Dynamic energy budgets of the oyster Crassostrea gigas
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
The Pacific oyster Crassostrea gigas has become an important aquaculture species in New Zealand. As a consequence, increasing use has been made of sheltered coastal waters for oyster culture. A carrying capacity model is required to quantify how these developments may affect ecosystem processes in a dynamically varying environment, and for the management of the oyster farming industry. An important component of carrying capacity modelling is the development of an energetic model, which is used for quantitative estimates of oyster growth and condition. In this study, a preliminary dynamic energy budget (DEB) model was developed based on published information relating to energy acquisition and expenditure of C. gigas. This model was used to simulate growth and condition of the oyster in response to varying environmental conditions (Chapter 2). The model was designed to incorporate endogenous processes (energy allocation) and exogenous factors (temperature, quantity and quality of food) over ranges which are applicable to a variety of coastal ecosystems in New Zealand. Two state variables (core weight and storage) were used in order to account for differences in physiological rates in the functioning of structural and reserve tissues. It was assumed that phytoplankton (chlorophyll) was the sole food source, although the saturating effect of total seston on feeding was represented. In Chapter 3, the model is applied to a published dataset from Marennes-Oléron Bay in France. Most parameters were estimated from literature sources. Where such estimates were unavailable, a 'free-fitting' procedure was employed. This involved changing the values of the parameters until a good fit was obtained for the data. Three parameters were varied in this way. This procedure achieved good agreement between model simulation and data for dry weight of oysters. To investigate the relative contribution of detrital material and phytoplankton, the above procedure was repeated using total organic material as the represented food source rather than phytoplankton. In this case, no agreement between model simulations and data was achieved. A further comparison was made with published datasets from inlets in Western Canada. In this case, little information was available on sediment concentrations, so these were assumed constant. Again a free-fitting procedure was used and reasonable agreements between model simulations and data were achieved. However, two of the parameters obtained by free-fit in both applications were different. From this initial study it is unclear whether these differences arose due to inherent differences between stocks , lack of information (no reproduction data available for Canadian stock), or inadequacies in the model formulation or parameterisation. The initial efforts to develop functions and parameterise the preliminary model provided an appropriate framework for investigating the dynamics of oyster growth, but also showed that there were major gaps in knowledge (Chapter 3). Field and laboratory experiments were undertaken to fill these gaps and to obtain appropriate functions and parameters for the model. Physiological measurements were conducted under different environmental conditions (Chapter 4). Clearance rates (CR) were higher at high seston concentrations compared to previous studies. CR was found to be a hyperbolic function of temperature. It was modelled as a 2-component function: pumping rate of water and extraction efficiency of particles from water. The filtration rate was found to be a Type 2 hyperbolic function of seston concentration within the range tested. Ingestion rate was described as a function of food quantity, quality and selective ingestion on organic particles. An effect of organic content on absorption efficiency was found only at very low organic content of less than 5%, while above this level, absorption efficiency was constant at 86%. Oxygen consumption rate had an allometric relationship to body size and increased over the range of experimental temperatures. The observed respiration rates were significantly lower than those previously reported in the literature. Overall the study provided systematic quantitative information on the feeding ecology of C. gigas over environmental (temperature, food, seston) ranges observed in New Zealand. Good agreement was obtained between all the chosen functions and the experimental data. In Chapter 5, an attempt was made to quantify core weight. This was done by starving the oysters for a period of 170 days and comparing length-weight-volume relationships before and after the starvation period. A set of allometric relationships was established between 'core' weight, body volume and length. In Chapter 6, histological and biochemical studies were done to determine the timing of gametogenesis, reproductive effort and utilisation of reserves. These showed that gametogenesis was initiated in July-August and spawning occurred in January. Gametogenesis was associated with an increase in gonad weight and protein content of the gonad. A decrease in glycogen content at the later stages of gametogenesis was strongly linked to increases in the size of oocytes. Reproductive effort was described as a function of dry flesh weight of oysters at pre-spawning. In Chapter 7, time-series of environmental data, dry flesh weight and length of oysters (3 size classes) were collected during growth experiments near a farming site in the Marlborough Sounds. Experiments were done over winter at two depths in order to vary the food supply. These data were used for calibration of the final DEB model. Of the environmental variables (chlorophyll, temperature, particulate organic matter and particulate inorganic matter), chlorophyll was the main factor influencing the growth and condition of oysters. The final DEB model is developed in Chapter 8. This model utilises the experimental findings of chapters 4-6 on model functions and parameters, and one dataset (of six) from Chapter 7 for model calibration. The resulting parameter set was used in the validation procedure which involved applying the model to the remaining five datasets from Chapter 7. The model was found to be capable of simulating the growth and condition of an individual at different life stages (immature individuals and adults undergoing processes of gametogenesis). A sensitivity analysis for each model parameter was conducted by varying each parameter value by ±10%. The study has achieved a quantitative description of the energy acquisition and allocation processes in oysters. It provides the basis for the development of carrying capacity models, oyster farm management models, and in general, provides a framework and tool to investigate quantitative physiology of bivalves. Overall, the study makes an initial, but significant, contribution to understanding the complex relationship between growth and environmental variation for a bivalve species in New Zealand.