumped in the trap and the natural out going current could have done a lot of the work.
Cost: a lot of time, rocks and cleverness and probably boats. Having people scratch their heads a thousand years later, priceless…