before these findings can be generalized to southern to mid-NSW. q 2000 Elsevier Science B.V. All rights reserved.
Keywords: Oysters; Mortality; Triploidy; Mikrocytos; Parasite; Culture
1. Introduction
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The Sydney rock oyster, Saccostrea glomerata Gould, 1850 formerly S. commer-
. cialis; Buroker et al., 1979; Anderson and Adlard, 1994 , is cultured intertidally by the
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stick and tray method in estuaries along the eastern seaboard of Australia Nell, 1993 , from around latitude 27
8S to 37.58S. Across the southern portion of the range of culture, Port Stephens to the NSW-Victorian border, the species is susceptible to Australian
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winter disease Lauckner, 1983 known locally as ‘winter mortality’. This may kill up to 80 of oysters in a local area, generally in their third winter and just before they reach
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market size Lauckner, 1983 . Roughley 1926 described the signs of the disease and Ž
. associated it with low temperatures. Wolf 1976 suggested high salinity as a contribut-
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ing factor. Farley et al. 1988 named and described the aetiological agent, Mikrocytos roughleyi, a protistan parasite of uncertain taxonomic affinity.
Growing level within the intertidal range was identified as a contributing factor to the Ž
. Ž
. Ž
. severity of winter mortality. Roughley 1926 , Wolf 1967 and Nell and Smith 1988
provided anecdotal evidence that mortalities could be reduced by raising the growing level. Oyster farmers currently deal with the disease by relaying their oysters to
upstream leases before May when infestation generally begins, andror by selling those oysters large enough to be marketed before the end of July and the onset of mortality
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. Nell and Smith, 1988 . The latter option is not available to many farmers from southern
growing areas and losses can be high in bad years. Triploid S. glomerata exhibited increased growth rates over that of their diploid
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siblings under commercial culture conditions Nell et al., 1994; Hand et al., 1998a . Ž
. Triploid production in hatcheries currently produces single seed cultchless oysters,
which require mesh trays or similar methods for growout. Oyster farmers have expressed the belief that trayed diploid oysters are more susceptible to winter mortality than
oysters on sticks, and feared that the single seed triploids might similarly suffer higher mortalities. For triploids to be commercially successful in NSW, it is necessary that their
resistance to winter mortality, when grown on trays, is at least as great as that of wild-caught oysters cultivated using the standard tarred stick method.
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Smith 1991 found no difference between Sydney rock oyster diploids and triploids on trays in their response to winter mortality in the Georges River. The work of Hand et
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al. 1998b , however, showed triploidy conferred greater survival when exposed to winter mortality compared with sibling diploids at some sites, and no difference at
others. This publication reports a study demonstrating the relationship between growing level
and mortalities in diploid S. glomerata on the Georges River in the northern part of the range of winter mortality. A second trial examines the effects on mortalities of growing
level, growing method and ploidy at Merimbula, in the southern part of the range of the disease.
2. Materials and methods
