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Certain coastal waters of the Netherlands, e.g. the Waddenzee and the Delta area, have a 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. To reduce the impact of the mussel seed fisheries on wild mussel beds mussel seed capture installations (MZI) (Dutch: MosselZaadInvang installatie) are developed. At the moment the musselfarmers are in a transition process from fishing seed to collecting seed. The aim is that in the long term the MZI installations can supply the mussel seed which is needed for mussel cultivation.
An EDD approach of using MZI's can be found in the EDD Case - Harbouring Opportunities. In this case MZI's are used to create ecological value (biomass) in the port of Rotterdam with an additional function of water filtering.
The MZI installations make use of the lifecycle of the mussel. 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. The MZI installations are made of hanging substrates such as ropes and nets (see design) and are placed in the water in the spring at a time when large numbers of larvae are present in the water. The best time for installation of the MZI in the Netherlands is in April.
The main aspect of a MZI installation is a hanging substrate in water. The MZI installation should provide materials on which mussel larvae want to settle and on which they can efficiently develop and stay attached. 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:
- Floating Jerry cans
The substrate used can be made out of ropes or nets, for more information on types of substrate see substrate materials.
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, see also suitable locations.
Photos of MZI-installations
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.
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.
Problem of drift litter
MZI installations can increase the amount of drift litter in the sea. 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 know, 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.
At moment of writing the mussel sector is improving the different MZI techniques.
- With the use of a hanging substrate it is possible to capture a large amount of mussel seed.
- The best results are gained with the use of a vertical hanging substrate, in comparison with horizontal stretched substrate.
- In the comparison of different types of ropes the New Zealand and Christmas rope give the best result. Both are quite ravelled on which the mussel larvae can easily attach.
- The construction of the MZI should be robust enough to withstand the local current and wave conditions. Therefore it is favourable to construct the MZI in relative sheltered areas. However there should be enough current for the supply of mussel seeds and food (algae).
- The best time to install a MZI in the Netherlands is in early spring, being April, due to the production of mussel larvae.
- One should take into account drift litter which can come from the MZI installation. An interesting option, which should be investigated is to use bio degradable materials as substrate, which are not harmful to the environment.
Kamermans P. & E. Brummelhuis (2002): Productie van mosselzaad met collectoren. RIVO Rapport C010/02.
Kamermans P., M. Poelman, E. Meesters, I. De Mesel, C. Smit & S. Brasseur (2008): Onderzoek naar Duurzame Schelpdiervisserij (PRODUS). Eindrapport deelproject 1c. Alternatieve mosselzaadwinning met MosselZaadInvangsystemen:variatie in zaadinvang en effecten van MZI's op het ecosysteem. Rapport C075/08
Poelmans M., P. Kamermans (2010): Inventaristatie MZI oogst 2009. IMARES rapport C033/10
Scholten M.C.Th., F.A. Veenstra & R.H. Jongbloed (2007): Perspectieven voor mosselzaadinvang (MZI) in de Nederlandse kustwateren. Een evaluatie van de proefperiode 2006-2007. IMARES rapport C113/07.
Wiersinga W.A., J.E. Tamis et al. (2009): Passende beoordeling voor Mosselzaadinvang (MZI) in Nederlandse kustwater. IMARES Rapport C089/09