Understanding reproduction and energy storage, for broodstock conditioning, in Mytilus galloprovincialis

Synchronous seasonal reproduction in Mytilus galloprovincialis has been well documented, however asynchronous reproduction is also observed in many populations. These reproductive events occur at different times of year, and therefore under different environmental conditions. Experimental evidence regarding the influence of environmental parameters, such as temperature and food, on reproduction and energy storage in this species is limited. Therefore, this research aimed to determine the important environmental conditions that influence the timing and rate of reproductive maturation in relation to energy storage and use in mussels, which can be used for broodstock conditioning in the mussel aquaculture industry. Monthly samples of the population were collected over two years, and the timing and rate of maturation and energy storage was different between years and at different depths. Mussels used energy from food for reproduction in 2006, and used stored energy (glycogen) for reproduction in 2007. The use of these different reproductive strategies was potentially influenced by differences in temperature and food (phytoplankton concentration). Diet composition was tested for its effect on reproduction and energy storage. Mussels were able to spawn more readily when fed a combination of Chaetoceros calcitrans and Pavlova lutheri. A monospecific diet of Pavlova lutheri provided PUFAs necessary for maturation, but may have provided less energy; and Chaetoceros calcitrans provided sufficient energy, which increased glycogen storage, but may have been limited in PUFAs. The effects of temperature and ration were also tested, and the rate of maturation was fastest at 7°C and reduced in warmer water, up to 19°C, when held at constant temperature. When held in temperatures that mimicked seasonal temperature changes in spring, summer and autumn, mussels produced oocytes and stored energy during summer, but during autumn production of oocytes was greater and stored glycogen was depleted. Mussels fed a higher ration in this experiment allocated greater amounts of energy to storage or energy demands, but oocytes were not increased, which suggests that temperature rather than ration regulates reproductive maturation. The effects of food and temperature may be synergistic during seasonal cycles of reproduction. Reproduction appears to be prioritised over energy storage in mussels, which should result in year round reproduction in this species if energy (food or glycogen) is not a limiting factor. However warmer seasonal temperatures limit reproduction, therefore available energy is stored as glycogen. This information can be used to control the rate and timing of maturation of broodstock in a hatchery.