Abstract:
Ostrea chilensis is a flat oyster species native to both the Pacific Coast of South America and New Zealand. This species has been commercially fished in the Foveaux Strait and Nelson/Marlborough area from the 1850’s, currently generating more than $23 million per year. However, overfishing and parasite outbreaks have greatly reduced wild production. Thus, aquaculture of O. chilensis represents a prime economic opportunity, and production of this species is currently carried out in Stewart Island and in the Marlborough Sounds. However, within the Marlborough Sounds, mortality levels of cultured O. chilensis are consistently highest in late summer following a spawning peak. Causative agents of this mortality are unclear. A warmer water temperature increases physiological rates which in turn increases energy demand. Primary production can be dependent on nutrient input from freshwater run-off, which in late summer is reduced, and unable to meet this demand. Thus, the aim of this thesis was to investigate the effect of food quantity on the condition of O. chilensis exposed to a putatively stressful summer water temperature in order to establish whether food limitation may be causing physiological stress and contributing to post-spawning mortality. Comparisons were also made among various selectively bred families in order to potentially identify families that are better able to withstand nutrient and temperature stress. Clearance rate, absorption efficiency and metabolic rates were measured in order to calculate the scope for growth in oysters exposed to a water temperature of 20 °C in combination with various algae cell concentrations. Simple gravimetric and visual (subjective) condition measures, as well as biochemical (protein, lipid and carbohydrate) assessment were also used to assess these same oysters, and to compare condition with oysters freshly obtained from the farm site. Results revealed a decline in scope for growth at the highest cell concentration. In general, the other condition measures used showed an improvement in condition as algal cell concentration increased. Examination of the separate components of the scope for growth equation indicated that the absorption rate was low at high food concentrations. This may be explained by saturation of the digestive tract, resulting in a lower absorption rate while in fact condition was positive. This highlights the importance of using multiple condition measures concurrently to gain a thorough understanding of response to different environmental conditions. Large inter-individual variation resulted in no significant differences between families in any of the condition measures. Further investigations using larger sample sizes or longer term trials may be required for any significant differences to become apparent. Scope for growth was more positive under low food environments, suggesting that nutritive stress is not a significant factor in post-spawning mortality. Future research should focus on other potential contributors to late summer mortality, for example warmer water temperature, or infection from parasites such as Bonamia ostreae which has recently been detected in New Zealand populations for the first time.