With only one exception, all existing mussel farms use pergolari suspended from either scaffolds or floating long-lines. Some growers collect seed on ropes and transfer to the pergolari when the seed have grown to 1–2 cm. Others obtain dredged ‘seed’ mussels already of this size.
These methods are well established and will not be discussed in detail. If mussel cultivation is to expand, however, Spanish-type rope culture is more amenable to mechanization and produces a more consistent grade of mussel. Knowledge of this technique should be made available to growers so that they at least have a choice of methods.
The ropes now used in Spain are spun from three strips cut from used fishing net. Twelve metres is the usual length, but in Greek waters 3–6 m would be appropriate. They can be hung either vertically or horizontally to collect spat in spring, depending on the local pattern of settlement. Once this has occurred, they are always hung vertically. Large rafts, as used in Spain, would be inappropriate given the weak currents in Greece, as stock nearer the centre of the raft would be starved.
In late summer the seed are stripped from the ropes either manually or by drawing the rope through a notch in a steel plate. It is then reseeded on 4–5 times the original number of ropes, using an ‘encording’ machine. This feeds a controlled flow of mussels from a hopper into a tube through which the new rope is drawn. As the rope and mussels leave the tube a light cotton, or 66% cotton 33% synthetic fibre, mesh is spun around both, binding them. Wooden pegs are put through the ropes for added support.
Once rehung in the sea, the cotton disintegrates in about one week, leaving the mussels embyssed on the ropes, with only the light synthetic fibre remaining. Ropes are lifted from the bottom end during handling, using a large steel basket hung from a specially designed hydraulic crane on the servicing boat. Stainless steel mechanical ‘declumping’ and grading machines are used, normally of a similar design to those of Franken B.V. of Netherlands.
All these techniques could be applied in Greece, except that the ropes would be hung from existing types of pole or long line. Cranes are not necessary for the short lengths able to be used on existing farms, but if deeper water (e.g., in N. Evoikos Gulf) comes to be used for long-lines, these would be required.
It would be advantageous to supplement spatfall in the Thessaloniki and Maliakos Gulf regions by the use of French spat collectors. The resulting seed could either be used for stocking the fished beds, or it could be grown-on in plastic mesh bags (‘poches’).
The spat collectors currently used in France are either plastic rods or tubes of plastic mesh about 1 m long. They are made up into bundles and placed sub-tidally in areas where good spatfall is known to occur, a few weeks before this is expected. They are left in place until any settled spat has grown to 1–2 cm. The bundles are then separated and the oysters removed by flexing each collector manually. The oysters are transferred to ‘poches’, c. 1 m × 0.5 m, closed at either end with a plastic stick threaded through the mesh. About 2 kg of seed is put in each bag. For Ostrea edulis the poches should be used without any internal spacer, as the slight pressure of the mesh directly on the oysters helps to prevent excessive movement by waves. Seed should be at least 30% bigger than the mesh of the poche. A good starting size is with 12 mm+ seed in 9 mm mesh poches. Smaller sizes quickly become fouled.
Poches are laid on steel tables (Fig. 2) and fastened down with strong rubber bands cut from old car inner tubes. In Greek waters the tables should be positioned so that they are just below the surface and do not uncover in tidal areas. Exposure would subject the oysters to excessive temperatures in summer.
The upper surfaces of poches become covered by weed. This is most easily controlled by turning them top-to-bottom two or three times during the year. When the oysters have grown to about 10 kg/poche, they are taken ashore, removed from the bags and graded. They can either be returned to clean 12 mm poches at 2–3 kg/poche, or laid on suitable ground within the area of the oyster fishery for subsequent dredging. The low price obtained for oysters in Greece (c. Dr 280/kg) may make it uneconomic to grow them completely to market size in poches.
The abundance of Ostrea edulis spat in Greek waters removes any necessity for hatcheries to be considered for oysters. There is no case for considering other species of oyster (e.g., Crassostrea gigas) as the major market in France is already well supplied with these and prices are low.
These can be planted in the sediment either at a low inter-tidal level, or in lagoons, provided these can be drained for harvesting. On inter-tidal sites 500–600/m2 can be grown, but in lagoons only c. 100/m2. Protection against crabs is essential. French methods of laying palourde under net or within a ‘French fence’ (Fig. 3) on the beach, and mechanized harvesting using a tractor, were discussed with the cooperatives at Alexandropoulis and Stilida. They hope to visit Brittany and Charante-Maritime (La Tremblade) to see these techniques.
At Alexandropoulis seed is reported to be available. This is not the case elsewhere, so hatchery seed would be required. Only the Manila clam (Tapes semi-decussatus) is currently produced by commercial hatcheries in the UK, France and Spain, as this is the species now favoured for cultivation.
Lagoon cultivation of Tapes, as practised in southern Spain and northern Italy, will present special problems of salinity control and water exchange in Greece. Extensive low-lying ground available on the coast near Alexandropoulis could be excavated into lagoons and provided with tidal exchange. This area was inspected by another consultant (D. Scott, in preparation). There may be conflicts with agricultural interests in this area.
Manila clams can be spawned in enclosed ponds and the spat then collected from the bottom. This is done extensively in China, but has not yet been attempted in Europe.
The limited number of sites suitable for palourde culture in Greece means that any hatchery established for them would only have a small home market. In view of the low success rate of newly established mollusc hatcheries in other countries, initial efforts at palourde cultivation should be based on imported seed from existing western European hatcheries. Pumped nurseries using the upflow system could be built to handle small ex-hatchery seed (1–2 mm) and raise it to a size suitable for laying in the bottom. Another consultant (see report of Richards) has recommended prohibition of imports destined for on-growing in Greek waters. The present consultant believes that any prohibition on the import of T. semidecussatus would prevent the development of palourde (Manila clam) aquaculture.
The native species, Penaeus kerathurus, was reported to take two summers (c. 15 months) to reach market size. It is possible that P. japonicus could be raised in a single year from hatchery production. This would have to compete in world markets with tropical production based on low pond construction costs, low food and labour costs and 2.5 crops/year. It should be possible to produce two crops of P. japonicus to a size of 20–25 g in Greece, one from April to July/August and another from July to October/November, but the economics of selling at this smaller size need to be assessed. The number of sites in Greece where ponds could be built and provided with good water exchange without pumping is very limited. Where natural lagoons exist, they support well-managed trap fisheries. In these, any attempts at penaeid on-growing involving construction of ponds or partitioning with nets, would have to be carried out by the cooperatives already working the lagoons.
Any prawn hatcheries built should operate with their own matured parent stock, not with bought in gravid females, a practice which creates conflict with wild fishery interests.
During visits the consultant was asked to comment on the possibilities of cultivating two other species of bivalve. One was the ‘horse mussel’ (Modiolus barbatus), fished in small quantities at Chalastra and Stilida. The other was a species of small scallop (Chlamys glabra).
The horse mussel has a specific market in Italy, to where 20–40 t/year are exported. These are obtained by dredging. Cultivation for this species does not exist, and it would only be if significant quantities of the spat could be obtained on ropes, not mixed with Mytilus galloprovincialis, that it could even be contemplated. With the abundance of Mytilus in Greek waters, this is unlikely to occur.
The fishery for Chlamys glabra in Kallonis Gulf, Lesvos, supplies a specialized market in Athens with 5–10 t year. The price is good (c. Dr 700/kg) but stocks are very low and the fishery, which is normally permitted in January, February and March, was not allowed this year in an endeavour to conserve stocks. A minimum size of 5 cm has been introduced. Spat collection trials using conventional methods for scallops (polypropylene potato sack collectors suspended from polystyrene floats) have been established by the Ministry of Agriculture Fisheries Department, in collaboration with the Prefecture and the Ministry of the Aegean. Spat have been obtained but almost total mortality occurred during early on-growth in suspended cages. This may have been due to excessive temperatures this summer. Earlier trials by the National Centre for Marine Research indicated that from settlement in April, Ch. glabra could reach a market size (5–6 cm) by June/July of the following year. The trials should be continued, but the existence of the small fishery should not inhibit the development of aquaculture for other species, such as mussels, in the gulf. However, any use of T.B.T. antifoulants would be very damaging in this enclosed body of water.
