Hatchery-reared juvenile Strombus gigas and S. costatus were used in a series of field release experiments to determine rates of growth, mortality and dispersion. These experiments, each approximately 8 weeks long, were conducted throughout the year at an offshore site,
17 m deep. Average shell length at release ranged from 3 cm to 4 cm. S. gigas was found to be considerably more active than S. costatus. The former showed greater rates of diffusion away from the release site, and noticeable migrations: inshore during late winter, offshore
during autumn. Movement of S. costatus was insufficient to detect directed migration. S. gigas consistently grew faster than S. costatus. Growth varied seasonally, being negligible in late winter and highest in autumn when temperature was maximum. Mortality was similar
between species and varied over the year, being higher in summer and lower in winter. A simple sine function fitted to the combined data yielded an integrated average instantaneous annual mortality rate of z = 8.62 or 2.3% mortality/day.
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