The semi-natural breeding process produces juvenile mussels to help prevent the threatened mussel stocks from dying out. With semi-natural breeding, the juvenile mussels grow in near-natural conditions. As such, this method is also suitable for obtaining additional insights into the ecology of juvenile mussels. Semi-natural breeding is based on the development cycle of the mussels and so is split into three phases:
Extraction of glochidia and “infection” of the host fish
The first step in the breeding cycle is obtaining the glochidia (= mussel larvae). There are several common ways to do this. The most common is in-situ extraction. Adult mussels are regularly checked in their natural habitat to see if they are gravid and to see the maturity level of the glochidia. Once they are fully mature, the glochidia are extracted from the adult mussels while they are in their natural habitat. The glochidia are then placed in a bath into which the host fish (brown trout) are placed and subsequently “infested”. Alternatively, there is ex-situ extraction. During gravidity, the adult mussels are usually removed from their natural habitat and placed in a breeding facility where the host fish are then infested directly.
Extraction of juvenile mussels ex situ/in the breeding facility
Once the larvae have fully developed, the juvenile mussels drop off the host fish. During this time, the fish are kept in circular flow tanks/aquaria and the discharged water is passed through sieves. The juvenile mussels are caught in these sieves. Circular flow tanks usually have stream water flowing through them. As such, harvesting in circular flow tanks is a natural, weather-related process that takes place over time, from the development of the larvae into juvenile mussels and then the subsequent harvest of the juvenile mussels. With harvesting in aquaria, is it possible to shorten the developmental stage on the host fish. This can be done by operating the aquarium in closed circuit which causes the water to warm up. This means that you can start harvesting the juvenile mussels after three months. Both methods will be used in the MARA Project.
Rearing stage ex situ
Particularly with an early harvest, the juvenile mussels are moved to special containers with the right food (detritus and algae) and kept in optimal conditions until early summer. This step allows the juvenile mussels to develop a good physiological constitution which will help them to survive during the later in situ rearing stage.
Rearing stage in situ
For the in situ rearing, the juvenile mussels are transferred to the rearing waters and kept there in rearing containers, Buddensiek plates and gravel cages, for several years. This allows the juvenile mussels to grow in a natural environment and means that they are adapted to the target water when they are released later. Regularly checking and cleaning the rearing containers allows the mussels to grow without their development being impeded by the fine sediment inputs in the water. The direct exposure of the juvenile mussels to the target water means that semi-natural breeding can also be used for bioindication experiments. One disadvantage of this method is that the water parameters cannot be regulated like they can in ex situ breeding. In addition to an optimal adjustment of the target water, in the breeding programme, great value is placed on preserving genetic diversity, which is why breeding books are established. These books are used to record the adult mussels used for breeding each year.