INDUCING DETACHMENT OF SILVER-LIP PEARL OYSTER (PINCTADA-MAXIMA, JAMESON) SPAT FROM COLLECTORS

A number of stress factors were assessed for their potential to induce detachment of silver-lip (or gold-lip) pearl oyster (Pinctada maxima, Jameson) spat from spat collectors, The factors assessed were: increa sed salinity to 40 parts per thousand (ppt) or 45 ppt, reduced salinit y to 30 ppt or 25 ppt, raised (to pH 10) and reduced (to pH 4) pH and exposure to air. All treatments promoted significantly greater(P < 0.0 01) percent spat detachment than the control treatment of 'normal' sea water (34 ppt, pH 8.0) The pH 10 treatment resulted in heavy mortalit y (86%) after 1 h and was abandoned. The greatest percentage of detach ed spat (92.3 +/- 0.6%, mean +/- s.e.) resulted after 1 h exposure to hypersaline sea water at 45 ppt, this was significantly greater (P < 0 .05) than for any other treatment. The next best treatment was pH 4, w hich resulted in 85.6 +/- 2.3% detached spat after 1 h. Exposure to th e treatments beyond Ih, except in the case of exposure to air, did not yield significant increases (P > 0.05) in the proportion of detached spat. Spat that had detached in the treatment baths within the first h our began to re-attach during the second hour. With the exception of t he pH 10 and air exposure treatments, detached spat in all treatments had firmly re-attached and had apparently adapted to the new environme ntal conditions after 24 h. Survival of spat left exposed to these tre atments for 24 h was 100% as was the survival of spat exposed to air f or up to 6 h. No mortality was recorded for any treatment 24 h after t he spat were returned to control sea water. (C) 1997 Elsevier Science B.V.