Jeffrey S. Ren

A dynamic energy budget model of the Pacific oyster Crassostrea gigas

A dynamic energy budget model has been developed to simulate the growth of Pacific oyster Crassostrea gigas in response to varying environmental conditions. The model is designed to incorporate the effects of endogenous (core weight and storage) and exogenous (temperature, quantity and quality of food) factors and to be applicable to a variety of ecosystems. Two state variables (core weight and storage) are used in order to avoid confusion between functional roles of structural and reserve tissues. Assimilation and metabolic rates are modelled as functions of core weight, while reproduction is entirely dependent of storage. In addition to reproduction, a variable in terms of energy requirements for gametogenesis is introduced in this model. Calibration of the model was done through sensitivity analysis and comparison of simulation outputs and observed data. The model is capable of simulating growth and condition of oysters in the ecosystem of Marennes–Oleron Bay. The simulations indicate that growth of oysters is strongly regulated by the phytoplankton concentration, while detritus has little contribution.

Seasonal variation in the reproductive activity and biochemical composition of the Pacific oyster (Crassostrea gigas) from the Marlborough Sounds, New Zealand

Abstract Reproductive cycles of the Pacific oyster Crassostrea gigas (Thunberg) from the Marlborough Sounds, New Zealand, were followed between June 1998 and January 2000. Histological examination of the gonad confirms an annual cycle with a winter inactive period followed by rapid gonad development and a single short spawning period. The population gonad index correlated with seawater temperature and changes in tissue dry weight, condition index, and biochemical components. In winter, few individuals with early gametogenic stages were present and rapid development of primary oocytes (diam. 11 μm) occurred during spring (September‐November). The developmental rate and the diameter of mature oocytes (37 μm) was similar for the 1998 and 1999 seasons. For a standard 110‐mm‐length oyster, maximal tissue body weight and condition index were recorded in December. Rapid weight loss in January was length dependent and was attributed to spawning. Temperature was the environmental variable which best correlated with the timing of gametogenesis. Food availability (phytoplankton biomass) may have been responsible for inter‐annual variations. The biochemical composition (% glycogen, lipid, protein) of separated gonad and somatic tissues were variable seasonally and annually. Gametogenesis (oocyte diameter) was associated with increased gonad protein and glycogen and a decrease in lipid concentrations. These changes are similar to those in Pacific oyster populations from other parts of the world.

COMPARISON OF ASSIMILATION EFFICIENCY ON DIETS OF NINE PHYTOPLANKTON SPECIES OF THE GREENSHELL MUSSEL PERNA CANALICULUS

Abstract The greenshell mussel Perna canaliculus is the most important species in aquaculture in New Zealand. Mussel energetics and growth rates are subject to the natural variability in phytoplankton biomass and species composition and thus understanding the influence of food type on assimilation efficiency is fundamental to the prediction of mussel production and planning farm management. In this study pulse-chase feeding techniques were used to assess the effect of diet on assimilation efficiencies for nine phytoplankton species including three diatoms Chaetoceros calcitrans, Skeletonema costatum and Thalassiosira sp., and three flagellates Eutreptiella sp., Pyramimonas sp. and Isochrysis galbana, and three dinoflagellates Akashiwo sanguineum, Alexandrium minutum and Gymnodinium catenatum. Assimilation efficiency varied with algal species, but it was significantly higher when mussels were fed dinoflagellates (84.5%) compared with diatoms (61.7%) and flagellates (77.9%). Assimilation efficiency of dinoflagellates and flagellates increased with gut passage time, whereas with a diatom diet, a negative correlation was evident. This finding has implications in understanding and predicting growth rates of mussels (and hence commercial yield) in tandem with natural variability in phytoplankton species composition.