Advancing the nursery culture of juvenile green-lipped mussel, Perna canaliculus

Abstract

The green-lipped mussel, Perna canaliculus, has become one of the most valuable seafood exports from New Zealand with current annual export earnings of US$308 million based on the production of around 200,000 t each year. However, the green-lipped mussel aquaculture industry in New Zealand relies heavily on juvenile mussels, or spat, harvested from the wild. Consequently, with the rapid growth of the industry, shortages of spat have become an increasingly major constraint for the industry. One of the solutions is to optimise the quantity and quality of juvenile mussels produced from a hatchery. However, the production of juvenile mussels in a hatchery is costly compared to harvesting wild mussel spat, with the production of microalgal food for hatchery spat contributing around 30 – 50 % of total hatchery production costs. There is still limited information on the feed requirements of juvenile mussels and the optimum environmental conditions required to obtain the best performance from spat in the hatchery situation. Therefore, the aim of the research presented in this thesis was to investigate the feeding abilities of juvenile P. canaliculus of a range of sizes when exposed to microalgal food particles of various sizes, and to observe some of the potential key environmental conditions that may affect the performance of the mussels in a nursery situation (i.e., growth, retention and survival). Flow cytometry (FCM) was used to count microalgae to determine the filtering abilities of juvenile mussels of a range of sizes when exposed to a variety of microalgae species of different sizes. Potential key environmental conditions (i.e., light exposure, water motion and oxygen levels) were experimentally manipulated and the subsequent performance of juvenile mussels was measured to determine the optimal culture conditions. The results showed that during a short observation period (1 h) the smallest size class of juvenile mussels used in this study (0.5 – 0.9 mm) was more efficient at filtering larger sized microalgae species (i.e., > 6 μm cell diameter) compared to microalgae species of smaller size. This suggests that the current commercial practice of feeding juvenile mussels with small microalgae species (i.e., 3 – 5 μm cell diameter) is highly inefficient. Over a longer observation period (24 h), when a range of sizes of P. canaliculus spat were provided with a high concentration (40 cells μl⁻¹) of four microalgae species with different cell sizes (i.e., Diacronema lutheri, Tisochrysis lutea, Chaetoceros muelleri, and Tetraselmis suecica), the particle capture ability was found to increase in proportion to the size of the juvenile mussels. The largest microalgae species T. suecica tended to be captured at the lowest rate compared to a smaller microalgae species D. lutheri. Using the longer observation period (i.e., 24 h) with FCM measures found that juvenile mussels in the size class of 0.5 – 0.9 mm in shell length were highly efficient at capturing all four of the tested microalgae species at ~2.4 – 2.9 × 10⁵ cells mussel⁻¹ 24 h⁻¹. Results from the experiments testing the effects of environmental conditions on the performance of juvenile iv mussels under culture conditions indicated that a combination of light exposure (i.e., some period of light exposure within 24 h) produced better growth, while high water motion (i.e., > 33 ml s⁻¹ of aeration or > 5 cm³ s⁻¹ water flow) and high oxygen levels (i.e., > 80 %) increased the growth, retention and survival of juvenile P. canaliculus. Overall, the results of the research presented in this thesis provide valuable information that has the potential to greatly increase the efficiency of commercial nursery culture of juvenile P. canaliculus in New Zealand.