Some years ago, off Waikiki beach at about 80 to 100 feet, I estimated
that there were several million Pinna muricata in rows more or less
parallel to the beach.  They were in sand with about a third their
length above the sand unless they were piled up on other Pinna.
 
We had a hurricane that raised considerable surf that destroyed perhaps
10 to 20 % of the Pinna.  Later there was a strong storm with winds from
the south, which is unusual in Hawaii.  This killed the remainder of the
Pinna to the extent it was difficult to find a live one.  The following
year you could find one a day protected by rubble in stead of being in
open sand.
 
On the Coast off Hawaii Kai, to the East, there were other Pinna beds.
One day, I saw at least several acres of very small Pinna in the sand
that appeared as a fuzz when you looked horizontally.  Obviously a great
number were involved.  A week or two after the hurricane mentioned
above, all the new Pinna were gone, and most of the adults were also
dead.
 
Aloha, Wes