Can the collection of "tyrian purple" from Plicopurpura pansa (Gould, 1853) (Prosobranchia, Muricidae) be blamed for its declining population?

Marked, sized and sex determined purple snails Plicopurpura pansa, (Gould, 1853) were distributed randomly among other snails in crevices of an intertidal rocky island splashed during high tides by high impact waves. After 89 days 18%, after 117 days 12%, after 145 days 8% and after 183 days only 3% of the marked snails could be recovered. There was no statistically significant difference between size and sex and the recovery rate. In the laboratory we determined the time needed for reattachment to the surface under different situations. Snails placed with the aperture down on a wet surface or in water reattached themselves after about 20 min, snails placed in water on their backs; in about 40 min, and snails left on a wet surface on their backs after 2 hours. After 4 hours only 50% of the snails placed on their backs in a wet surface were found to be reattached. Great differences were noticed in the period needed for reattachment among individual snails. The time needed for the snails to overcome the stress of being detached from the surface and to reattach themselves again can be blamed for the loss of animals during the increasing incoming tides combined with the high impact wave actions. The prohibition of "milking" P. pansa to obtain "Tyrian Purple" and to collect the snails as a bait for fishery or as a special food for foreigners should be enforced and should be extended to the removal the snails from the crevices of intertidal rocks.