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Relationship between water quality parameters (nutrients, seston, chlorophyll a), hydrodynamics and oyster growth in three major Pacific oyster (Crassostrea gigas) growing areas in southern Tasmania (Australia)


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Mitchell, Iona Margaret 2001 , 'Relationship between water quality parameters (nutrients, seston, chlorophyll a), hydrodynamics and oyster growth in three major Pacific oyster (Crassostrea gigas) growing areas in southern Tasmania (Australia)', Research Master thesis, University of Tasmania.

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An assessment was made of three Pacific oyster ( Crassostrea gig as) growing areas in
southern Tasmania (Pitt Water, Pipeclay Lagoon and Little Swanport) with respect to
water quality parameters, oyster growth and hydrodynamic characteristics. This was
done in an order to explain differences in reported oyster growth rates and hence address
the issue of shellfish productivity in each area. Water samples were collected monthly
for 13 months from several sites along the length of each area from a marine site to the.
upper reaches of the estuary, or coastal embayment. These were analysed for
chlorophyll a, nutrients (NOX, P04-P and Si04-Si), and seston quality and quantity (i.e.
total particulate matter (TPM) and particulate organic and inorganic matter (POM &
PIM)). Temperature, salinity and secchi disk depths were also recorded. Oyster growth
and condition were assessed from studies conducted over three consecutive periods at
two sites within each area. Hydrodynamic characteristics were calculated from tide
gauge data obtained. Additionally, a biodeposition study was conducted at one area
during two seasons to determine rates of deposition and composition of biodeposits.
Seston quantity was similar among areas, but seston quality, as expressed as %POM,
showed variation attributed to the characteristics of each area. Chlorophyll a
concentrations were generally low in each area, ranging from 0.2 to 4.0 μg L-1
Interestingly, chlorophyll a levels measured were high in winter to early spring months
within each area. Higher levels of chlorophyll a were measured following periods of
flooding and freshwater inflows, particularly in two of the study areas. Considerable
variation among areas was shown in oyster growth, with respect to shell length, width,
depth and live weight of oysters. Differences in growth are largely attributed to the
water quality and hydrodynamic characteristics noted within each of the areas. Mean
biodeposition rates varied from 39.6 g DW m-2 d-1 in winter to 180.5 g DW m-2 d-1 in
summer. The average organic content of biodeposits (approximately 19.2% POM) was
similar in summer and winter. The organic matter content of sediments under oyster
baskets was low(< 2.6 %), and it was concluded that biodeposits were being transported
and deposited elsewhere.
The overall findings from the study indicated that growth rates and productivity of each
area were largely influenced by the supply and availability of food. It appeared that stocking density and spatial arrangement of leases provided the greater limitation on
growth rate in Pitt Water and Pipeclay Lagoon. Little Swanport was characterised as
having the better growth rates and conditions for growth. Food quality, as measured by
chlorophyll a and %POM in particular, was higher than the other two sites, and flow
rates indicated that a greater quantity of food was reaching a larger proportion of the
cultured population. The marine nature of Pipeclay Lagoon suggested that the main
source of food supply to the cultured oyster population is of marine origin. However,
flow rates and transport of this material over the culture area is insufficient to provide
faster growth rates. Stocking density of oysters, and spatial arrangement of the culture
area, is most likely responsible for limitation on available food supply to the majority of
the population. Sufficient food is available for maintaining metabolic processes, but is
insufficient to enable greater storage and hence growth rates. Similar processes
appeared to be occurring in Upper Pitt Water, though it seems the greater fraction of
food is sourced from within the estuary, rather than being of marine origin. Sampling
during this study was fortunate to coincide with infrequent events of heavy and
prolonged rainfalls in the latter part of the year, resulting in flooding of this estuary.
The beneficial effects of this were elevated nutrient, chlorophyll a, seston levels and
greater increase in oyster dry meat weights, confirming the concerns raised by the oyster
farmers with respect to the negative effects of the Craigboume Dam.
Shellfish production estimates as used overseas were found to be not applicable to
Tasmanian conditions. Differences in culture environments between overseas oyster
growing areas and those found within Tasmania are discussed.

Item Type: Thesis - Research Master
Authors/Creators:Mitchell, Iona Margaret
Keywords: Crassostrea, Oyster culture, Shellfish fisheries, Aquaculture, Oyster fisheries
Copyright Holders: The Author
Copyright Information:

Copyright 2001 the Author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (M.Sc.)--University of Tasmania, 2001. Includes bibliographical references

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