I agree with Don. On Okinawa the natives have collected the green turban
shell for food, for centuries. They collect them at night on live reefs
during the minus tides, and don't seem to be doing any damage. For four years
on the minus tides we always went out with them. We were looking for
different shells, and not once did any of us not find the reefs alive with
shells. Even the Okinawan's had their sacks full. Of course these minus tides
only occur three months a year. Usually two, three days cycles each month so
the reef did have plenty of recovery time.

Frank (rain, rain go away. Now there's thunder) in Massachusetts