Bivalves are active filter feeders and at the high temperatures attained in Greek waters require good concentrations of phytoplankton in the water to sustain growth and condition. Because the majority of Greek waters are low in productivity, a first approach to assessing broad areas in which potential sites might exist can be based on an examination of the productivity, after which the ecological and cultural requirements of individual species need to be considered.
Any development of penaeid culture will depend on the availability of suitable lagoon areas, or coastal land where ponds can be created. These are localized and are considered in a later section (4.4).
Areas of potential high productivity are most easily identified by their nutrient levels, preferably taken during winter. Chlorophyll levels are difficult to interpret without associated measurements of rates of production, as grazing by filter feeders can reduce the standing crop even in areas of high production.
Inorganic nutrient concentrations have been determined in most of the areas where bivalves are currently harvested. Friligos (1987) related these to ‘background’ levels in the Aegean Sea, expressing them as a ratio with the Aegean water concentrations. Using phosphate as an indicator of potential productivity, and using data from some additional sources, major areas can be listed as follows:
Ratio of PO4 with that in Aegean Sea*
| Maliakos Gulf | 5.6 |
| Elefsis Bay | 5.1 |
| Inner Thermaikos Gulf | 4.9 |
| North Evoikos Gulf | 2.8 |
| Geras Gulf (Lesvos) | 2.8 |
| Inner Saronikos Gulf | 2.5 |
| Western Saronikos Gulf | 2.2 |
| Kavala Gulf | 1.8 |
| Outer Themaikos Gulf | 1.5 |
| South Evoikos Gulf | 1.4 |
| Pagassitikos Gulf | 1.0 |
| Patras Gulf | 1.0 |
* Aegean Sea Concentration 0.12 μgm/atom/1 PO4-P
In all areas where this value is at least 1.5, molluscs are either commercially harvested, or trials (with mussels) have yielded satisfactory growth rates.
For commercial scale culture to be maintained, water movement is also necessary, as bivalves can quickly remove all phytoplankton from the water even in highly productive areas. Where a good exchange of water exists, bivalves can be produced in less productive water. In most areas in Greece the lack of current will limit the density at which bivalves can be cultured. Highly intensive systems such as Spanish rafts (c. 180 t mussels/stremma) will not be possible, but long-line systems for mussels, permitting a less intense loading, are feasible. The pole culture of mussels around Thessaloniki and in west Elefsis Bay do achieve similar levels of harvest to Spain, allowing for the shallower depth of water column used 15 t/stremma per m water column used).
Productivity and water flow are not the only criteria in site selection, and shelter from storms is essential. Long-lines and rafts should withstand 2-m waves, but mussels will be lost from the pergolari or ropes by agitation during storms. Much of the coast of northern Greece is very exposed, and first trials with pole cultivation near Alexandropoulis have suffered from storm damage. The possibility of using an outer tube of coarse mesh in the way that ‘catinage’ is applied to ‘buchot’ cultivation of mussels in France was discussed by the consultant.
There is clearly room for expansion in the areas where mussel farming already exists. The question of maximum holding capacity of an area was discussed specifically in relation to the enclosed Kallonis Gulf in Lesvos and the consultant referred the Ministry of Agriculture ichthyologist to work at La Tremblade laboratory of IFREMER in France. However, at the present stage of development of cultivation in Greece, this is an academic question. Existing studies in phytoplankton and chlorophyll levels carried out by NCMR can serve as a baseline for future analysis of this question as cultivation expands. Reductions in growth rate in an area would be the first indication of excessive stock levels.
A large area of productive water with good tidal exchange exists in Maliakos Gulf, enriched by nutrients carried down by the River Sperhios. There is some industry around the Gulf, but a previous major concern about pollution from a cable factory has been diminished by improved effluent treatment. The consultant found live oysters within 20 m of the effluent pipe. Friligos (1976) reported Cu concentrations in mussels in Maliakos Gulf to be only slightly higher than in other Mediterranean areas, at 28 ppm.
a. Mussels
Being the most active feeders of the three bivalve species under consideration, and the only ones which can be grown practically in three dimensional suspended culture, mussels require the most food, so high productivity combined with water movement is essential. Given these conditions they can reach a market size of 7 cm length in 11–14 months from settled spat, or 9–11 months from dredged seed of 1–2 cm. Mussels can tolerate lower salinity than the other bivalves before growth rate is diminished, 20% is adequate.
The types of site suitable can be divided into three groups:
Shallow estuarine areas suitable for pole culture. High productivity. Water movement mainly by wind. e.g., Chalastra, Stilida (W. Maliakos), Keramoti.
Protected open water areas with some elevation of productivity, good water exchange. Suitable for long lines. e.g., Makrigialos, Stilida (E. Maliakos), N. Evoikos.
Localized productive bays. Good small-scale culture, but holding capacity soon exceeded by large-scale cultivation. e.g., Kyparissi Bay.
Large enclosed bays such as Kallonis (Lesvos) can be included in (2), even though exchange with the open sea may be restricted. Water movement is wind induced, so farms must be more widely spaced.
In view of the paucity of sites in Groups (1) and (3), the greatest potential for expansion is in areas with the criteria of Group (2), though the requirements for open, moving water and some shelter conflict.
Most of the coast of N. Greece has productive water with good exchange, but few sites offer shelter. The site being used at Makrigialos is very exposed to the south east, and some damage can be expected in storms, which often occur during summer. Limited shelter exists north of the island of Thassos. Trials with pole culture at Alexandropoulos have suffered from storm damage.
There remains room for expansion of pole cultivation in the shallow estuarine areas around Chalastra in Themaikos Gulf and around the more open coast in the area (e.g., Mihaniona). This is already happening, and cultivation in the area can be expected to increase.
The best combination of productive water, currents and shelter for long lines exist in the N. Evoikos Gulf/Malaikos Gulf area, which has one of the largest tidal ranges in the Mediterranean and which is enriched by nutrients, mainly from the Sperhios River. Sites suitable for pole culture exist around the western end of Maliakos Gulf, with deeper water long-line sites in the central and eastern end. The extent to which productive conditions extend into Diavlos Orean to the north east have not been ascertained, but this area should be investigated. Very enclosed bays within the area, such as Kyparissi, can sustain small long-line units, but have a restricted carrying capacity.
A mussel growth trial has been carried out at Kyparissi (N. Evoikos Gulf), where in a joint project between PASEGES, the local Agricultural Cooperative and NCMR, a small quantity of mussels were grown in pergolari and reached 7.1 cm from 3.4 cm between September and May, i.e., mainly over winter (Metaxatou, NCMR), exceeding growth in a similar trial in Elefsis Bay. The mussels were of good quality and sold for an exceptional price of Dr 480/kg. The trial was in the most enclosed part of the bay, and another consultant in the present study recommended this area for development, with mussel cultivation within the innermost bay and fish cages offshore (see Guelorget, 1987). The present consultant believes that the holding capacity of this enclosed area would quickly be exceeded, and that mussel growth would be threatened by any antifoulants (especially T.B.T.) used on fish cages. The consultant therefore advises that the main efforts in mussel cultivation should be seaward of the area recommended for fish cages.
The two Gulfs of Lesvos (Kallonis and Geras) offer excellent protected water conditions. The latter has a large leather tannery near its mouth, but beyond c. 2 km from this conditions should be good. Close inside the mouth of Kallonis Gulf should be an excellent area. The use of this bay for mussel culture was discussed with the ichthyologist from the Ministry of Agriculture, Fisheries Department, in Mitilini. They had been cautious about encouraging mussel cultivation as the Gulf sustains a small (5–10 t/year) but unique fishery for a small scallop, Chlamys glabra. The department's concern had been that additional larvae resulting from the spawning of cultivated mussels might compete with the Chlamys for settlement surfaces, and overwhelm them. The consultant advised that the wild stocks of mussels which exist in the gulf must already provide surplus numbers of larvae, and hence colonization of mussels must be restricted by the substrate requirements of Mytilus. The introduction of additional numbers through farming would only add to this surplus and should not endanger the Chlamys stocks.
In another mussel trial in Amvrakikos Gulf, carried out by the Preveza office of the Ministry of Agriculture, growth has been very slow and mortality high. The water appears to be very productive. High temperature (29°C) and excessive plankton blooms may have produced this poor result. Winter growth is reported to be better than summer growth in Amvrakikos, and the trial is being continued. Reports of mussel growth on the south shore of Amvrakikos are more encouraging. T.B.T. must not be permitted here.
The origin of the high productivity in Saronikos Gulf is mainly domestic sewage discharged from an outfall near Piraeus, with lesser sources around the gulf. A number of protected sites exist around the gulf and could be used for mussel cultivation. However, depuration has not yet been established outside the Thessaloniki area.
b) Oysters
In Greece, all production of oysters is exported and growers need to be able to market a minimum consignment of 20 t a time, so it becomes difficult to start small-scale operations in areas remote from the existing exporting centres of Thessaloniki and Stilida. It is therefore recommended that initial efforts at spat collection and subsequent on-growing be concentrated in those two areas. When cultivating Ostrea edulis it is advisable to operate in an area where wild stocks grow successfully. Increased organic load in the River Axios may lead to some existing spat settlement grounds becoming fouled and unsuitable for larval settlement when the new sewage scheme is completed. If this happens it will accelerate the need to supplement spatfall by collection in clean areas.
c) Palourdes
The European species, Tapes decussatus, has specific substrate requirements for firm, slightly muddy, coarse sand. This greatly restricts its distribution. As in the case of the oyster, cultivation efforts should be concentrated in those areas where there is natural production. The inter-tidal beach currently fished at Alexandropoulis is the prime site for this. The inter-tidal flats at the head of Maliakos Gulf should also be suitable, but much of the ground here is too soft. Also there is no obvious source of surplus seed as found at Alexandropoulis. Manila clams would be more likely to succeed in Maliakos. At other sites visited (e.g., Geras Gulf, Lesvos) the lack of tidal exposure would make cultivation difficult. At Kyparissi, growth of native palourde appeared to be very slow and no large individuals were found. The site on Salamis, close to Piraeus, should not be used.
d) Penaeids
These could be raised in lagoons, such as those at Keramoti and Agiama (Kavala) where there is a good tidal exchange of water and salinity is controlled by natural springs and artesian wells. In other lagoons, such as those on the north side of Amvrakikos and at Messolonghi, salinity control and provision of adequate water exchange without recourse to pumping would be a major problem.
Sites exist where ponds could be excavated for either penaeids or palourde. At Alexandropoulis one such area has the advantage of sufficient tide to provide water exchange. There will be conflicts with agriculture if ponds are built, especially over fears of saline water intrusion into surrounding soil.
