Sargassum is also an appropriate species to cultivate offshore. It even has certain
advantages over kelps, such as a higher market value ($500 per t) and the unique
cultivation technology of using a perrenating holdfast for multiple years.
2.5.3 Bivalves
2.5.3.1 Mussels
Mytilids present a high fecundity and the larval phase is mobile and free-living, a
fact that facilitates a widespread distribution. The culture technique used is greatly
influenced by the seed availability. Mussel culture worldwide is mainly relying on
wild seed, either by capturing competent (=ready to settle) larvae by means of
mussel seed collectors or by fishing young mussel seed (2 cm) from natural mussel
banks, which is predominantly applied in The Netherlands, Germany and Denmark.
Mussels can be farmed on-bottom or off-bottom. On-bottom culture is based on
the principle of transferring wild seed from areas where they have settled naturally
to areas where they can be placed in lower densities to increase growth rates, and
that facilitate harvest and predation control (Seed and Suchanek 1992; Seaman and
Ruth 1997). Off-bottom cultivation methods includes (1) the bouchot method,
where piles with seeded mussel ropes are planted in the intertidal zone and mussel
spat grows to market size, (2) rafts, where ropes are seeded with young mussels and
suspended vertically from a floating raft construction, and (3) longline culture,
where mussel spat is attached to vertical ropes, called seed collectors, or on continuous ropes (e.g. New Zealand), which hang on a horizontal backbone including
floating devises to support buoyancy (Buck et al. 2006b). Seed collectors may
consist of coconut fiber ropes placed in the intertidal zone (France), short polyethylene droppers from rafts (Spain), specially designed collector droppers with
increased surface (e.g. “christmas tree”), fixed on longlines or collector nets (e.g.
Smartfarm®). Production time from settlement to market size depends very much
on the species and production location (temperature and phytoplankton concentration) but varies between 18 and 36 months.
Longlines and raft systems are the two commonly and most successfully used
mussel-farming systems. The longline system consists of one to two suspended
horizontal headlines made of 12–15 cm polypropylene rope about 100–220 m in
length and anchored at both ends. Single headlines are sometimes preferred as the
gear required to lift and harvest a single line is far more economical and less labor
intensive. The headlines are connected to large floats (usually of plastic, each with
about 300 kg displacement), which together support a large number of 5 m culture
droppers (ropes). The number of droppers used varies with hydrological and biological conditions but is normally in the order of 400–500 per (double headline)
longline. Secure anchoring is necessary to prevent the longlines shifting in heavy
seas or strong currents. In soft bottom sediments, traditional concrete or steel
anchors of 250–500 kg are used. Depending upon the amount of movement
2 Offshore and Multi-Use Aquaculture with Extractive Species…
33
advantages over kelps, such as a higher market value ($500 per t) and the unique
cultivation technology of using a perrenating holdfast for multiple years.
2.5.3 Bivalves
2.5.3.1 Mussels
Mytilids present a high fecundity and the larval phase is mobile and free-living, a
fact that facilitates a widespread distribution. The culture technique used is greatly
influenced by the seed availability. Mussel culture worldwide is mainly relying on
wild seed, either by capturing competent (=ready to settle) larvae by means of
mussel seed collectors or by fishing young mussel seed (2 cm) from natural mussel
banks, which is predominantly applied in The Netherlands, Germany and Denmark.
Mussels can be farmed on-bottom or off-bottom. On-bottom culture is based on
the principle of transferring wild seed from areas where they have settled naturally
to areas where they can be placed in lower densities to increase growth rates, and
that facilitate harvest and predation control (Seed and Suchanek 1992; Seaman and
Ruth 1997). Off-bottom cultivation methods includes (1) the bouchot method,
where piles with seeded mussel ropes are planted in the intertidal zone and mussel
spat grows to market size, (2) rafts, where ropes are seeded with young mussels and
suspended vertically from a floating raft construction, and (3) longline culture,
where mussel spat is attached to vertical ropes, called seed collectors, or on continuous ropes (e.g. New Zealand), which hang on a horizontal backbone including
floating devises to support buoyancy (Buck et al. 2006b). Seed collectors may
consist of coconut fiber ropes placed in the intertidal zone (France), short polyethylene droppers from rafts (Spain), specially designed collector droppers with
increased surface (e.g. “christmas tree”), fixed on longlines or collector nets (e.g.
Smartfarm®). Production time from settlement to market size depends very much
on the species and production location (temperature and phytoplankton concentration) but varies between 18 and 36 months.
Longlines and raft systems are the two commonly and most successfully used
mussel-farming systems. The longline system consists of one to two suspended
horizontal headlines made of 12–15 cm polypropylene rope about 100–220 m in
length and anchored at both ends. Single headlines are sometimes preferred as the
gear required to lift and harvest a single line is far more economical and less labor
intensive. The headlines are connected to large floats (usually of plastic, each with
about 300 kg displacement), which together support a large number of 5 m culture
droppers (ropes). The number of droppers used varies with hydrological and biological conditions but is normally in the order of 400–500 per (double headline)
longline. Secure anchoring is necessary to prevent the longlines shifting in heavy
seas or strong currents. In soft bottom sediments, traditional concrete or steel
anchors of 250–500 kg are used. Depending upon the amount of movement
2 Offshore and Multi-Use Aquaculture with Extractive Species…
33