Hi, Alan.
 
The shells do come in various degrees of degradation from fishes'
guts. We hear about and see the best of these.
 
There are articles on-line in the Archive site of THE CAPTURED COWRY
( http://www.geocities.com/~makuabob/ ), though I can't point to
them right this minute. Mostly, it seems the best shells have been
recently ingested and the fish caught soon after. This is likely
due to the fish being in an active phase, looking for food.
 
In one article, four or five of the same species (labrolineata, I
think) were taken from one fish. All were in very good condition.
 
I suspect that many cases of shells found in fishing nets (where
catching fish was the main intention) are cases of regurgitated
meals that didn't fall through the mesh.
 
 
Aloha,
 
makuabob (a.k.a. Bob Dayle)