SCALLOP CULTURE

Scallops are bivalve molluscs of worldwide commercial significance. Twenty years ago, a total of 175,000 tonnes were landed on the world fisheries market. However, because of overfishing and increasing market demand, capture fisheries alone cannot meet market demand and cultivation of scallop has therefore become an important mariculture activity. For example, in Japan, the quantity of cultured scallop was only 1,743 tonnes in 1967. By 1977, the culture yield had attained 74,778 tonnes. Similarly in China, about 120,000 tonnes of cultured scallop were harvested in 1988.

4.1 Economically important species

There are over 30 species of commercially important scallops in the world. Their distribution and significance are listed in Table 4.1.

Table 4.1. Distribution and economic significance of major scallop species.

This paper concentrates on the culture of Chlamys farreri which is the most important cultured species in China.

4.2 Habitat and feeding

The scallop, Chlamys farreri, is widely distributed in Italy, Greece, Algeria, Chile, Japan and China. In China, it is the most important among the species cultured (Fig. 4.1). It can be found in the sub-tidal areas down to 30 meters in depth within an annual temperature range of 0–28 °C.

Figure 4.1. Seven species of scallops found in Chinese waters: a) Chlamys farreri, b) C. nobilis, c) Pecten plica, d) C. radula, e) P. laqueatus, f) Amussium japonica, and g) A. pleuronectes.

Under normal conditions, the scallop attaches to rock or gravel underwater with its foot-thread. However, if ambient conditions are not suitable, the molluscs give up their old foot-thread and move to a new place through propulsion caused by the opening and shutting action of the shells.

Scallops take up food from the surrounding seawater with their gills. The volume of food taken in depends on the amount of food in the water and gill filtration rate. The filtration rate of an animal with a shell length of 40 mm is 3.26 l/hr, while an individual with a shell length of 65 mm has a filtration rate of 4.72 l/hr. Filtration rate is also temperature related.

4.3 Reproduction

Chlamys farreri is a dioecious animal. In the reproductive season, the male and female gametes are extruded out of the spawner's gonads directly into the water, where egg fertilization and development take place.

4.3.1 Reproductive season

The reproductive season of scallop depends on species and region. In general, for Chlamys farreri, as water temperature rises to 11–12 °C, the animal's gonads ripen. Most of them will spawn at 14–16 °C. In the provinces of Shandong and Liaoning, the spawning seasons cover 5 months, from May to September. The gonad index is shown in Fig. 4.2.

Figure 4.2. Gonad Index (GI) curve of Chlamys farreri.

4.3.2 Larval developmental stages

The fecundity of a mature scallop is as high as 3–6 million eggs. Eggs size is 69–72 μm in diameter. The fertilizing capacity of scallop sperm lasts longer than that of abalone. At 16–19 °C, sperms retain their fertilizing capacity for 6 hrs in seawater. Developmental stages of larvae are shown in Table 4.2.

Table 4.2. Developmental stages of Chlamys farreri larvae.

4.4 Hatchery

As mentioned above, scallops enter the sexual maturation phase when the seawater temperature rises to 12–14 °C, while spawning starts at 16–17 °C. Based on this knowledge, spawners are collected from the wild when the seawater temperature is between 14–16 °C and moved into indoor tanks for further conditioning for 7–10 days. The sexual ratio (female:male) is 20:1.

4.4.1 Artificial spawning

After conditioning, the spawners are desiccated for 2 hours at room temperature. The desiccated spawners are then placed into a tank filled with running seawater for about 1 hour. Following the desiccation shock, the scallops are moved to a spawning tank where the water temperature is 2–4 °C higher than the ambient seawater. In 1–15 minutes spawning starts and may continue for 50 minutes (Fig. 4.3). After spawning, the broodstock can be reconditioned over a period of 7– 10 days, after which a second spawning stimulation can be carried out.

4.4.2 Fertilization and hatching

Provided that female and male animals are separated from each other, pure eggs and sperms can be obtained. In this way, the number of sperms used for fertilization is easily controlled so that the phenomenon of multi-sperm fertilization can be avoided. Ideally, good fertilization occurs with one or two sperms around one egg, as can be observed under a microscope.

After fertilization, the culture density of fertilized eggs is kept at 20–30/ml, and in order to prevent the developing eggs from piling up, a gentle aeration is provided to stir the water.

As the larvae develop into trochophores or veligers, the first sorting is carried out in order to separate healthy specimens from slow growing and dead larvae. As healthy larvae are capable of swimming actively and are usually found in the upper water layer, they can be easily separated by using a siphon. One-half or three-quarters of the surface water in the hatching tank is decanted to another tank for further rearing.

Figure 4.3. Relationship between the number of spawning individuals and stimulation time.

4.5 Larval rearing

The development and rate of larval growth is influenced by the larval density itself, water quality and nutrition. In order to guarantee a high survival rate of the developing embryos it is essential to maintain suitable environmental conditions of the culture medium.

4.5.1 Water temperature

The scallop larvae (Chlamys farreri) can develop normally at 14–24 °C, however the optimum water temperature ranges from 17–20 °C. Although the larvae develop faster at higher water temperatures, lower values are often maintained as high temperatures usually cause rapid water quality deterioration.

4.5.2 Water exchange

As the larvae enter the veliger stage, the rearing water must be changed. The exchange rate during the first 7 days is 30–50% of the tank volume twice a day. Afterwards, the rate should gradually increase to 200% over the same period of time.

Seawater pumped from the sea is allowed to settle in a reservoir tank for 2–3 days and then passed through a sand filter before use.

It is important to ensure that the difference between inflowing and outflowing water temperature should be minimal and, if possible, not exceeding 1 °C. If the difference exceeds 3 °C, the scallop larvae tend to sink to the bottom of the tank and die.

4.5.3 Feed and feeding

Two- or three-day old veliger larvae can already feed on their own. At this stage, it is necessary to feed the larvae with suitable unicellular algae. Algal species such as Platymonas sp., Dicrateria sp., Phaeodactylum sp., etc. are suitable food organisms for scallop larvae. The feeding rate of scallop larvae in relation to their size is shown in Table 4.3.

Table 4.3. Feeding rate of scallop larvae at different sizes* (Chlamys farreri).

* Larvae density : 2–5/ml

4.5.4 Installation of substrate

Larvae reared at 17–19 °C attain a shell length of 165–180 μm in 18–20 days. During this period, the appearance of the eye-spot indicates that the larvae are about to end their pelagic phase and settle. At this stage the substrate for larval attachment should be placed into the rearing tanks.

Materials such as palm thread, nylon fiber or polyethylene fiber can be used as substrates, as long as they have the common feature of being non-toxic, easy to scatter and with a large surface area.

Experiments have shown that the larvae tend to favour substrates with either a brown, red or yellow coloration.

4.5.5 Transplant of spats

When scallop spats attain a shell length of >500 μm, they easily detach from their substrate and sink to the bottom. When this happens under artificial culture conditions, a high degree of spat mortality usually occurs due to poor water quality and unsuitable substrate (Fig. 4.4). In order to reduce such loss, the larvae are transplanted at this stage to a nursery site located in the sea.

When spats attain a shell length of 250–500 μm (24–28 days), they are placed into netcages and then transplanted to the nursery site. Experimental results show that spat growth rate in the sea is higher than in the culture tanks (Fig. 4.5).

Figure 4.4. Percentage of dead scallop spat in a rearing tank in relation to shell size.

Figure 4.5. Growth rate of scallop spats (Chlamys farreri).

4.6 Culture method

There are two methods of scallop culture practiced in China: bottom culture and raft or long-line culture. For both methods, the culture site should be selected in areas where the seawater is clear and the current is swift. Bottom culture is rather simple, but effective and economical. Fishermen spread the scallop spats from a moving boat over a pre-selected area, where they are left until the marketable size is attained. This method is restricted by water depth, shore profile, seabed composition and predators.

The raft and long-line culture method is used in deep water areas. One long-line unit is usually 60 m long. The buoyancy and anchoring arrangement depends on the local environmental conditions, load, type of hanging structure and degree of fouling. Usually 40–60 netcages are suspended from one long-line. Several types of containers for culturing scallop are being used, however pearl nets and lantern nets are the most popular. Lantern netcages are usually 1 m in height and 40 cm in diameter with 6–7 chambers (Fig. 4.6). Each chamber has a lateral opening through which the scallops are introduced. The stocking density per compartment is 31–35 scallops.

Figure 4.6. Lantern netcage and a long-line unit.

4.7 Harvesting and processing

Eight to 12 months after stocking, scallop spats attain marketable size with a shell height of 6–7 cm. Experiments have shown that late spring and early summer are the best seasons for harvesting the market size scallops, as the ratio of fresh to dry meat is higher than in autumn and winter (Table 4.4).

Table 4.4. The relationship between harvesting season and dry meat weight.

After harvesting, the scallops can be processed into three kinds of products: fresh scallop on shell, frozen fresh or dried.

In view of the high culture yields (35–40 tonnes/ha) and economic benefits, scallop culture has become a significant seafarming practice in China.