Certain coastal waters of the Netherlands, e.g. the Waddenzee and the Delta area, have great ecological value. This value is expressed by the protection of these areas by European law (Nature Conservation Act or the European Birds and Habitats Directive). Mussel seed fishery in these areas may impede with these laws. By using mussel seed capture installations (MZI) the impact of the mussel seed fisheries on wild mussel beds can be reduced. At the moment of writing the musselfarmers are in a transition process from fishing seed to collecting seed.
Mussel produce larvae in the spring. These larvae drift in the water for several weeks before latching onto a hard substrate. The survival rate of the larvae is much higher if the larvae can latch onto a substrate that hangs in the water, rather than latching directly onto the seabed. After latching on the hard substrate the mussel is known as brood and develops into seed. When the seed is large enough the mussel farming sector can use them for bottom culture.
Different designs of MZI-installations can be distinguished:
- Fixed construction to the seabed, being:
- Vertical capture installation around a pole or in a rack (fence).
- Horizontal capture installation which are stretched above the sea bottom (table).
- System which is hanging on a floating construction (curtain). These floating constructions are attached to the sea bottom using cables with anchors or concrete blocks. The floating constructions can be anything that floats, for example:
- Barrels
- Floaters
- Buoys
- Platforms
- Pontoons
- Floating Jerry cans
Conducted pilot studies showed that ropes and nets hanging down vertical from the water surface give the best result. Vertical constructions are more productive than horizontal constructions. This is due to the lack of protection in the horizontal constructions against predators, which eat the larvae once attached to the substrate.
In terms of technical feasibility the construction must be robust enough to withstand the local wave climate and currents.
Substrate materials
Different types of substrate materials (ropes) can be used. Pilots with different types of ropes (see figures) are executed. The New Zealand and Christmas tree rope gave the best results. Both ropes have a lot of filaments.
To make a design more sustainable one could use recycled materials as substrate. Another interesting alternative is a substrate which consists of degradable material. However the material used should not dissolve too quickly in the salt water environment.
Left figure: Left to right: Spanish, New Zealand, Fries, Japanese ropes and completely right, a cotton net to sock mussels
Right figure: From top to bottom: Coconut rope (CCR), Polipes rope (BQR), Christmas tree rope (XTR) and Galician traditional rope (GTR)
From top to bottom: Coconut rope (CCR),
Polipes rope (BQR), Christmas tree rope (XTR)
and Galician traditional rope (GTR)
Left to right: Spanish, New Zealand, Fries, Japanese ropes
and completely right, a cotton net to sock mussels
Suitable locations
MZI installation can be used anywhere where mussel larvae are present. However the construction itself is quite vulnerable to currents and waves. Therefore an area which is sheltered is favourable. On the other hand a current is required for larvae and food (algae) supply.
Most MZI installations are constructed from plastics. Due to wave action part of the MZI installation (e.g. jerry cans, ropes) can become loose and start drifting. This is called macro-plastic pollution. The macro-plastic pollution can be reduced by a robust construction which is able to withstand destructive forces, such as waves.
Another type of contamination by the MZI installation is micro-plastic pollution. The mussel seed attaches onto ravelled parts of the ropes. Very small ravelled parts can become loose, e.g. during harvest. Hereby micro elements of the used materials may disappear into the environment. In case of synthetic materials (plastics, polymers) this may have a negative effect on the ecology. The micro plastics may be taken in by filter organisms. These organisms are mostly on the base of the marine food chain. The micro plastics itself are toxic and in addition to this organic pollution in the water (DDT derivates, PCB's and modern relatives) may attach to the micro plastics. When the micro plastics go through the digestive system these contaminations can be absorbed by the organism. Because the effects of micro plastics are not completely known, more research should be conducted to investigate the effects of these micro plastics. More research needs to be conducted into base material which in case of fragmentizing in the seawater are fast degradable. And materials which are their total lifespan harmless.
Currently the mussel sector is improving the different MZI techniques.
Several lessons can be learned from Mussel seed capture installations (MZI).
