A recent report by the Healthy Rivers Commission to the NSW Government has recommended that areas in estuaries suitable for the cultivation and safe harvesting of oysters should be identified and protected. Sydney rock oyster production in NSW and State wide average productivity (oysters/ha/y) has consistently declined since the late 1970’s despite the introduction of new technologies. The decline is not product demand-driven. Only one of the state’s 30 production estuaries shows an increasing trend in production over the past 10 years. The impact of diseases, acid sulfate soil drainage and the introduction of Pacific oysters into the Port Stephens estuary do not account for the overall decline. One hypothesis is that oyster production is decreasing because of limited food (seston) supplies. Oysters probably make up the largest fraction of the total biomass in NSW estuaries and process most of the primary production. Spatial variation in the quantity and quality of seston could limit cultured oyster growth. Farmers have identified the need to investigate oyster production decline and to identify food conditions required for optimum oyster growth and production. We propose to examine the components of seston, their spatial distribution and origin between oyster farming areas and their dynamics over time. The study will determine the spatial distribution of oyster growth rates in farming areas in response to resource availability and environmental conditions. This information will be used to assess the adequacy of food in estuaries, the identification of areas with optimal oyster growth rates and enable establishment of benchmark oyster growth rates for comparative analysis temporally and spatially. This will contribute to improving farming strategies and to identifying farming areas for protection. Oyster growth and food sources (which are intimately related to the concept of carrying capacities) in estuaries are a high priority in the ORAC 2000-2005 strategic plan.
The primary outcome of this study has been to increase the understanding of the environmental drivers that influence the southern NSW Sydney rock oyster (SRO) industry, in particular in the Clyde and Crookhaven/Shoalhaven estuaries and to identify some of the factors that limit the production of SRO. Increased amounts of nitrogen and organic carbon are delivered by increased river flows following rain events and these were found to significantly enhance oyster growth in the two south NSW estuaries. During normal and/or dry conditions, the estuaries were nitrogen-limited suppressing primary production and, potentially, oyster growth. On the other hand, during heavy rain periods, large amounts of nitrogen entered the estuaries, which then became phosphorus-limited. Optimally an intermediate level of Nitrogen:Phosphorus ratio is desired for enhancing SRO production in the south coast of NSW so that neither nutrient is limiting.
An important outcome has been to identify the diet of the SRO in the Clyde River. Prior to this study, diet preferences for the SRO were assessed only under laboratory conditions and using a narrow range of food sources limited to some specific phytoplankton species. In this study a wider range of natural food sources were used as field experiments took place at the oyster cultivation grounds where oysters are exposed to a much wider array of food sources. Through the use of carbon and nitrogen isotopic signatures it was found that seagrass’ debris, its epiphytes and seasonal filamentous green macroalgae played little part in the SRO diet in the Clyde River. However, benthic diatoms were the main contributors of the diet. In addition, the signature of mangrove debris was found to be within the isotopic SRO diet range. Consequently, resuspension processes reflecting wind, currents and water depth play an important role in making benthic food sources accessible to the oysters and thus in coupling benthic and pelagic processes.
Another outcome of this study has been the identification of oyster condition index as an useful indicator of oyster performance in terms of stocking densities in order to assess production carrying capacity levels in an area. Condition index levels were found to decrease with increasing stocking density even when there was no statistically significant trend in oyster growth. These experiments suggested that the lowest experimental stocking density in the tray experiment (1 kg / m2) produced oysters with the highest condition index, at certain times up to 16% higher than the other two experimental stocking densities (2 and 3 kg / m2). The difference in oyster performance during winter was much lower (1%) as a result of oysters spawning at the end of autumn and due to drops in temperature levels. A similar relationship between condition index and oyster density was found in floating cylinders at the Shoalhaven/Crookhaven River. At densities above 0.5 kg / per cylinder there was a consistent and significant drop in condition index. Stocking densities used in this project were lower than the typical stocking density level used in NSW cultivations except for the highest level. However, biomass gain increased with stocking density without reaching a plateau. If a plateau would have been reached this would have indicated there would not be any advantage in having higher stocking densities.
The above environmental drivers were incorporated into a computer model that combined the hydrology and nitrogen levels in the Clyde River. The model was used to investigate the consequences of changes in phytoplankton levels, as the main component of the diet, and oyster growth. The output of this model suggested that an additional food source– a carbon source – in addition to phytoplankton was needed to reach the observed growth rates and that the nutrient deliveries into the estuary from rain events played an important role in enhancing oyster growth.
In addition, a series of simple environmental indices were investigated to assess the carrying capacity of areas or estuaries. The indices chosen are easy to calculate by oyster growers and they can give an indication of when the ecological and production carrying capacity are exceeded.
The above outcomes contribute to the ecological sustainability of SRO farming by identifying an optimum level of stocking density under which growers could maximize condition index of SRO. Overall, mud flat habitats, due to the presence of large biomass of benthic diatoms mainly, have been identified as a key parameter for maximizing oyster growth due to their contribution towards the SRO food source.