Culture of the Pacific Oyster (Crassostrea gigas)

in the Republic of Korea TABLE OF CONTENTS

PREPARED FOR THE OYSTER CULTURE TRAINING COURSE CONDUCTED BY THE FISHERIES RESEARCH AND DEVELOPMENT AGENCY

(REPUBLIC OF KOREA)

AND ORGANIZED BY THE REGIONAL SEAFARMING DEVELOPMENT AND DEMONSTRATION PROJECT

(RAS/86/024)

IN COOPERATION WITH THE REGIONAL SMALL SCALE COASTAL FISHERIES DEVELOPMENT PROJECT, BAY OF BENGAL PROGRAMME FOR FISHERIES DEVELOPMENT AND THE

INDONESIA SEAFARMING DEVELOPMENT PROJECT JUNE 1988

CULTURE OF THE PACIFIC OYSTER (CRASSOSTREA GIGAS)

IN THE REPUBLIC OF KOREA

Byung Ha Park Mi Seon Park

Bong Yeoul Kim Sung Bum Hur Seong Jun Kim

Prepared for the Training Course on Oyster Culture conducted by the National Fisheries Research and Development Agency

Pusan, Republic of Korea June 1988

Seafarming Development and Demonstration Project (RAS/86/024)

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TABLE OF CONTENTS

IntroductionStructure and Biology of Korean OystersGenus Crassostrea Genus OstreaEcology of the Korean OystersA. Geographical Distribution B. Ecological Distribution C. Shell Growth and Growth Variations of Soft Part D. Maturation and Spawning E. Fertilization, Embryonic Development and Planktonic Phase of Oysters F. SettlingSeed ProductionA. Basic Factors in the Production of Spat Oyster B. Collection of Spat C. Collecting Ecology D. Preparation of the Collectors E. The Manufacture of Collectors and Establishment of Collecting Beds F. Spatfall ForecastingCultureA. Culture Methods B. Culture UnitsDiseasesA. Fouling Organisms B. Diseases C. Eutrophication of Oyster Farm D. Red TideHarvestingProcessing

A. Fresh Shucked Oysters B. Frozen Oyster C. Canned Smoked Oysters in Oil D. Canned Boiled Oyster E. Dried Oyster F. Salt-Fermented Oyster G. Oyster JuiceReferences

LIST OF TABLES Table No.

3.1 Geographical distribution of some commercially important oyster species3.2 Number of days required for the maturation and spawning of the American oyster (C. virginica) in relation to water temperature3.3 Optimum salinity for the development of C. gigas and C. angulata3.4 Embryonic stages of Crassostrea gigas in relation to time at water temperature of 20–21 degrees Celsius3.5 Influence of decreasing salinity on settling of Ostrea edulis larvae4.1 Estimated number of oyster brooders and egg spawned at two sites in relation to water depth4.2 Comparison of mortality occurrence between hardened and ordinary oyster spat4.3 Comparison of mortality occurrence between hardened and ordinary oyster spat in relation to exposure time4.5 Vertical distribution of oyster larvae in Namhae and Changwon coastal areas5.1 Culture steps in the oyster hanging culture methods in relation to ordinary and hardened spat5.2 Number of oyster culture establishments permitted per hectare of water area for hanging culture in Korea5.3 Details of materials for one raft5.4 Details of materials for one long-line5.5 Characteristic differences of the Pacific oyster (C. gigas) cultured in different ongrowing sites6.1 Number of oysters preyed upon during a given time period6.2 Comparison of lethality (%) between oyster (C. gigas) and mussel (M. edulis) under different heat treatment6.3 Lethality of H. norvegica in relation to freshwater treatment8.1 Standards of size for individually frozen oysters8.2 Inspection standards for canned smoked oysters in oil8.3 Inspection standards for canned boiled oysters8.4 Standards of count size for dried oysters

LIST OF FIGURES Fig. No.

2.1 Soft parts of Crassostrea sp.

2.2 Internal anatomy of C. gigas2.3 Internal anatomy of C. nippona2.4 Soft parts of Ostrea sp.2.5 Internal anatomy of C. denselamellosa3.1 Tolerable salinity ranges for various oyster species3.2 Vertical distribution of several oyster species3.3 Developmental stages of C. gigas3.4 Larval setting time in relation to salinity levels4.1 An example of geographical and environmental situation for mobile spat collection4.2 An example of geographical and environmental situation for fixed spat collection4.3 Changes in water temperature in some oyster-growing areas4.4 The proportion of gonad size to total size in cross-section of spat oyster4.5 Gonad sections of female Pacific oyster (C. gigas) showing seasonal changes4.6 Gonad sections of male Pacific oyster (C. gigas) showing seasonal changes4.7 Depth range where natural setting of the Pacific oyster occurs4.8 Distribution of oyster larvae by water depth4.9 Structure of a rack for oyster seed collection4.10 Seed collecting rack of the Pacific oyster5.1 Bottom culture of Pacific oyster5.2 Stone culture of Pacific oyster5.3 Schematic drawing of stone method5.4 Stick culture of Pacific oyster5.5 Raft culture of Pacific oyster5.6 Long-line culture of Pacific oyster5.7 Rack culture of Pacific oyster5.8 Structure of a raft for oyster culture5.9 Structure of a long-line for oyster culture5.10 Shell growth in terms of height from the start of the hanging culture method5.11 Meat weight increase from the start of the hanging culture method5.12 Comparison of growth of oyster in different locations of the growing area5.13 Difference in the external appearance of Pacific oyster (C. gigas) in different regions6.1 Typical fouling organisms in oyster culture grounds6.2 Settlement frequency (%) of major fouling organisms in Hansan-Goje Bay in relation to the settlement of the Pacific oyster (C. gigas)6.3 Bacterial foci (focal necrosis) from Pacific oyster6.4 Bacillary necrosis of oyster larvae showing typical bacterial swarming6.5 Infected ovum of Pacific oyster (C. gigas) with Maeteilioides chungnuensis6.6 The causative organisms of red tide7.1 Harvesting of oyster by crane7.2 Yield of oysters from a 3m-long ren8.1 Flowsheet showing the various stages of processing frozen oysters8.2 Flowsheet showing the various stages of processing canned smoked oysters in oil8.3 Flowsheet showing the various stages of processing dried oysters

INTRODUCTION Oyster is consumed worldwide. It is a delightful and nutritious food taken in a variety of forms: fresh, frozen, canned and dried as well as juice. By popularizing the hanging culture method since the early 1960s, the culture practices appropriate to a region have been gradually evolved and established. At present, most of the oyster production comes from long-line culture. In Korea, the Pacific Oyster (C. gigas Korean kang-gul (C. rivularis), Korean pawit-gul (C. nippona), spiny oyster (C. echinata) and the densely lamellated oyster (C. denselamellosa) are grown. Among these, the Pacific oyster is the main species for commercial farming. In 1986, the Korean oyster culture industry showed there were 10,736 hectares of licensed growing water and a production of 255,000 metric tons shell-on, or a per-hectare yield of about 23.75 metric tons. Oyster production was 25 to 30 percent of total mariculture output in the 1980s, compared to 50 to 60 percent in the 1960s. Most of the processed Oysters and some of the fresh produce are exported to North America, Japan, Southeast Asia, Middle East and other countries. The purpose of this Manual is to extend the technology of oyster culture to farmers and extension workers. It covers the biology, theory and practice of oyster culture, and the findings from applied research and field studies. It also includes processing.

STRUCTURE AND BIOLOGY OF KOREAN OYSTERS Oysters fall under the family Ostreidae, Filibrachia, Bivalvia. More than 100 species and three genera have been indentified. Nevertheless, their taxonomic position has not been clearly established. Oysters are included in three genera: Ostrea, Craassostrea, and Pycnodonta (Ranson, 1950, 1960). The classification is based on the form and structure of larval shell, mode of reproduction, life history, adult shell, morphology and foot shape. Korea has five species of oysters - four of genus Crassostrea, namely, Crassostrea gigas, C. nippona, C. echinata and C. rivularia, and one of genus Ostrea. Ostrea denselamellosa. The major species are C. gigas, C. nippona and O. denselamellosa. The characteristics of these three species are as follows:

1. Genus Crassostrea (Figure 2–1)

This species is relatively big and covered with scale-like shells. Oysters of this genus have small or big umboral cavities without chomata. Adductor muscle scar is on the opposite side of the abdominal region along with the central part of the shell. The inner side of the shell is covered with an ivory-like material known as conchiolin. Tentacles of the inner fold vary in size and are the longest among oysters. The auricles of the heart are connected to the anterior end of the shell.

Figure 2–1. Soft parts of Crassostrea sp. Shape of the mantle tentacles: A, tentacles of inner fold; B, mid-tentacles and C, outer tentacles; AD, adductor muscle; L, labial palps; V, ventricle; Au, auricle; AP, anus and anal pailla. a. Crassostrea gigas (Figure 2–2) ShellThis species is relatively big and irregular in shape. It is long and oval. Both shells are concave but the left one is more concave than the right. Radial ribs are on both shells starting from the umbo. Attached line of both shells and commissural shelf is wavy and narrow. There is no chomata. Adductor muscle scar is reniform or kidney-shaped. Dorso-anterior border is concave. Umboral cavity is commonly deep. The outer side of the shell is whitish yellow and has brown radial rib. Its inner side is white and partially whitish milky color. Muscle scar is of polished white color or light yellow. The muscle scar of right shell is sometimes violet. Soft PartTentacles of the mantle's inner fold are conical and their length is almost four times their width. Tentacles of the middle fold have two layers, inner and outer. Inner layer tentacles are round conical and their length is three to five times their width. Outer layer ones are club-like. All of the tentacles are ivory or whitish yellow with brown or black spots. Gill is ivory in color and the filament number is 13 ± 2. Heart is ivory or whitish yellow. The auricles of the heart are light brown. Rectum is black. The coloration of soft parts of this species is the darkest among genus Crassostrea. b. Crassostrea nippona (Figure 2–3)

Figure 2–2. Internal anatomy of C. gigas. Figure 2–3. Internal anatomy of C. nippona. A, anus; AD, adductor muscle; G, gill; L, labial palps; M, mouth; ME, mantle edge P, pericardium; R, rectum; S, stomach.

A, anus; AD, adductor muscle; G, gills; L, labial palps; M, mouth; ME, mantle edge; P, pericardium; R, rectum; S, stomach.

ShellThis species is relatively big and its shape varies from oval to round. The left shell is flat or slightly concave while the right is concave. The right shell has wavy growth scales, growth lines and a big commissural shelf without radial rib and commissural plication. This species has a small umboral cavity without chomata. Adductor muscle is reniform. Dorsal border is concave. External coloration of both shells is while. Growth line is dark brown and its basic part is light brown. Left shell is light violet and part of the inner side is milky white. Muscle scar is polished white with violet yellow and has growth line. When the shell is cut across its dorso-ventral side, there are some concave chambers on the right shell. Left shell consists of whitish ivory sediments. The cutting line is white. Light violet pigments are sometimes found on some parts of the left shell. Soft PartThe soft part of this species is similar to that of C. gigas, but different from that of genus Crassostrea when analyzed under electrophoresis (Torigae and Inaba, 1975).

2. Genus Ostrea (Figure 2–4)

The species of this genus are relatively medium or small in size and have thin, dense and leaf-like growth scales. Commissural plications are very gentle in slope. Radial ribs are very weak or sometimes absent. Umboral cavity is small. Adductor muscle scar is reniform and placed on the central part of the shell. The number of chomata varies from a few in some species to many in others. Both shells are concave and consist of whitish ivory sediments. Tentacles of the middle fold have two to four layers. Its anal pailla is longer than that of genus Crassostra (Figure 2-1). Anus is placed on abdominal part.

Figure 2–4. Soft parts of Ostrea sp. Shape of the mantle tentacles: A, tentacles of inner fold; B, mid-tentacles; C, C, outer tentacles; AD, adductor muscle; L, labial palps; V, ventricle; Au, auricle; AP, anus and anal pailla. a. Ostrea denselamellosa (Figure 2–5) ShellThe shell of this species is medium in size and round in shape. Both shells are concave, but the right one is flatter than the left. Both shells have radial ribs. The ribs of the right shell are covered with many fragile growth scales while the left has none. Commissural shelf is narrow. This species has no chomata, or, if it has, limited to both sides of the ligament. Adductor muscle scar is reniform and placed on concave dorso-anterior border. Umboral cavity is small. Outer side of the shell is unpolished white with light yellow spots while conchiolin is dark brown. Inner side is polished white with light brown or gray spots. Adductor muscle scar is polished white with light brown growth line. When the shell is cut across its dorso-ventral axis, concave chambers can be seen. Both shells have whitish ivory sediments. Soft PartsTentacles of the inner fold of mantle are club-like and their length is two times their width. Tentacles of the middle fold have five layers, i.e inner and outer. Inner ones are club-like and bead-like in one layer. Outer ones are club-like in four layers. Lengths of the middle fold tentacles are 1.5 to 2 times their width. All tentacles are whitish yellow with some black spots. Gills are ivory or ivory with gray. Normally, the number of gill filament is 13 ± 2. Anal pailla is slightly long.

Fig. 2.5. Internal anatomy of C. denselamellosa A, anus; AD, adductor muscle; G. gill; L, labial palp; M, mouth; ME, mantle edge; K, pricardium; R. rectum; S, stomach.

ECOLOGY OF THE KOREAN OYSTERS

A. Geographical Distribution

Oysters are found everywhere in the world except in the north and south poles. The vertical and horizontal distribution and dispersion of oysters are linked to the planktonic-stage period which is during one to four weeks after spawning, and also by the artificial transplantation of young oyster, For instance, oyster larvae settle in shallow coastal waters and in deeper areas in the open sea. The distribution of C. gigas has been extended to the Pacific coast of America through transplantation from its original habitats in the Korean and Japanese coasts. Distribution of oysters is wide, as shown in Table 3-1. 1. Ostrea edulis (European flat oyster) occurs widely in European coastal areas, particularly in France, England, Denmark, Norway and Italy. 2. Ostrea lurida occurs along the Pacific coast of America. 3. Crassostrea angulata occurs in Portugal, Spain, France and England. 4. Crassostrea virginica (American oyster) occurs widely in the Atlantic from south Canada to the USA. A small amount is found in certain Pacific regions. This species adapts to widely varying environmental conditions. 5. Crassostrea commercialis (Sydney oyster) is oviparous and occurs widely from east to soutwest Australia. The majority of this species live in shallow water, but sometimes in deeper areas. The species and distribution of Korean oysters are as follows: 6. Crassostrea gigas occurs in Korea, Japan and China. Through transplantation of oyster, its distribution has been extended to America, Australia and Europe. 7. Crassostrea rivularis is oviparous and occurs at the mouth of Nakdong River in Korea. 8. Crassostrea nippona is an oviparous species which occurs from the southern part of the east coast to the south coast of Korea. 9. Crassostrea echinata occurs from the south to the west coast of Korea, and in the coasts of Japan, Australia and Indonesia.

10. Ostrea denselamellosa occurs in the southern part of the east, south and west coast of Korea.

Table 3.1. Geographical distribution of some commercially important oyster species

Species Distribution area

Ostrea edulis 40 – 60°N

European flat oyster Europe : England and coastal area from the Mediterranean to Scandinavia

O. lurida 33 – 50°N

Olympia oyster Pacific coast of America

Crassostrea angulata 37 – 45°N

Portuguese oyster Portugal, Spain, France

C. virzinica 27 – 47°N

Virginia oyster Atlantic coast of America

American oyster

Eastern oyster

C. commercialis 22 – 28°S

Sydney oyster Australia

Australian oyster

C. gigas 30 – 45°N

Pacific oyster Korea, Japan, China

B. Ecological Distribution

Oyster species could be subdivided further according to their distribution in water of different salinities and temperature from shallow areas to the open sea. Figure 3-1 shows the salinity range tolerance of some oyster species. Generally, Ostrea species are distributed in areas with full strength salinities. Those belonging to the genus Crassostrea tend to be more euryhaline (they can survive and develop in fresh to saline waters) and they are found in shallow water with fluctuating salinity levels. Figure 3-2 shows the vertical distribution of several oyster species. Generally, oyster species belonging to the genera Ostrea and Pycnodonta are found in tidal zones and Crassostrea species in the intertidal zones.

Figure 3-1. Tolerable salinity ranges for various oyster species.

Figure 3-2. Vertical distribution of several oyster species.

H.T.L. - High tide level. L.T.L. - Low tide level.

C. Shell Growth and Growth Variations of Soft Part

Oyster growth is indicated by shell size. Oyster shell is composed of calcium carbonate as with other shellfish, and conchiolin, a kind of protein. The shell is excreted by the mantle which absorbs calcium ion from sea water (0.4g/l). Absorption is accomplished by the outer layer of the mantle. Thus shells grow without food, except when the level of calcium ion in the seawater is below 20 percent. The growth rate of shell varies from one site to another and is influenced by environmental conditions. Water temperature has a great influence on shell growth. Water temperature in winter are often too low for shell growth but growth could be stimulated in warmer water

bodies. Water temperature may also cause differential growth on different parts of the shell. The growth of soft part is poor during summer and early autumn mainly due to spawning. The severity, however, depends on the abundance of nutrients. The weight variations of the soft part is related to the rate of gonad development against glycogen; accumulation during a one-year cycle. Follicular cells dispersed in connective tissue develop rapidly during spring to summer. After that, the central part of soft body is filled with mature eggs or sperms except the outer layer of follicle cell wall. After spawning, the connective tissue increases rapidly and the remnant of gonads is absorbed. A thick connective tissue containing glycogen is formed during winter. The weight increase in the soft part of the organism due to the above mentioned process is generally known as “full up.” Filling up of the soft part varies from season to season. This phenomenon proceeds as follows:

1. Gonad develops from spring to summer. 2. Spawning. 3. Soft part becomes invisible by spawning and high water temperature. 4. Start of filling up of the soft part when water temperature drops below 20

degrees Celsius. 5. Filled up with glycogen in the winter.

The degree of glycogen accumulation varies from one place to another. However, the process begins generally towards the end of September and the flesh becomes distinctively white in appearance. According to the increment of accumulated glycogen, the style sac becomes white by November, and the white part is extended to the whole part except the gills. Flesh becomes thick. The economic value of the soft part varies with the degree of fill up. The degree of fill up is the rate of dry weight of the soft part versus the volume of the shell multiplied by 1,000. This is called “condition index.” The factors that influence oyster flesh growth are water temperature, food quantity, feeding rate, spawning and population density. The most important one is food quantity which is influenced by water circulation and such climatic conditions as rainfall, wind velocity and tide. Oysters suffering from malnutrition grow slowly or not at all. Generally, high stocking density could be the reason,. Oysters from a nursery with low water velocity or from high density population or at the center of a culture structure, such as raft or line, slowly fill up so that the product is comparatively leaner than those placed in favorable conditions. The soft part of cultured oyster under poor nutrition and high population density becomes transparent; the vacuum left by spawning is filled with water rather than glycogen. The style sac is clearly visible. Generally, oyster do not fill up during summer and autumn because of the spawning process. Quantitative aspects of spawning such as duration, amount of seed, etc. influence the rate of the next fill up.

D. Maturation and Spawning

Maturation of oysters mainly depends on water temperature. Table 3-2 shows the number of days required for the maturation and spawning of cultured C. virginica in relation to different water temperatures (Loosanoft, 1945).

Spawning can be induced by rapid rise of temperature. Orton (1926) and Korringa (1947) reported that a full moon induced spawning. However, recent observations have attributed induced spawning to salinity, temperature and water pressure rather than to lunar and tidal cycles. Salinity induces oyster to spawn with the rapid rise of temperature. On the other hand, spawning induced by the use of diluted sperm medium was reported by Galsoff (1938). The physiological responses of oyster to various forms of physico-chemical stimuli have not been fully understood, however.

E. Fertilization, Embryonic Development and Planktonic Phase of Oysters

Environmental factors influencing fertilization development and growth of juveniles are mainly water temperature and salinity. Fertilization occurs in a wide range of temperature, but development up to the D-stage is limited within a certain range. At low temperatures, growth is slow and morphological variations, e.g. projection of umbo, do not occur easily. Therefore, settling time of larvae cultured at low temperature is longer than that at high temperature, and so is the planktonic period. Table 3-2. Number of days required for the maturation and spawning of the American

oyster (C. virginica) in relation to water temperature. Water temperature(°C)

Habitat 12 21 24 27

Long Island Sound 68 15(18) 5 5.5

Now Jorney - 55(78) 32 22.5

( ) : Time required in days for spawning - : Immature The planktonic period of C. gigas larvae differs with water temperature. It is usually more than three weeks at 19–20 degrees Celsius and ten days at about 27 degrees Celsius (Yoo et. al., 1974). Salinity has a strong influence on the rate of development of larvae. Optimal salinities of some species have been determined in experiments as shown in Table 3–3. Optimal salinities for juveniles are similar to those for adult specimens. Salinity tolerable to C. gigas and to the Portuguese oyster (C. angulata) ranges between 14 to 37 ‰ The time required for the embryonic development stage of the Pacific oyster is shown in Table 3–4. The shape of eggs before and after spawning, and its developing stages, are shown in Figure 3–3. Prior to spawning, the shape of the eggs in the gonad is irregular; after spawning it becomes spherical with a 50 μm diameter.

Table 3-3. Optimum salinity for the development of C. gigas and C. angulata. Species Optimum Salinity (‰) Remark

20–26 Amemiya (1928) 23.3–28.5 Seno et al. (1926) C. gigas

12–23 Ranson (1948) 26–35 Amemiya (1928)

C. angulata18–23 Ranson (1948)

Table 3–4. Embryonic stages of Crassostrea gigas in relation to time at water temperature of 20–21 degrees

Celsius. Developmental stage Time required

1st polar body releasing 50 minute - 1 hour 10 minute 2nd polar body releasing 1 hour 00 minute - 1 hour 20 minute

1st division 1 hour 20 minute - 1 hour 40 minute 2nd division 2 hour 00 minute - 2 hour 20 minute Morula stage 3 hour 5 minute - 3 hour 25 minute Blastula stage Rotary movement 5 hour 20 minute - 5 hour 50 minute Free swimming 5 hour 30 minute - 6 hour 00 minute Gastrula stage 6 hour 10 minute - 6 hour 30 minute Trochophore 9 hour 40 minute - 10 hour 00 minute

D-shaped larva 15 hour 00 minute - 28 hour 00 minute

Newly spawned eggs are in the first oocyte stage before maturation division occurs. The germinal vesicle does not disappear until this stage. As the sperm enters the egg, the fertilization membrane forms on the surface of the egg and the first polar body is released through the maturation division (Figure 3-3c). This stage takes about 50 to 70 minutes. The egg nucleus is at first fertilized and the egg then undergoes cleavage (Figure 3-3d, 3-de, 3-3f). Cleavage continues to give rise to the morula and blastula stages. At the blastula stage, the egg is ciliated at the surface and begins to rotate.

Table 3–5. Influence of decreasing salinity on settling of Ostrea edulis larvae.

Salinity (‰) No. of settled larva (Mean No./disk)

26–27 141

25.0 325

22.5 146

20.0 76

17.5 15

15.0 2

12.5 0

10.0 0

7.5 0

Figure 3-3. Developmental stages of C. gigas. A, egg prior to spawning; B, newly-liberated egg; C, fertilized egg; D, 3-celled stage; E, 8-celled stage; F, 16-celled stage; G, multi-celled stage; H, ciliated gastrula stage; I, trocophore stage; J, D-shaped larvae; K, early umbonate larvae; L, mature pediveliger larvae. Following the blastula stage is gastrulation at which stage the blastopore depressions become evident. It takes six and one-half hours to develop until this stage. After that, the larvae becomes trochophore with cilia ring. The velum then develops and the larva becomes D-shaped with shell. To develop until this stage, it takes 25 to 28 hours. The larva at this stage is known as veliger.

F. Settling

The time it takes for D-shaped juvenile to settle depends mainly on water temperature. Settlement of the juveniles begins with the appearance of the foot gland, foot and eyespot. Movements are aided by the velum which enables the larvae to creep until it finds a suitable place for permanent attachment. Settling of juvenile is also affected by salinity. In flat oysters, the optimal salinity range is 22.5 to 25.0‰; settling drastically decreases as shown in Table 3–5. In the case of American oyster, settling time also varies with salinity, and greatest rate occurs at 15–20‰ (Figure 3–4). Spat settling also depends on water current and concentration of copper ion. Settling does not occur where water current is more than 5 to 7 cm/sec.

Figure 3–4. Larval setting time in relation to salinity levels.

It has been observed that there is plenty of spatting where the speed of water velocities is slowed down by a barrier (Korringa, 1941; Thomson, 1950). Clean substances are preferred to muddy substances (Needler, 1941) and the concentration of settled spats is generally higher on the lower part of the cultches (Knight, 1951; Wilson, 1941), mainly believed to be due to the absence of sediments and seaweeds (Korringa, 1941). The density of settled spats is directly related to the density of the planktonic stage. Theoretically spats settle in a wide range of depth. However, this range is restricted by certain environmental factors that could induce mass mortality at the early stages of spat development. For instance, newly-settled spats cannot withstand three to four hours of direct sunshine in summer, and cannot settle on muddy surface or in turbid areas. Consequently, oyster settling is determined by these and other phenomena. However, the settling layer varies somewhat with depth, depending on species. After settling, foot, velum and eye spot degenerate, gill and adduction muscles develop and the chitin shell is replaced with calcium. The density of planktonic larvae indicates the degree of future settling levels (Lambert, 1946). The number of spats per million spawned eggs is estimated to be in the range of one percent. In order to improve the collection of natural spat, further studies are needed on the developing processes of the larvae, and on the environmental conditions that influence the movement of the larvae in the water.

SEED PRODUCTION Oyster culture begins with seed production. The production of oyster seed can be carried out on a large commercial scale, depending on the biological characteristics of oyster, the environmental conditions, and economic factors. Most of the spats for culture are collected along the southern coast of Korea.

A. Basic Factors in the Production of Spat Oyster

The southern coast of Korea is the major producing area of spats due to favourable biological and environmental conditions; it has a large broodstock population, water temperature is suitable for spawning, the environment is suitable for larval growth and the geographical condition is such that collection of oyster larvae is easy. 1. Geographical condition. The major producing area of oyster spat along the southern coast is largely divided into two areas: one is a comparatively deep place, as in Changwon and Goje Bay (Figure 4-1). The other is a shallower place, as in Namhae Island and Gohung (Figure 4-2). The range of water movement in Changwon and Goje Bay is narrow due mainly to the small tidal range. Thus, the distribution range of oyster larvae is also narrow

Figure 4.1. An example of geographical and environmental situation for mobile spat collection.

Figure 4-2. An example of geographical and environmental situation for fixed spat collection. The distribution of larvae varies according to tidal current and water movements between the numerous islands near these bays. Dense communities of larvae are formed in these bays (Yoo et al, 1971). The collecting area is the place where a high density community is formed. The vertical and horizontal distribution of larvae is nearly constant in the bay which relatively shallow. 2. Water temperature. The surface temperature of the Korean coast, increasing from March, is highest between July and September and decreasing gradually from there on. It is lowest between January and February. The tendency in water fluctuation is related to latitude and water depth. For example, the surface temperatures of Sinhuk-Ri and Sani-Myon in the western coast are below 10 degrees Celsius and those of Ogu-Ri and Limyong-Ri in the southern coast are about 12 degrees Celsius in mid-April. The tendency of water temperature to rise at Sani-Myon is similar to that of Limyong-Ri after mid-April. The temperature in Ogu-Ri, where the water is relatively deep, increases more slowly than in other areas. The temperature in northern Sinhuk-Ri is below 8 degrees Celsius until mid-April and about 12 degrees Celsius until mid-May, after which the water temperature increases rapidly. At its highest, the surface temperature of Sani-Myon and Limyong-Ri is about 25 degrees Celsius. The decreasing tendency of water temperature during October is similar in all the stations; it gradually increases within the range of 16 to 20 degrees Celsius. In Ogu-Ri and Sani-Myon in the western coast, the surface water temperature decreases rapidly from November until, in winter, it is approximately 0 to 6 degrees Celsius. However, the surface water temperatures of Ogu-Ri and Limyong-Ri decrease more slowly than in other areas; the lowest water temperature in winter is 9

degrees Celsius at Ogu-Ri and 11 degrees Celsius at Limyong-Ri (Figure 4-3). Maturation and spawning phases of oyster undergo changes with the rising pattern of water temperature.

Figure 4-3. Changes in water temperature in some oyster-growing areas.

3. Oyster broodstock and fertility. The origin of the oyster broodstock varies among different localities. In the inner coast and deep waters of Changwon and Goje Bay, oyster broodstock are cultivated from hardened oyster spats. In the tidal zone where the water is shallow, they are mostly from nature. In oyster broodstock cultivated from hardened spats the amount of spawning can be estimated from the number and density of facilities (Figure 4-4). However, the number of brood oysters changes because the number of facilities for culture varies every year.

Figure 4-4. The proportion of gonad size to total size in cross-section of spat oyster.

The fertility of an oyster is also related to the size of the oyster itself. For example, a two-year-old oyster does not only give a bigger spawn; it also yields more eggs than a one-year-old oyster. Generally, a female oyster can spawn tens of millions of eggs.

When an oyster meat weighs about 50g, the number of eggs can be estimated to be more than a billion. A two-year-old oyster cultivated by hanging culture spawn later in the rising period of water temperature or early in the period of falling water temperature (Yoo et al, 1971). Therefore, the number of larvae liberated from a two-year-old adult specimen decreases gradually due to the lowering of the water temperature. In the inner coast where the water is deeper, oyster broodstock are mostly two-year-olds. At Goje Bay where water is deep and at Namhae-Do (Cha-Myon) where it is shallow the number of brood oysters and the amount of eggs spawned are shown in Table 4-1. The table shows that the number of brood oyster increases annually. Therefore, the amount of oyster that can be collected increases. The amount of spat produced may also change annually due to environmental factors. The effect of natural conditions on the production of spats is inconsistent.

Table 4-1. Estimated number of oyster brooders and egg spawned at two sites in relation to water depth (Goje Bay, deep site; Namhae-Do, shallow site).

Gŏje Bay Namhae-Do Year

Brood oysters Eggs spawned Brood oysters Eggs spawned 1969 1,725 × 104 375 × 1012 63 × 104 128 × 1010

1970 6,025 × 104 1,309 × 1012 68 × 104 139 × 1010

1971 13,075 × 104 2,842 × 1012 76 × 104 156 × 1010

Table 4-2. Comparison of mortality occurrence between

hardened and ordinary oyster spat. Spat Mortality (%)

Hardened 18.2

Ordinary 42.6

Table 4-3. Comparison of mortality occurrence between hardened and ordinary oyster spat in relation to exposure time.

Time of exposure Spat Hardening period (Sep. to Mar.)

Culturing period (Apr. to Sep.)

8 Hardened 7.3% 18.2%

5 Hardened 21.8% 27.6%

0 Ordinary 32.7% 42.6%

B. Collection of Spat

One method of producing oyster spat consists of the fixed collecting method (a collecting table is set up and collectors are hung on the collecting table). It used to be the only method for producing spat oyster. Recently, however, the movable method (hanging collector) has become popular. The success of spat collection is influenced by the environmental conditions of the collecting ground. Selection criteria for collecting grounds are as follows: 1. Fixed collecting ground

a. The place is the natural habitat of oyster and where oyster can be cultured. b. Southward coastal areas which are under little influence of waves. c. Bottom is not a steep slope and has estuarine silt mixed with sand. d. The wind blows towards the collecting sites during the collecting period. e. here is little inflow of fresh water without wide variation in salinity values. f. Where eddy occurs. g. There is abundant phytoplankton for oyster to feed on. h. There is no factory in the neighborhood, no industrial or domestic pollution, and

the red tide does not occur. i. Current is smooth and velocity is in the range of 5 to 7 cm/sec. j. Not infested with predators like starfish, gastropod, barnacle, polychaeta and

mussels, as well as fouling organisms. k. Labour is abundant.

2. Movable collecting ground

a. The place is a natural oyster ground and where oysters can be cultured. b. The place where one can establish facilities made of ropes and rafts in water

more than 5m deep. c. The wind blows towards the collecting sites, and the inner coast is calm during

the collecting period. d. There is little inflow of fresh water without wide salinity variations. e. Current is slow and velocity is 5 to 7 cm/sec.

C. Collecting Ecology

Pacific oysters are separate in sex although hermaphrodites occur occasionally. The sex is determined only by examining the reproductive tissue. Adult oysters which can spawn are mostly over one year old. With the transition of autumn into winter, the water temperature begins to fall and glycogen storage begins. Under normal weather conditions, full ripeness is attained in most of the Korean waters by the end of June. Eggs appear as tiny cream-coloured granules, barely visible to the naked eye; the sperm which is extremely minute is a pure white material and runs in thin streams. The fertility of a Pacific oyster is related to its size and the state of its nutrition. The number of eggs produced by an average market-sized oyster has been estimated at 50 to 100 million; the number of sperm at much more. During winter, the surface of a fat oyster is smooth and even, but with the onset of sexual maturity it becomes deeply veined. This veined gonad covers both sides of the anterior end of the oyster and takes up a considerable portion of the body weight. Figure 4-5a shows a section of eggs tightly packed in blind sacs called follicles, and the tubules through which the eggs are discharged. Details of the seasonal gonadal changes are as follows (Figure 4-5, 4-6): Spawning of the Pacific oyster may occur anytime between late June and early September but most often in late July and early August. After complete spawning, the body of the oyster is nearly transparent and the gonad follicles tend to collapse and contain only a few gamete cells and tissue fragments. The condition of the oyster at this stage is at its lowest level. By November the level of winter conditions has been established: the follicles have shrunk to small compact islands of germinal tissue scattered throughout the mass of

vesicular connective tissue, which has filled in the inter-follicular spaces as well as the area between the gonad and the epithelium. The relative amount of this connective tissue determines the condition of the oyster. The main outer gonaduct separates the gonadal area from the outer connective tissue area where no germinal material occurs. At this stage some of the follicles that have not completely closed up may contain a few eggs or sperm; in the few cases of partial spawning, only the outer follicles would be involved.

Figure 4-5. Gonad sections of female Pacific oyster (C. gigas) showing seasonal changes. A, ripe female with gonad covered with a thin layer of glycogen-rich tissue; B, partially spawned female with a few relict eggs; C, female in fall condition with follicles closing in on the relict ova; D, female in spring condition with early-stage developing eggs on follicle margins. (x100)

Figure 4-6. Gonad sections of male Pacific oyster (C. gigas) showing seasonal changes. A, ripe condition prior to spawning; B, partially spawned gonad with the follicles nearly empty; C, early development of male gonad; D, gonad approaching ripeness with developing cells in the outer portion of the follicle. (x100) The gonad is generally undifferentiated as to sex at this time (November) and this condition prevails throughout winter. It is not until April that early stages of gonad proliferation and differentiation may be noted. The maximun stage of development has been observed in late April in female specimens. About 25 percent of the gonad area is occupied by follicular material. By mid-May gametogenesis is well underway, and about 50 percent of the potential gonadal area is filled with expanding follicles. By the end of June all animals are fully ripe with the follicles tightly packed with eggs and sperm. Only a very thin layer of vesicular connective tissue covers the gonad. This condition persists until spawning although an occasional partially spawned individual may show proliferation of connective tissue between the spent follicles. During the spawning process, the female discharges the ova into suprabranchial chambers, from which they are forced through the gill ostia (apertures) into the mantle chamber. From this chamber, the eggs are ejaculated into a small cloud. This process is accomplished by adjustment of the mantle edges and by vigorous action of the adductor muscle. Discharge of eggs is intermittent, five to ten times per minute. They are propelled at a distance of 12 inches or more from the oyster. The male oyster discharges its sperm in a thin steady stream, also into the suprabranchial chamber. However, instead of passing through the gill apertures against the current, as do the eggs, they are carried out in the normal exhalant stream of water. Thus the eggs and sperm are discharged on opposite sides of the oyster. Spawning may be brought about by temperature shock, by chemical stimulation, or by a combination of both. It is thought that the large mass spawnings that occur in molluscs such as the oyster, indeed in a number of other marine animals, are

necessary to create the concentration of gametes needed to ensure fertilization when sexual products are discharged freely into the open water. The presence of sexual products of the oyster in waters where other oysters are feeding is usually enough to stimulate spawning. This fact makes it possible to induce large quantities of oysters to spawn. In Korea, the salinity range for development (breeding) of the Pacific oyster is between 11 ‰ and 32 ‰ the optimum is considered to be between 20 ‰ and 25 ‰ development can take place within the temperature range of 14.5 to 30 degrees Celsius, with the optimum at about 24 degrees Celsius. In Korea, the salinity requirements are easily met, but the optimum temperature occurs rarely.

D. Preparation of the Collectors

Oyster and scallop shells are used as collectors. Other types include PVC tubes, cement and other materials but these are not practical because spat oysters become detached from the surface during culture. Oyster shells are used for both long-line and raft culture methods and scallop shells for the long-line method. Besides PVC tube, polyethylene string, wire and other materials are used as collector accessories. The oyster cultch that is mainly used in Korea is the uneven right shell. A hole is drilled on the center of the shell and a string is threaded through it. With the scallop shells, a PVC tube is placed between shells. A desirable collecting string can generally accommodate from 60 to 80 shells per meter of hanging string. If scallop shells are used, the PVC tube between shells should be two to three cm long.

E. The Manufacture of Collectors and Establishment of Collecting Beds

The water layer that Pacific oyster can adhere to (except in special waters with vertical mixed current) is from the high tide line to 1 and 2 m below the ebb tide line. In this range, the middle water layer provides the best opportunity for Pacific oysters to adhere. In the coast, the natural settlement layer of Pacific oyster is generally the exposure time between three and six hours (Figure 4-7). The tendency of Pacific oysters to adhere varies according to water depth or collecting method (Figure 4-8). If the water in a bay is shallow, the fixed collecting method is generally used. If water is shallow, the vertical adherent range is narrow. Conversely, it is wide where water is deep. On the other hand, where the water depth in a bay is deep, the hanging method for collection is generally used. In this case, Pacific oysters tend to adhere in a concentric manner. After considering the water depth and the collecting method that best suits the site, collectors are placed in the desired site. There are two methods for collecting spat: the fixed (stick collecting method) and the hanging (raft collecting method). The fixed collecting facility such as a stick is used in muddy tidal areas. The facility is about 25 m long, 2 m wide and 2 m high. Based on the results of spatfall forecast, the cultch strings are vertically hung to collect pacific oysters. If the collecting bed is narrow and the depth is shallow, the cultch strings is horizontally laid across the collecting bed (Figures 4-9, 4-10). The cultch string used at this time is about three meters long and, if it is suspended vertically, half of it is laid across the collecting bed.

In a floating unit, the cultches are vertically hung, totally submerged in the water. For culturing one-year-old oysters, the string culture method is used from the beginning. The string used for collection is also used for culture.

Figure 4-7. Depth range where natural setting of the Pacific oyster occurs.

Figure 4-8. Distribution of oyster larvae by water depth.

F. Spatfall Forecasting

1. Environmental Factors and SpawningThere are physical and chemical factors that induce oysters to spawn but the most common are water temperature and specific gravity. Pacific oysters from Korea spawn mainly at 20 to 25 degrees Celsius and specific gravity of 1.014 to 1.021. However, the period of reproduction varies according to the locality. Depending on the spawning period of oyster at various sites, the collecting period can be divided into prior-collection and post-collection.

a. Prior seed collection Prior seed collection is carried out before water temperature begins to rise, which is in early summer. After seed collection, either of these activities is gradually carried out:

• After two to three weeks, the untrained spats are transported to the culture sites.

• The spats are left at the collecting site until the end of September, and when they reach a size of 10 mm, they are placed in hardening racks.

b. Post seed collection Post seed collection is carried out towards the end of an ascending period, or at the beginning of a descending period of water temeprature, usually around August to September. If left for a long time after collection, adhered spats become feeble or a lot of them are lost by current movement. Thus spats have to be trained at hardening racks two weeks after settlement. Merits of hardened seed (Tables 4-2, 4-3)

• It is treated easily in relatively small size. • None is lost in handling. • Mortality during culture is less than with ordinary spat because it is more

resistant. • Culture period is relatively short because of rapid growth. • Culturist can make plans for culture management.

2. Relation of exposure time and adherence of spatsThe fixed collectors may become fouled with organisms like barnacles if exposed for some time. The hanging method faces no such problem because the collecting string is suspended in water. The best place where spat oysters adhere is exposed from two to four hours (Table 4-4). 3. Vertical distribution of oyster larvaeWith the hanging method, the collecting layer is determined by the vertical distribution of oyster larvae. Oyster larvae are not always uniformly distributed vertically and horizontally because of rapid changes in environmental conditions. These larvae generally gather in a place where current flows across or eddy occurs. The distributional layer of oyster larvae also depends on the tide. Oyster larvae usually concentrate in the upper 2m of the water column as indicated in Table 4-5 and Figure 4-8.

1. colleoting string: 488 ea. 2. standing beam: 27 ea. 3. latral beam: 32 ea. 4. supporting beam: 9 ea. 5. clamp: 54 ea. 6. cultches per string: 80 shells

Figure 4-9. Structure of a rack for oyster seed collection

Figure 4-10. Seed-collecting rack of the Pacific oyster

Table 4-4. Setting of oyster seed at different tidal zones.

Period of time for exposed Description

2 hrs. 4 hrs. 6 hrs.

Collecting strings 59 ea. 55 ea. 54 ea.

Settled seed oysters 1,829 ea. 1,829 ea. 1,695 ea.

Settled seed oysters for each cultch 2.38 ea. 3.10 ea. 2.41 ea.

Table 4-5. Vertical distribution of oyster larvae in Namhae and Changwon coastal areas.

No. of oyster larvaea

Station Namhae Changwon

Remark

1 26 24 Inner coast

2 17 31

3 9 3.1

4 3 3.7

5 4 31

6 2 6.9

7 1.6

8 53 Outer coast 4. Investigations needed for collection of seed oysterSpatfall forecasting predicts the proper times, water areas and water layers for collecting oyster larvae. The season for spat collection is when the number of settled spats increases more and more. For the forecasting of seed collection, the following activities are performed:

• Environmental investigation: Water temperature, salinity, current direction and velocity, wind direction, turbulence and rainfall are investigated, if possible.

• Larval investigation • horizontal investigation - after selecting a given station, a sample is

collected by towing a plankton net (diameter 30 cm, mesh size XX 20) at a depth of 30 cm to 100 m.

• vertical investigation - after selecting a given layer, a sample is collected by towing vertically a plankton net (diameter 30 cm, mesh size XX 20).m

CULTURE Culture refers to growing seed oyster until harvestable. The place where seed oyster grows is an oyster farm. It is important to understand the entire growing process from collection to harvest. Growth of oyster depends on the hanging period, condition of oyster ground, condition of oyster seed and other factors. The general procedure is shown in Table 5-1.

Table 5-1. Culture steps in the oyster hanging culture methods in relation to ordinary and hardened spat. Month 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 Remark

Preparation for seed collection

Saad collection Hanging Culture

←-----------------------→ Harvesting Ordinary

spat Process of culture

Seed collection

Hardening ←-----------------------→

Hanging←--------

Culture ←-------------

Harvesting-------------→

Hardened spat

The various culture methods are listed and described below:

A. Culture Methods

1. Traditional MethodsBottom sowing. The simplest of oyster culture methods. What is basically required is that the sea bottom of the farm be hard. Otherwise, the shells get buried and lost in the mud. This method has been used mainly in the intertidal zones of Korea (Figure 5-1). Stone culture. In this method, stones are used as collecting materials in an intertidal zone where the bottom is soft. A lot of attention must be paid to selecting the growing

site. The bottom must be “combed” or raked to make it smooth and even one month before setting the collecting materials (Figure 5-2, 5-3). Stick culture. To use this method, the oyster ground must have a smooth bottom and be located in protected coastal areas with little influence of wind and waves. Sticks are 1.2 to 1.8 m long. They can be made out of pine trees, a kind of oak, bamboos or other materials. The advantage of this method is that the facilities for culture are easy to establish.

Fig. 5-1. Bottom culture of Pacific oyster.

Fig. 5-2. Stone culture of Pacific oyster.

Fig. 5-3. Schematic drawing of stone method.

2. Hanging MethodThis method promotes the growth of oyster by prolonging the feeding time. However, the cost of the facilities is higher. Operating it also requires more people. Yield is high, though, and because of this, hanging ground is continuously expanded. This method includes raft and long line culture. Raft culture. Wooden poles are laid parallel to each other about 0.5 m. apart and fastened by wire lashing to lateral beams. Each beam is used to suspend the seed oysters from the raft. The floats for the raft are usually made from styrofoam. A raft is normally 18 m long and 9 m wide and the float materials range from 30 to 40 pieces. About 400 to 500 strings of oysters can be suspended from the raft. One string is 9 m long (Figure 5-5).

Long-line culture. The long-line culture is a modification of raft culture. This method is used for offshore culture. The basic feature of a long-line unit is a series of styrofoam floats arranged in a row. The long-line is secured at each end with two anchors. One long-line is 100 m long and consists of about 51 floats connected by a polyurethane rope 15 mm in diameter. A series of strings of oysters called “rens”, each about 5m long (the exact length varies with the depth of water) is suspended to each rope (Figure 5-6). 3. Other MethodsRack culture. The rack method is a shallow water adaptation of the hanging culture method. To construct a rack, wooden poles are driven into the bottom, two to four metres apart. These uprights are connected by horizontal poles. The horizontal poles support the suspended strings of cultch, which are placed about 30 cm apart. Umbrella-type culture. This is a modification of the rack method. A unit consists of one standing pole driven into the bottom to which is tied a series of strings of oysters spread radially as an open umbrella.

B. Culture Units

Establishing the culture unit is one of the most important tasks in oyster culture. A general description of the culture units has been briefly mentioned; this section describes the long-line and raft methods. 1. Number of units per areaThe level of culture units permitted per given area is set by the Ministry of Agriculture, Forestry and Fisheries mainly to prevent an excess of intensive culture. This is shown in Table 5-2. The units are established on 5 to 10 percent of the established oyster ground areas.

Fig. 5-4. Stick culture of Pacific oyster.

Fig. 5-5. Raft culture of Pacific oyster.

Fig. 5-6. Long-line culture of Pacific oyster.

Fig. 5-7. Rack culture of Pacific oyster.

Table 5-2. Number of oyster culture establishmentspermitted per

hectare of water area for hanging culture in Korea. Long-line culture methoda

(set) Raft culture methodb

(set)

6–20 1–4 a One set of long-line indicates 100m of line. b One set of raft indicates 9×18m of raft.

Table 5-3. Details of materials for one raft. Material Specification Quantity Remark

Bamboo pole ø 10cm, 4.5m long 94 ea.

Float Styrofoam, 450 13 ea.

Rope for anchor Wire rope, ø 24mm 70 m Anchor Reinforced concrete, 0.6 M/T 4 ea. Hanging string #13 galvanized wire 3,600 m 9m × 400 strings Pipe PVC, ø 1cm, 20cm long 16,000 ea. 40 cultches × 400 strings Cultch Oyster shell 16,000 shells 40 cultches × 400 strings Table 5-4. Details of materials for one long-line.

Material Specification Quantity Remark Rope PE rope, ø 15mm 100 m Rope for anchor PE rope, ø 15mm 60 m 30m for a string

Float Styrofoam, 60 51 ea. At 2m intervals

String ø 5mm, PE coated 142 m At 70cm intervals Anchor Stick or iron 2 ea. Both sides Cultch Oyster shell 2,850 shells 20 cultches × 142 strings Line for fastening float PP twist line 255 m 5m × 51 ea. 2. Methods of establishing a culture unitRaft culture. Raft is made of buoyant and pliant bamboo. Floats are usually made of styrofoam. Sizes of rafts vary but the most common is 18 m × 9 m. One needs 45 pieces of floats for this raft size. This number would normally weigh about 400 kg. The raft can accommodate 400 hanging strings. A hanging string is 9 m long and is ordinarily reinforced by two pieces of No. 13–14 galvanized wire wound around its length. The number of cultches per hanging string is about 40. The cultches are held about 20 cm apart by a PVC pipe (Figure 5-8 and, Table 5-3). Long-line culture. This has been described in the preceding section and is indicated in Figure 5-9 and Table 5-4.

Figure 5-8. Structure of a raft for oyster culture.

Figure 5-9. Structure of a long line for oyster culture.

Figure 5-10. Shell growth in terms of height from the start of the hanging culture

period.

Figure 5-11. Meat weight increase from the start of the hanging culture period.

3. Oyster growthGrowth of Pacific oysters greatly depends on the conditions of the culture ground and on the hanging period. Effects of hardening on growth. Oyster spat trained on hardening bed for more than six months after collection are resistant to disease and grow well. Tests showed that the mortality rate of hardened oyster spat was only 18.2 percent in eight months after hanging while that of ordinary oyster spat was 42.6 percent. Effects of the hanging period on growth. Growth rate varies with hanging period. For example, oyster of 1.5 cm shell height hung in March grew to 9.3 cm by April of the next year while oyster of 2.4 cm in shell height hung in July grew to only 7.6 cm by April of the next year (Figures 5-10, 5-11). The environment and oyster growth. Gonad development and growth of the Pacific oysters differ according to the habitat, physiology and ecology. In the southern part of Korea, oyster grows rapidly from June when the water temperature is over 20 degrees Celsius, and the rate of fattening becomes faster than the growth of shell from November. Specific gravity and speed of water current also influence the growth of oyster shell; oysters in the outer coast where water current is rapid grow more slowly than oysters in the inner coast (Figure 5-12).

Fig. 5-12. Comparison of growth of oyster in different location of growing area.

Culturing density and oyster growth. Culturing density also influences yield. With the long-line and raft culture methods, suitable size of the unit and number of strings per long-line or raft depend on the condition of the oyster ground. In the southern coast of Korea, hanging strings are set 70 cm apart. The characteristics of growth according to culturing area. The external shapes of Pacific oysters in Korea differ with habitat (Figure 5-13), small in the western coast and large in the eastern coast. Pacific oysters from the tidal zones are generally small and slow in growth, while those in deeper waters are large. Also, cultured Pacific oysters in the southern and eastern coasts are oblong while those from the tidal zones of the western and southern coasts are oval (Table 5-5). Shell weight also varies with the water depth; Pacific oyster from the southern coast is lighter than those from the eastern coast.

Table 5-5. Characteristic differences of the Pacific oyster (C. gigas) cultured in different ongrowing sites.

Characteristic Eastern Southeastern Southwestern Western Growth Most rapid Rapid Slow Slowest Size Largest Large Small Smallest Shell width Narrow Narrow Wide Wide Rate of meat weight to total weight Low Low High High

Colour of shell Light gray Dark gray Dark purple Purplish brownSpawning season Latest Earlier Earliest Later

Figure 5-13. Difference in the external appearance of Pacific oyster (C. gigas) in different regions (A, western; B, southern, C, eastern).

DISEASES

A. Fouling Organisms

1. PredatorsThere are many species of oyster predators such as Sparus swinhonis, Octopus vulgaris and Asteria amurensis, etc. Carnivorous gastropods such as the following drill into the oysters: Purpura clavigora, Purpura bronni, Ocenebra japonicum, Goratostoma founari, and Rapana thomasiana, etc. Drill damage usually occurs from April to November. Greater damages have been observed when water temperature goes up. R. thomasiana causes drill damage all year round even if the water temperature is below 10 degrees Celsius. The damage by Rapana thomasiana had been the most serious (Table 6-1). In a ten-day test, R. thomasiana, P. clavigora and O. japonicum drilled 12, 5 and 2 oysters, respectively. Predators and their egg sacs should be removed by hand-picking at low tide. Starfish should be thoroughly removed from the oyster growing area because their revival ability is strong and they feed on oysters all year round. 2. Sessile fouling organismsThere are many species of sessile fouling organisms. Among these are mussels, Hydroides norvegica, sea squirts, flat worms, fungi, diatoms, sponges, seaweeds, hydroids, annelids, and other crustaceans (Figure 6-1). Some are microscopic in size. Most predators attach themselves to the growing facilities or to oyster shells; others like Polydora, Protella and Caprella species, are food competitors.

Table 6-1. Number of oysters preyed upon during a given time period.

Description Thais clavigera

Thais bronni

Chicoreus asionus

Ceratostoma burnetti

Rapana venosa Starfish

Number of oyster fed by predator 18.1 14.3 22.4 24.4 61.7 208.9

Period (days) 307 292 235 236 350 365

Figure 6-1. Typical fouling organisms in oyster culture grounds. A, Codium sp.; B, Porphyra sp.; C, Ulva sp.; D, flat worm; E, sponge; F, sea anemone; G, Bagula sp.; H, barnacle; I, M. edulis; J. H. norvegica; K, Ciona intestinalis; L, Styela sp.; M, Didemnun noseleyi. Ecology and life cycle of important sessile fouling organismsMytilus edulis (mussel). Though this is native to North Europe, it is now widely distributed over the world. It is prolific. The mussels compete for attachment space and food, and their growth and reproduction may be superior to those of the oyster, This species matures in about one year, and one-year-old spats have the ability to spawn all year round. Hydroides norvegica. It is one of the Polydora species and lives in its own calcious tube. This species feeds on microscopic food. The larvae develops into the trocophore stage 24 hours after hatching, freely float for a week, and develop into the

nectrochaetal stage, when it settles down to attach to some solid substrate. Settlements are abundant in places where water current is slower than 1.8 knots. This species is not resistant to fresh water with salinity below 15%. Halocynthia spp. Halocynthia spp. are of two types, solitary and colonial. Among them, the Ciona intestinalis does great damage to oyster, competing for space and food. It spawns from April to December, the main season being June to September. C. intestinalis larvae freely float for a month and then cling to some substrate with their adhesive ciliated feed. Immediately after settlement, their anatomy changes drastically. Balanus sp. The shell of Balanus sp. consists of solid compounds of calcium. If spat collectors are set out too early, great numbers of Balanus spp. with habits similar to those of oysters may be attached. Balanus has the same life cycle in larval stages as that of shrimps and crabs. It undergoes eight stages of metamorphosis before attaching to solid substrates. The larvae develop from nauplius stage to cypris stage with transparent shell. Cypris stage larvae have a unique organ which is called a cement line between ear and gullet. The larvae secrete a mucous glue and attach themselves firmly to a solid substrate. 2. Appearance of Fouling Organisms and Fluctuation of their Biomass The sessile fouling organisms that infest cultured oyster include Hydroides spp., Bugula neritina, Ascidians, sponges, Balanus spp., and Mytilus spp. Most of these organisms attach during the period May to September in warm water season, but the period and depth of their attachment are different according to the species.

Figure 6-2. Settlement frequency (%) of major fouling organisms in Hansan-Goje Bay in relation to the settlement of the Pacific oyster (C. gigas). The quantity of attached fouling organisms in oyster farms located in Hansan and Goje Bay differs by species. For example, Ascidians largely settle in May, B. neritina in June to September, massel and sponges in June, and barnacles in September (Figure 6-2). 3. Control of Fouling Organisms Oyster farmers need to eradicate fouling organisms from oyster spats being harmful and from growing oysters before setting the oyster facilities. Some effective control methods are as follows:

Hot water treatment. Heat up sea water to 55–60 degrees Celsius in a great can or oven on a barge. Soak the strings of oyster in the heated sea water for 10 to 15 seconds. This method is effective in eradicating Mytilus spp., Balanus spp., and Ascidians (Table 6-2). Freshwater treatment. The method makes use of the osmotic pressure of organisms; in effect, the lesser density fresh water “drains out” the “thicker” body fluid of the organism, thus killing it. Soak the strings of oyster in stream water or in a tank of fresh water. Soaking should be at least for 50 hours in 15 to 20 degrees Celsius of water temperature, or 30 hours in 20– 25 degrees Celsius. Hydroides is easy to eradicate by this method (Table 6-3).

Table 6-2. Comparison of lethality (%) between oyster (C. gigas) and mussel (M. edulis) under different heat treatment.

Water temperature 50°C 55°C 60°C Oyster Mussel Oyster Mussel Oyster Mussel

Treated time (second)

(1–2.5) (1–2) (4–5) (1–2.5) (1–2) (4–5) (1–2.5) (1–2) (4–5) 1 - - - - - - 0 0 0 3 - - - - - - 0 70 0 5 0 0 0 0 0 0 0 100 0 10 0 0 0 0 60 0 0 100 0 15 0 10 0 0 100 0 8 100 20 20 0 30 0 0 100 0 13 100 30 30 0 100 0 0 100 10 20 100 60 60 0 100 0 10 100 20 60 100 100 ( ) : Shell height in centimeter. Table 6-3. Lethality of H. norvegica in relation

to fresh water treatment. Soaking time (min.) Lethality (%)

20 8.0

60 44.6

120 64.9

B. Diseases

The more oysters are cultured, the more disease problems encountered. It is difficult to find out the cause of various diseases affecting oyster farms. Researchers have not clearly pinpointed the causes. In 1960, a considerable number of oysters died of an unknown disease in Chesapeake Bay, USA. It took almost ten years for researchers to confirm the pathogens, a parasite known as Michinia nelsoni and a mold known as Labyrinthomyxa marina. The classification of oyster diseases, as reported by researchers, is as follows: 1. Viral diseases Herpes virus diseases. The digestive organ of oyster infected with this virus usually changes into white colour and the oyster dies of the herpes virus. The shape of the virus is hexagon and its diameter ranges from 70 to 90 mm.

In an experiment, a great number of oysters reared in water temperature of 12 to 18 degrees Celsius died when transferred to water of 28–30 degrees Celsius. The following methods are used to prevent virus diseases:

1. Water temperature of oyster growing area must not rise above 27 degrees Celsius.

2. Infected oysters should be removed from the oyster farm.

2. Bacterial diseases Some marine microorganisms have also been known to cause oyster diseases. Researchers have carried out experiments which showed microorganisms to be a major cause of high shellfish mortalities. In 1967, American conchologist Colwell succeeded in making pure isolation of Pseudomonas enalia which is believed to have caused mass mortality in young oysters. In 1977, Sindermann reported that Vibrio anguillarium and V. angullarum-like species were the causes of severe mortality in young oyster. In general, a deterioration of environmental conditions in oyster grounds makes it easier for these disease-causing microorgansms to infect oysters,.

Fig. 6-3. Bacterial Foci (Focal Necrosis) from Pacific oyster.

Fig. 6-4. Bacillary necrosis of oyster larvae, showing typical bacterial swarming.

In 1953, when a large number of cultured oysters died at Hiroshima Bay, gram-negative bacteria showed an incidence of 22 percent; this incidence was the highest among. all microorganisms found in the dead oysters. It was presumed to be Achromobacter sp. although the organism was not clearly identified (Figures 6-3, 6-4). 3. Protozoa Sporozoa. Among oyster diseases, the damage caused by sporozoa and Minchinia nelsoni are very serious. From 1957 to 1960, 95 percent of all oysters in the U.S. east coast died from Minchinia nelsoni infection. The pathogen was not identified until about four years after the incident. Researchers named it as MSX (multinucleate sphere unknown) due to the presence of many nuclei. Infected oysters in an oyster farm were easily picked out at that time. It was also easy to determine whether they were infected through microscopic test. In oyster farms in the U.S.A., Minchinia nelsoni frequently appear at low water salinity conditions of around 15% and in sheltered waters of high temperatures. On the other hand, Minchinia costalis is frequently observed in high salinity waters. A sporozoan similar to this organism is occasionally observed in Korea. Egg disease. The egg disease occurs only in Crassostrea gigas, its causal organism having been thought to be an amoeba (Chun, 1979). Recently however, the pathogen has been identified as Marteilioides chungmuensis (Comps et al, 1986) of Phylum Ascetospora. In Korea, this disease chiefly appears from August to November. The parasites attach to the eyes of the oyster. The adult is 10 to 12 um in length (Figure 6-5).

Figure 6-5. Infected ovum of Pacific oyster (C. gigas) with Marteilioides chungmuensis (p = parasite).

C. Eutrophication of Oyster Farm

The productivity of oyster varies with the location and conditions of the ongrowing ground. If the oysters are cultured fairly densely, productivity of the oyster ground could decrease and certain environmental factors that favour growth and fattening may disappear. Oyster ground deterioration is mainly caused by the waste products from the dense oysters and other fouling organisms. Spoilage caused by fouling organisms also affect the water quality of an oyster growing area. Toxic gases such as ammonia and hydrogen sulfide are produced by microbial decomposition of fouling organisms. Deterioration of oyster grounds may be more serious during summer months.

D. Red Tide

Red Tide is caused by a sudden increase in the population of certain planktonic microalgae. These organisms exhaust the dissolved oxygen so that the red tide is extremely harmful to fish-growing areas. Recently, adjacent sea waters have been polluted by industries on the southern seacoast of Korea. Water pollution may also lead to the development of red tide. As a matter of fact, red tide has occurred often in the Chinhae-Masan Bay in the southern part of Korea. The planktons which are related with red tide are Chaetoceros, Gymnodinium, Gonyaulax, Ceratium, Peridinium, Prorocentrumn, Noctiluca, ciliate, etc. (Figure 6-6). Among these, the phytoplankton Gymnidium is poisonous to other living organisms, including oyster by releasing toxins. At present, there are no known methods to adequately protect fishing and culture grounds from red tide. Measures should be tried in order to lessen the damage from red tide, but unfortunately, the best that can be done at present is to prevent water pollution.

Figure 6-6. The causative organisms of red tide.

HARVESTING Harvesting of Pacific oysters for the domestic market begins in October or November, but most of the oysters from hanging culture are harvested from December to May as shown in Table 5-1. Oysters from long-lines are harvested by raising them up with a winch. Hanging rens are 9 m long (Figure 7-1). Since cultches are threaded through by a galvanized wire, a cut at the end of the hanging string could cause the oysters on cultch to slip off the wire.

Figure 7-1. Harvesting of oyster by crane.

Harvested oysters are passed through a washer. Because the hanging strings in a long-line are placed at a relatively narrow interval between ropes, a small harvesting boat which can pass through the narrow spaces is used. Yield depends on harvesting period and on the condition of oyster ground, but in the raft method meat weight per hanging string of 9 m is generally 7 to 11 kg (Figure 7-2).

Figure 7-2. Yield of oysters from a 3m-long ren.

PROCESSING

A. Fresh Shucked Oysters

1. Washing of harvested oysters

Oysters harvested and transported directly to the processing plants should be reasonably free of sediments and other foreign substances. They can be cleaned with the use of an octagonal-cylindrical continuous rotary washing machine. The washing container is made of a rotating perforated cylinder of heavy construction with both ends open. The cylinder generally measures 4.4 m in length and 1.5 m in diameter but the dimension differs from plant to plant. A perforated water pipe is built into the cylinder from which water jets outward. The position of the cylinder is fixed at an angle of 7 degrees slope to allow oysters to slide down. A 3 HP power motor is needed to rotate the cylinder. Sea or fresh water can be used for washing the oysters, but they must be clean enough from the standpoint of hygiene. The oysters from the washer are automatically rinsed when passing the high pressure water spray through the conveyor. They are placed in clean plastic baskets for shucking. 2. Shucking Oysters are shucked in a manner that does not get them contaminated. Stainless steel tables are used for shucking. The table tops are smooth, well drained and high enough above the floor to prevent product contaimination. Shucking knives, which have fairly long blades and pointed tips are used. Shucking containers are most often plasticware or stainless steel vessel with a capacity of about 3 liters. The opening process begins by placing the oyster on the table or holding it with the left hand with the concave or left shell down and with the hinge pointed toward the side of the opener. With the oyster in this position the adductor muscle is located about two thirds of the distance from the hinge towards the right. The knife is inserted between the shells at this point with a slight twist of the knife to keep the handle of the blade elevated. After the knife point has entered, moving the knife to the right and to the left will cut the adductor muscle on the flat or right shell. The knife is then turned until the blade is vertical. A prying motion will break the hold of the hinge to separate the two shells. Next, the adductor muscle on the other shell is cut and the oyster meat is flipped into the shucking pail. Careful attention must be paid when the knife is slipped into the shell so that the knife blade will be against one shell or the other, in order not to injure the oyster meat and ensure that the adductor muscle is cut right against the shell. 3. Washing of Shucked Oysters The air blowing system is used in washing shellfish meats. Shucking and packing operations are carried out in separate rooms or in sufficiently separated areas so that no shucked product or packing room equipment is contaminated by splashes or other activities. The shucked oysters are conveyed from the shucking room to the packing room where the blower is located, through a delivery window or by other means of conveyance. In large scale plants, the oyster meats are collected into 18-liter capacity plastic containers. The meats flow with the running water to the blower through the stainless steel chute which runs from the shucking room to the blower tank in the packing room. For more effective washing, the chute can be modified as it is built in one or two trap sections on which a grid is placed. Through this grid, shell particles, sand and others drop into a collecting box. The blower is about 300 cm long, 40 cm wide, and 30 cm tall. The bottom section of the portion 21 cm below the top rim of the frame is built in the shape of a trough with a discharge valve on it. There must be a passage way at one end of the brim of the tank to enable the oyster meat to flow with the water. Two lines of perforated air pipe are laid above the true bottom, and a perforated plate is placed above the air lines. This plate serves as a false bottom. This horizontal type of blower has the advantage of

continuous operation. The entire machine is made of stainless steel and built for easy cleaning. For the washing operation, the tank is filled with water, the oyster placed in it, and the air turned on. The air bubbles agitate the water and the oysters, providing a cleaning action without damaging the meats. Small scale operators, however, use a tub of continuously overflowing water. The oysters placed in a shallow basket are stirred gently by hand or with a paddle in the tub. 4. Packing After washing, the oysters are poured into a perforated stainless steel table. Bits of shell and other substances and rejected oysters such as those which are badly cut or of poor condition are removed. Grading is also done on the table. After grading, oysters are rinsed by spraying or dipping, or by soaking them in chilled sea water for a while. They are then drained and packed. In general, packages are of two types, small and wholesale. For the small sized packaging, 250 or 300 g of shucked meat with some amount of chilled sea water is put into a polyethylene tube and sealed. This practice is carried out at shucking plants located in the field. Chain food companies use a small flat styrofoam tray on which 350 g of shucked meats purchased from a wholesale market is placed and sealed with thin vinyl wrapper. For wholesale use, 8 kg of meat with some sea water contained in sealed polyethylene bag is placed in a polyethylene sack which serves as the inner pack. This is then packed into a styrofoam box with a capacity of 10 kg.

B. Frozen Oyster

The operations involved in processing frozen oysters are the same as those in processing fresh shucked oysters except for the steps after freezing. Processing frozen oysters for export is limited to the certified plants which must use oyster produced from approved grwing areas. This is in accordance with the “Regulations Governing Sanitary Control of Shellfish, Their Growing Areas, Harvesting and Processing of Shellfish Products for Export (MAFF Ordinance No. 699).” Washed and graded oyster meat are individually placed on plastic plates designed for freezing oyster and frozen at -35 to -45 degrees Celsius. Freezing time varies with the type of freezing system: 50 to 60 minutes for air-blast freezer, 30 to 40 minutes for spiral freezer, and 15 to 20 minutes for belt freezer. After freezing is completed, the oysters are removed from the freezing plate by twisting the plate and immersing it in water at temperature of 0 to 5 degrees Celsius for glazing. The oyster are packed in polyethylene bags sized as a unit package (see Table 8-1). These unit bags are then packaged in cartons and stored in the cold storage room at -25 degrees Celsius.

Table 8-1. Standards of size for individually frozen oysters

Grade of size No. of individual oyster per pound

Weight of individual oyster (g)

Extra large (XL) Under 25 Over 18

Large (L) 25 – 35 Over 13

Medium (M) 36 – 45 10 – 13

Small (S) 46 – 55 8 – 9

Tiny (T) 56 – 75 6 – 7

Figure 8-1. Flowsheet showing the various stages of processing frozen oysters.

C. Canned Smoked Oysters in Oil

1. Preparation of raw materials Most processing plants have been using the steam opened oysters for canning but fresh shucked oysters can be used after cooking at 95 degrees Celsius for 10 minutes. Oysters washed by the rotary washer slide from the washer into an empty steaming car. The loaded oysters are steamed into a horizontal or vertical retort at 105 degrees Celsius (at 5 lb steam pressure) for about 18 minutes. The steaming time varies, depending on size and condition of the oysters. The oysters are shucked using the knife. Shucked oysters are conveyed from shucking room to the blowing washer directly or through a chute in which water is running. Shell, sand and other debris are washed away from the oysters without damaging the meat. The washed meats are drained and graded. 2. Smoking Graded oyster meats are placed in galvanized or stainless steel wire mesh bottom trays. Only one layer of oysters should be placed in a tray, spread so that the smoke can penetrate uniformly. The filled trays are brought into the smokehouse and smoked at 100 to 120 degrees Celsius for 10 to 20 minutes. Most processing plants have been recently employing the conveyor system continuous automatic smoking equipment which is 12.5 m long, 2 m high and 1 m wide and equipped with a five-step stainless steel wire mesh conveyor. The conveyor is designed in such a way that each succeeding step is longer than the previous one. In the mechanical smoking unit, oysters are smoked at 130– 150 degrees Celsius from 20 to 30 minutes depending on oyster size. Oak tree wood and oak tree sawdust are used mostly for smoking. Proper smoking produces oyster tinged with a light chocolate color. 3. Canning After smoking and cooling, the meats are filled into No. 3-B Square Cans classified by count size into large, medium, small and tiny. Drained weight means the solid weight contained in the can after cutting. The fill-in weight, therefore, must be decided taking into account moisture content of the oysters after smoking and the changes that occur during processing. For these reasons, the smaller oysters have less shrinkage so that the amount of oil agglutinated with oysters after processing is more in smaller oysters than in larger ones. The standards of net weight, drained weight and count size for canned smoked oysters in oil are shown in Table 8-2. The amount of oysters to be placed in each No. 3-B Square Can is about 88 g for the count size of large, medium and small, and 86 g for the tiny size. After filling, about 20 g of edible vegetable oil

(cotton seed oil, soybean oil or olive oil) heated at about 100 degrees Celsius is added into each container. If necessary, 1 to 2 g of table salt is added.

Table 8-2 Inspection standards for canned smoked oysters in oil No. of individual oyster per can by size

Can name Net Weight Drained Weight Large (L)

Medium (M)

small (S)

Tiny (T)

No. 3-B Square Appellation:106-2 Dia.:106.2×74.6mm Height:22.0mm Vol.:120.9ml

Over 105 Over 90 Under 13 14–20 21–30 Over 31

No. 5-C Square Appellation:103-1 Dia.:103.4×59.5mm Height:19.0mm Vol.:71.7ml

Over 70 Over 55 Under 8 9–15 16–25 Over 26

Note : Inner side of the can must be coated 4. Sealing and Processing The filled containers are sealed by a vacuum sealing machine, maintaining a vacuum of 30 cm Hg in the vacuum chamber. The sealed cans are stacked in iron trays of heavy construction and taken to the retort for processing. Oysters are packed in No. 3-B Square Cans and if the initial temperature of can contents is higher than 60 degrees Celsius, sterilization takes 70 minutes at 115 degrees Celsius (at 10 lb steam pressure). The pack is water-cooled rapidly in the retort to about 35 degrees Celsius. When cutting the cans after processing, the vacuum in the containers should not be less than 7 cm Hg.

D. Canned Boiled Oyster

Steam opened oyster meats arely used in the processing of canned smoked oysters, but fresh shucked oysters can also be used after cooking. The washed and graded oyster meats are placed in the cans (required drained weights are shown in Table 8-3). Taking into consideration shrinkage during processing (a 10 to 20 percent loss, for example), the amount of oysters to be placed in a No. 7 fruit can (8 oz.) in commercial practice is about 185 g for large, 180 g for medium and small, and 178 g for tiny size. Passed under a perforated pipe, the filled cans are added with 2.5 to 3 percent salt brine. The filled containers are sealed by a vacuum sealing machine. The conditions of retorting and cooling are the same as in processing smoked oysters. The vacuum in the cans, when they are cut, should not be less than 20 cm Hg.

Table 8-3. Inspection standards for canned boiled oysters No. of individual oyster per can by size

Can name Net Weight (g)

Drained Weight (g)

Large (L)

Medium (M)

Small (S)

Tiny (T)

No. 7 Fruit (8 oz Tall) Over 225 Over 145 Under 15 16 – 20 21 – 30 31 – 55

Appollation: 211-4 65.4 dia.x81.3 mmH Vol.: 249.3 ml No.7 Appellation: 211-5 65.4 dia.x101.1 mmH Vol.: 318.1 ml

Over 290 Over 187 Under 19 20 –26 27 – 40 41 – 60

No.4 Appellation: 301-7 74.1 dia.x113.0 mmH Vol.: 454.4 ml

Over 440 Over 273 Under 30 31 –40 41 – 55 56 – 85

Note: Inner side of the can must be coated

Figure 8-2. Flowsheet showing the various stages of processing canned smoked oysters in oil.

E. Dried Oyster

1. Preparation of raw material Cooked freshly shucked oysters can be used for the product, but most of the factories use steam-opened meats. They use the same washer described in the processing of fresh shucked oyster for washing the oysters. The washed oysters are loaded in steaming cars, and cars are pushed along the track to a horizontal retort of heavy construction with doors at each end. The oysters are steamed at 110 5 degrees Celsius for 7 to 10 minutes, and then shucked. During shucking, careful attention is taken to prevent the breakaway of the adductor muscles. The shucked meats are placed onto shallow mesh plastic trays and washed by shaking the tray in a water tank in which water is continuously overflowed. Clean seawater or 2–3 percent salt brine is used for washing the oyster meats. 2. Drying The washed oyster meats are spread on the bottom of bamboo trays 80 cm by 100 cm in size. The trays are stacked on the cars for steaming. The loaded oysters are

steamed in the retort for 5 minutes at 80 degrees Celsius to increase its firmness and reduce drying time. After steaming, the oysters in the trays are dried in the sun. At sundown the drying trays are stacked up to a convenient height and covered by tent or canvas if necessary. During the spring season the drying process requires four to seven days depending on the size of the oysters, for example four to five days for small and six to seven days for medium to bigger sizes. If the weather is unfavourable, the oysters are dried in a hot-air dryer. The drying tray on which oysters are spread are stacked on the cars designed for holding the trays and taken into the drying chamber. At the beginning of the operation, temperature of the chamber is maintained at 27–38 degrees Celsius for one to two hours and then increased gradually to 60 degrees Celsius. For hot-air drying, the drying process is repeated based on the assumption that an hour's drying at 60 degrees Celsius is equivalent to one day of sun-drying. Because hot-air drying results in product dicoloration, it is recommended that drying be completed under the sun.

Figure 8-3. Flowsheet showing the various stages of processing dried oysters. 3. Coating Coating means spraying the oysters with oyster extract during drying to improve brightness and prevent the product from hardening. The drained liquid obtained from the process of steaming the oyster meats before drying is used. Generally, spraying is done once or twice a day after one or two days of sun-drying. A total of four sprayings is generally made. In hot-air drying, spraying is done once after every drying operation. 4. Grading and Packing After drying, the oysters are graded by count size (Table 8-4). The graded oysters in 3-kg units are put in a polyethylene bag as a unit pack and sealed. Ten bags of the unit pack are packaged in a carton box. The unit of weight for export trade is expressed in terms of pickle (60 kg). Two carton boxes containing 20 unit packs each weigh one pickle.

Table 8-4. Standards of count size for dried oysters Weight of individual oyster by size (g) Extra large (LL)

Large (L)

Medium (M)

Small (S)

Tiny (SS)

Over 5.5 4.5 – 5.4 3.5 – 4.4 2.5 – 3.4 Under 2.5 5. Storage The packaged products are normally stored in cold storage rooms at -20 degrees Celsius until shipping out.

F. Salt-Fermented Oyster

Fresh shucked oysters are washed thoroughly to remove bits of shell, sand and other foreign substances, and placed on baskets for draining. For low-salt fermentation, 10% table salt is added to the oysters and mixed evenly with or without spices (minced garlic, shredded green onion, red pepper powder, roasted sesame seeds and chopped ginger are commonly used as spices). The mixture is packed in a jar and aged for 3 to 10 days, depending on temperature. Those fermented without spices are seasoned by adding some spices before serving. For high-salt fermentation, about 20% table salt is added to the oysters. After it is properly fermented, the liquid of hydrolysis that accumulates on the upper layer of the jar is collected and filtered through cloth. The filtrate is heated to interrupt the fermentation process, and then cooled. The heat-treated filtrate is put back into the jar and mixed. The jar is sealed and kept in a cool place.

G. Oyster Juice

Oyster juice is a concentrated oyster extract utilizing either the drained shell fluid, a by-product from steamed shellstock oysters, or the extract that comes from cooking fresh shucked oysters. Oyster juice is used as the raw material of oyster sauce, a popular seasoning. 1. Extraction Extract from steamed oysters. This extract can be obtained as a by-product when the oysters are steamed for processing into canned and dried products. The fluid that drains out of the oysters while the retort temperature is coming up to about 110 degrees Celsius is discarded because the liquid drained at this temperature zone is mostly sea water in the shells. The liquid drained at or above 110 degrees Celsius is collected for the extract. About 3 percent of diatom earth is added to the extract and the mixture stirred. The mixture is either filtered or settled in a vertical tank to remove foreign substances. Extract from shucked oysters. The boiled water used in cooking fresh shucked oysters processing is used as the extract. The extract is filtered and concentrated. Broken or rejected oyster meats may also be used as raw material for the extract. In this case, the oyster meat with about three times as much water is cooked slowly until the temperature reaches 80 degrees Celsius. Thereafter, the cooking is carried out vigorously for 30 to 40 minutes, and the extract is filtered and concentrated. 2. Concentration There are two methods of concentrating the extract. The more common one is evaporation using a simple stainless steel tank equipped with steam pipes or steam jacket kettle. The other method uses a vacuum concentrator to reduce pressure concentration. The concentration is carried out until the product comes to 37% in brix and 22–24 degrees in Be (baume). The product quality standards for oyster juice, as prescribed by the Fishery Products Inspection regulations, are as follows: salt content should be less than 15%, and extractives and total nitrogen should be more than 21 and 1% respectively.

3. Packing After completing the concentration, the warm extract is poured into 18-liter square-shaped tin cans, with a small opening on top for pouring the liquid in. After the opening is capped and seamed, the packs are stored at either room temperature or in a cold storage room.

REFERENCES Anon. Statistical yearbook of agriculture, forestry and fisheries. Ministry of Agriculture,

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seting behaviour of larvae of Ostrea edulis J. du Conseil, 14. Comps, M., M. S. Park and I. Desportes. 1986. Etude ultrastructurale de Marteilioides

chungmuensis n.g. n. sp. parasite des ovocytes de l'huitre Crassostrea gigas Th. Protistologica, T. XXII, fasc, 3. 279–285.

Galtsoff, P.S. 1938. Physiology of reproduction of Ostrea virginica I. Spawning reactions of the female and male. Biol. Bull., 74.

Galtsoff, P.S. 1938. Physiology of reproduction of Ostrea virginica II. Stimulation of spawning in the female oyster. Ibid. 75.

Kafuku, T. and H. Ikenoye. 1983. Modern methods of aquaculture in Japan. Kodansha Ltd., Tokyo.

Kim, B. Y. 1982. Seasonal settlement of benthic fouling organisms in oyster culturing area. Bull. Fish. Res. Dev. Agency 30. 91–102.

Knight Jones, E. W. 1951. Aspects of the setting behaviour of larvae Ostrea edulis on Essex oyster beds. Rapp. Cons. Explor. Mer. 128 (II).

Korringa, P. 1941. Experiments and observations on swarming, pelagic life and setting in the European flat oyster Ostrea edulis L. Arch. Neerl. Zool., 5.

Korringa, P. 1947. Relations between the moon and periodicity in the breeding of marine animals. Ecol. Monoger., 17.

Korringa, P. 1976. Farming the flat oysters of the genus Ostrea Elsevier Scientific Publishing Company, New York.

Korringa, P. 1979. Farming the cupped oysters of the genus Crassostrea, Elsevier Scientific Publishing Co., New York.

Loosanoff, V. L. 1945. Precocious gonad development in oysters induced midwinter by high temperature. Science, 102.

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Orton, J. H. 1926. On lunar periodicity in spawning of normally grown Falmouth oyster O. edulis in 1925, with comparison of the spawning capacity of normally grown and dumpy oysters.

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Ranson, G. 1960. Les prodissichonques (COquilles larvaires) des Ostreides vivans. Ibid., 1183.

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http://www.fao.org/docrep/field/003/AB706E/AB706E00.htm#TOCviernes, 18 de febrero de 2005