Steve, to answer your initial question, no I don’t know how they collected those shells. 
Personal experience has taught me that certain strombus will gather in relatively shallow water to mate. We collected Strombus thersites on Okinawa in a certain location at a specific time of year. We collected some really gem specimens for our private collections in 30-40 feet. 

Sent from my iPhone

> On Jan 14, 2021, at 4:17 PM, Steve Rosenthal <[log in to unmask]> wrote:
> 
> To follow-up on this interesting thread, I have seen the offerings  of three Indonesian dealers on online auction sites peddling Cypraea, Olives, Strombus etc, along with Conus including  Conus nobilis gisellelieae, and the aforementioned Cymbiola chrysostoma......if the data listed for the shells I saw is to be taken at face value, they were mostly gathered in fairly shallow water anywhere from under rocks along the shore at low tide to a few feet of water for the Olives and others. The  only exceptions were the Conus nobilis subspecies and the Cymbiola, at something like 10-15 mtrs and 20-25 meters respectively "By local divers"......are the original posters of this thread certain that trawling is involved here? Not to minimize the potential impact of over-harvesting these two species by divers , but diving has different potential impacts vs trawling. Or intertidal foraging (which can also potentially  diminish  local populations pretty quickly).   Perhaps the local folks there have just 'figured out' they have a resource that they can utilize/exploit/whatever? 
> 
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