Mortality estimates of the intertidal purple snail Plicopurpura pansa in the Baja California Peninsula, Mexico

摘要

The intertidal purple snail Plicopurpura pansa was intensively exploited for "Tyrian Purple," leading to declining populations and prohibition by the Mexican government of commercial exploitation. A probable cause for the decline is the difficulty of the snails to reattach themselves fast enough before being washed away by the high impact waves during high tides after being removed from the rocks to obtain `ink.' To assess this hypothesis, six experiments were conducted at two rocky beaches in the Baja California Peninsula. Total mortality rates were estimated by a recapture probability model. Mortality rates ranged from 6.34 y(-1) to 14.23 y(-1) at Playa Cerritos and 5.04 y(-1) to 7.79 y(-1) at Punta Perico. Male mortality rate values were lower than females. These rates show high vulnerability of purple snail after being removed from rocks. For management and conservation of snail populations, it is not sufficient to handle the snails with care and to "milk" them only periodically. Detachment from the rocks seems to be the main reason for high mortality and should be prohibited.