Scallop culture has been greatly enhanced since the 1950s due to the success in collecting natural scallop spat. At this time synthetic materials were first available for fishing nets and were being incorporated as spat collectors from the 1960s (Tsubata et al., 1972). Until then the settling materials, mainly cedar leaves, scallop shells, and nylon mesh, were hung or tied in bunches to lines or nets. It was at this time that an important contribution to the technique of spat collection was made by a Mutsu Bay fisherman. He fastened a small-mesh onion-bag around the cedar leaves, allowing the scallop larvae to enter the mesh and settle on the cedar leaves. The bag retained the settled spat which normally fell off the collectors as the abyssal attachment phase finished (Fig. 2).
A long-line system for spat collectors is now widely used. The lines vary in length but are usually 50–100 m long, and the number of bags on branch lines varies according to the working depth. The arrangement of the sub-surface buoys is not fixed, and varies according to the number of branch lines per section of rope and the number of bags attached. The polyethylene-mesh bag typically used in Japan measures 80 × 37 cm, with a mesh size of 5 × 2 mm (Fig. 2). Various artificial substrates are inserted in the bag depending on the spat numbers required per bag. In coastal areas where larval densities per m2 are known to be high, heavy spat settlement in the collectors is undesirable as it results in poor growth and subsequent survival. Two commonly used bag fillings are large mesh monofilament or soft netlon. However, a great variety of other materials (nylon and plastic meshes, rubberized fibre ‘Hairtok’, teased polypropylene rope, Hyzex film, etc.) are being used. For the giant scallop, P. magellanicus, monofilament gill netting is nearly twice as efficient as polyethylene strips. It has also been suggested that the amount of each substrate is critical for maximizing spat collection. Using polyethylene sheets, the best spat density occurs at 0.75 m2, whereas additional substrates result in only marginal increases in the number of spat. The phenomenon of diminishing returns is even more striking in the monofilament substrate where spat number actually decreases with an increase in substrate volume (Naidu et al., 1981).
Fig. 2. Spat collector bag from Mutsu Bay
For the two major European scallop species (C. opercularis and P. maximus), no particular material has any great advantage over the others, as both species settle on most materials, providing it is a clean, silt-free surface (Paul et al., 1981). The mesh size of the outer bag is important with regard to collector design, as it must be small enough to prevent spat from escaping, yet not cause excessive reduction in water flow. A 1.5 mm mesh bag has been used for collecting P. magellanicus and a 2 mm mesh can collect more P. maximus spat than a 5 mm mesh (Naidu and Scaplen, 1976; Buestel, 1976). On the other hand both C. opercularis and P. maximus can be collected successfully using a 6 mm mesh (Pickett 1978; Brand et al., 1980). A 2 mm mesh has also been used but the collectors become badly clogged with silt. No generalization can be made on the optimum size range, as this varies according to species and hydrographic conditions, particularly the degree of siltation.
The importance of setting out the collectors at the correct time has been recognised for many years by the Japanese who use a variety of forecasting techniques. These include gonad index, cumulative water temperature, and size composition of the swimming larvae (Ito et al., 1975; Yamamoto, 1975).
One problem related to spat collection and distribution is the transportation of spat out of water. Often they are shipped considerable distances before resettlement. Because of transportation difficulties scallop culture has not developed in some areas where it could be carried out. Scallop, during transportation, require moisture and low temperatures ranging between 6–8°C.
An intermediate culture stage for scallop has became necessary as high mortalities occur if early spat are transferred too soon. The requirements for an interim stage were developed by Kodera et al. (1961), who reared spat in boxes hung vertically in seawater until shell length reached 3 cm. The survival rate of these spat when transferred to the culture grounds was more than 50%. The interim rearing box spread with Salan net devised by Kodera et al. was eventually replaced with rearing baskets spread with Clemona. In 1959 this was also replaced with pearl net, still in use at the present time (Fig. 3).
Fig. 3. Pearl nets used for scallop intermediate culture. Mesh can be between 2–7 mm according to spat size.
The long-line for intermediate culture is similar to the spat collectors, except they are often up to 200 m in length. They can be set to the same depth, with the branch lines 0.5–1 m apart and the pearl nets (7–10 in number) 0.5–0.75 m apart according to mesh size. There is no fixed system, and many variations can be observed in different geographical areas. Naidu and Cahill (1986), in their guide to scallop culture in Newfoundland waters, give specifications which suit that particular environment.
The pearl net is an important feature of the intermediate culture process, and it is now used for ongrowing. These small-mesh nets offer protection and good flow characteristics with no critical silting for small vulnerable spat. Pearl nets may be pyramidal or conical, but the latter is preferred as it prevents bunching of scallops in the corners (Fig. 3). There is a range of mesh sizes available from 2 to 7 mm. Changing nets may be necessary at times in this fast-growing stage, and the correct stocking density is necessary for the spat to realize their potential growth. During intermediate culture, growth decreases significantly above a density of 100 spat per pearl net (Kanno, 1970). Mortality of juvenile king scallops (P. maximus), cultured in pearl nets rise with increasing density at the optimum depth of 1–3 m (Howell, 1987). Net changing and spat thinning may be necessary in those areas where growth is rapid. However, although it is now appreciated that frequent handling may be a source of disturbance, it is good practice to check the cultures periodically and, if necessary, remove predators such as crabs and starfish.
The water depth for hanging spat during interim culture is 2 to 10 m. In Maine Bay, Miyagi Prefecture, growth of scallop is poor when hanging depth is shallow, and improves with depth of a few meters. Growth of the Iceland scallop (C. islandica) is most favourable at 12 m and least favourable at 30 m. In the one case it appears to be surface water turbulence which affected growth, and in the other the result of differences in nutritional conditions of the water column (Wallance and Reinsnes, 1985).
After the intermediate culture phase, during which time spat develop a harder shell, a wide range of techniques are used for ongrowing to commercial size. These include lantern nets, pearl nets, book nets, and attachments, etc.
The long-line system, developed and widely used in Japan, has been adopted worldwide, and development has concentrated on adaptations of the system rather than developing new techniques.
Long-lines in Mutsu Bay in Japan vary in length from 50–200 m. Buoyancy and anchoring arrangements depend on the local conditions, loading, kind of hanging structure used, and degree of fouling. In highly productive areas, fouling considerably increases the cumulative hanging weight and precautions are taken. Glass and hyzex polyethylene floats (30–40 cm diameter) are both used. Although expensive, glass can withstand high pressure and are still used in deep lines. The latter are used on the surface and on branch lines down to 10 m. The anchor weights vary between 40 to 90 kg, according to the length of the long-lines, which are usually 15 m apart (Fig. 4).
Most hanging culture facilities include long-line systems of varying complexity, although rafts have also been adopted. Rafts are suitable in protected areas such as sheltered bays with little wave disturbance. In Japan, rafts are used in the bays of Iwate and Miyagi, where scallop culture is combined efficiently with oyster culture. Simple long-lines which are 50 to 60 m long are commonly used inshore in shallow areas, and are often installed above natural grounds. Off-shore long-lines are usually 100 to 120 m in length.
Of increasing importance in highly exploited culture areas is the efficient use of available space. A recent development in Japan is the multi-line systems, and mid- and deep-water systems have been designed and tested. These multi-lines, known as “Jumbo” systems, have been developed mainly in Funka Bay, Hokkaido, where wave action can be strong during winter months. Lines may be as long as 480 m, and are held securely in position by anchors weighing 3–8 tons. Half a million shells can be cultured in an area of 9 ha; this is achieved by reducing the distance between long-lines, and placing lantern nets at alternate depths so as to reduce the distance between lanterns from 1.0–1.2 m to 0.4 m. One major disadvantage of these structures is the installation costs; however, the stock is protected from typhoon damage and temperature thermoclines. It is reported that growth is considerably enhanced compared with shallower systems, and frequent cleaning is not necessary.
The Andon baskets, more commonly known as lantern nets, are widely used for on-growing (Fig. 5). These are usually constructed in 5 mm plastic-coated or galvanized wire hoops of 50 cm diameter and monofilament netting (usually 12 or 25 mm mesh) to give ten or more compartments 15 cm high. Each chamber has a lateral opening through which scallops are introduced. They are suspended in water from a nylon rope and, although heavy fouling may occur, good growth rates are achieved due to the efficient flow characteristics. Other advantages of these nets are their flexibility in the water and ease of transportation, handling, and storage. Each unit is a relatively cheap structure and has a 4–5 years life span. The stocking density per compartment is critical, as overstocking reduces growth and damages the soft growing-edge of the shell. Twenty scallop per compartment produce the best results (Ventilla, 1982). Growth of the Bay scallop, A. irradians, at densities above 25 specimens/square foot is reduced, probably due to greater competition for food as well as mechanical disturbance (Duggan, 1973). Lantern nets are used extensively in Mutsu Bay, Japan, where they were first developed, and have been widely and successfully adopted elsewhere.
Fig. 4. Typical long-line and hanging culture
Fig. 5. Pocket and lantern nets.
A more traditional method still widely used in the coastal regions of Iwate Prefecture and Lake Saroma, Japan, are pocket nets (Imai, 1977) (Fig. 5). The pocket net consists of a plastic-coated or galvanized wire framework (usually 140 cm high by 45 cm wide) on which a 2 to 3 cm polyethylene mesh is fixed. On the mesh itself several pockets 2 or 4/level are stitched, into which the young scallop are inserted. The pocket net is also an efficient on-growing system and produces unmarked shells. A further advantage is that growth and survival rates of the scallop are high, even in localities with rough waves.
The ear-hanging techniques is a traditional Japanese method first practised in the mid-50s (Fig. 6). A hole of 1.0–1.5 mm diameter is drilled in the left or right anterior ear of the shell and a nylon thread or stainless-steel hook is passed through and attached to a branch line. The scallops are usually spaced 8–15 cm apart, and one branch line (6–10 m long) may hold 100–120 shells. Although this method is cheaper compared with lantern nets, the labour is greater. This technique is good in areas with calm waters, and unsuitable for areas with strong wave action (Akahoshi et al., 1968).
In sowing culture, young scallop are released from a moving boat and spread over a pre-selected area. This method is mainly restricted by depth, shore profile, bottom currents, and seabed composition. Sowing culture is practised extensively in Japan, especially in Hokkaido where production by sowing accounts for twice that of hanging culture. In Mutsu Bay, sowing uses only 30% of the total area for culture, due to unsuitable bottom conditions. Sowing is carried out during the summer months. In Saroma Lake, the sowing season is from the end of May to the beginning of June, while in Mutsu Bay 3 cm seed are sown between March and April. Newly-sown seed have a survival rate of 25–30%, and 80% are subsequently recovered by scallop dredges.
Growth is closely related to the stocking density of the seed. The optimum density in Mutsu Bay for seed to reach minimum commercial size (5–6 shell/kg) is 5–6/m2. Overstocking usually results in reduced growth (Kanno et al., 1974).
Fig. 6. Ear-hole hanging culture technique.
