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)

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 ˜í¬¿g 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.