Year-round spawning and seed production of the rabbitfish, Siganus guttatus

Aquaculture, 59 (1986)259-272 Elsevier Science Publishers B.V., Amsterdam -

259 Printed in The Netherlands

Year-Round Spawning and Seed Production of the Rabbitfish, Siganus guttatud SHIRO HARA2, MARIETTA

N. DURAY, MONINA PARAZO and YASUHIKO TAK13

Aquaculture Department, Southeast Asian Fisheries Development Box 256, Iloilo City 5901 (The Philippines)

Center (SEAFDEC),

P.O.

‘Contribution No. 198 of the SEAFDEC Aquaculture Department. ‘Present address: Overseas Fishery Cooperation Foundation, Akasaka Twin Tower, 2-17-22 Akasaka, Minato-ku, Tokyo 107 (Japan) 3Present address: Tokyo University of Fisheries, 4-5-7 Konan, Minato-ku, Tokyo 108 (Japan) (Accepted 20 October 1986)

ABSTRACT Hara, S., Duray, M.N., Parazo, M. and Taki, Y., 1986. Year-round spawning and seed production of the rabbitfish, Siganus guttatus. Aquaculture, 59: 259-272. A series of experiments on the spawning and larval rearing of Siganus guttatus was conducted during a ll-month period in 1984-1985. Spawning occurred every month throughout the year, without hormonal treatment, between the first quarter and the full moon. Fertilization rates and hatching rat&swere high, with means of 84.2% ( n = 38) and 89.6% ( n = 34)) respectively. Females that had been fed diets rich in cod liver oil or in a cod liver oil/soybean oil/soybean lecithin mixture spawned repeatedly for at least 4 consecutive months. Larvae reared in 20,26, and 32%0 salinities showed no significant differences in survival rates at day 21. Survival was higher for larvae fed during days 2-4 with rotifers strained through an 80-m-mesh plankton net than for those fed unstrained rotifers. Larvae readily accepted Artemia nauplii and artificial diets when these were first introduced on day 15 and day 23, respectively. Higher larval survival was obtained in large tanks ( 15 m3) than in small tanks (500 1). Survival rates of 3.5-16.6% (f= 7.5% ) at day 45 were obtained in six trials of mass larval rearing and 5500-50 100 (f=27 700) juveniles per female were produced at day 45, ready for stocking in grow-out farms.

INTRODUCTION

Despite the widespread interest in siganids, commercial culture of these fishes is hampered by the lack of dependable year-round supplies of fry for stocking. Artificial propagation is seen as the long-term answer to this problem. Eight species of siganids have been spawned in the laboratory, either naturally or by hormonal induction, but only four of these have been successfully reared to the juvenile stage (Bryan and Madraisau, 1977; Popper et al., 1976; Shinhata and Shima, 1980; Juario et al., 1985). Sigunus g&tutus is one of the rabbitfishes that attain a large adult size. This 0044-8486/86/$03.50

0 1986 Elsevier Science Publishers B.V.

species has a wide distribution in the Indo-Pacific and is a favorite food in many areas. Various aspects of its biology have been studied (Von Westernhagen, 1974; Von Westernhagen and Rosenthal, 1976; Tahil, 1978; Carumbana, 1983) including spawning and larval rearing (Palma, 1978; Alcara and Luchavez, 1980; Juario et al., 1985). However, there have been no detailed accounts of seed production for this species. A series of experiments on spawning and larval rearing of Siganw guttutus has been conducted since February 1984 at the SEAFDEC Aquaculture Department. In this paper, results of these experiments are described to provide information on year-round seed production of S. guttutus. MATERIALS AND METHODS

Source and maintenance of broodstock Broodstock used in the study came from two sources. The first group consisted of 2.5year-old hatchery-bred adults, while the second one consisted of wild juveniles from Pandan, Panay Island, reared to maturity at the Tigbauan Research Station, SEAFDEC AQD. Mature females were distinguished from males by their swollen and soft abdomens, whereas milt flowed from the urogenital pore of the male under light pressure. The broodstock including both males and females was kept in roof-covered outdoor circular canvas tanks (4 m diameter, 0.8 m water depth) or in roof-covered rectangular concrete tanks (about 5-10 m3 capacity, 2 x 3 m and 2 x 4 m, 0.85 m depth). Water temperature and salinity in these tanks ranged from 26 to 30°C and 29 to 34%0, respectively. The first group was fed with diet A (Table 1),containing 25% protein, from February to June 1984 and then with diet B (Table 1),containing 38% protein, from June to October 1984. The second group of fish was maintained on commercial crustacean pellets (Universal Robina), containing 42% protein, from February to May 1984. To investigate effects of lipid contents in the diet on gonadal maturation, the two groups, after being fed with the above-mentioned diets, were fed diets containing different levels of lipids. Cod liver oil, soybean oil and/or lecithin were used as main lipid sources. Compositions of these diets (C-G) are given in Table 1. Females from the first group were divided into three sub-groups: one sub-group was fed diet C, the second was fed diet F and the third diet G, from mid-October until mid-March 1985. Females from the second group were also divided into three sub-groups and fed diets C, D and E, respectively, from mid-May until mid-October 1984. Each female was marked by cutting the posterior tip of the free margin of the dorsal or pelvic fin or both dorsal and pelvic fins, and was designated by a letter (corresponding to the diet) and an individual number. Feeding level was at 3% body weight daily. Males used for the

261 TABLE 1 Percent composition of seven formulated diets fed to Siganus guttatus broodstock Ingredients

Fishmeal (tuna) ’ Squid meal Shriip meal Soybean meal Corn gluten meal Wheat germ Brewer’s yeast Rice bran Ipii-ipiI meaIb Corn starch Cellulose” Cod liver oil Soybean oil Lecithin (soybean ) Vitamin mixd Mineral mix” Moisture Crude protein Crude fat Crude fiber Nitrogen-free extract Ash Calcium Phosphorus

Percent composition in diets A

B

C

D

E

F

G

13 0 0 26 0 0 0 3 4 38 7 5 0 0 5 0 1.9 27.9 11.0 0.1 54.1 7.0

10 10 10 8 8 8 6 0 0 32 0 5 0 0 2 1 6.9 37.5 8.0 4.1 43.5 7.1 1.1 1.2

10 10 10 8 8 8 6 0 0 37 0 0 0 0 2 1 9.3 37.5 0.3 1.9 53.0 7.3 1.3 1.4

10 10 10 8 8 8 6 0 0 27 5 5 0 0 2 1 6.6 37.5 7.4 4.9 43.4 6.9

10 10 10 8 8 8 6 0 0 17 10 10 0 0 2 1 6.6 36.8 13.2 8.6 34.6 6.9 1.0

10 10 10

10 10 10 8 8 8 6 0 0 5.5 17.5 8 3 3 2 1 8.3 36.7 16.5 11.3 29.0 6.9 1.3 1.6

1.1 1.2

1.1

8 8 8 6 0 0 21.25 8.75 4 1.5 1.5 2 1 8.2 37.4 9.9 7.4 38.0 7.4 1.2 1.2

“Peruvian fish meal. ‘Ipil-ipil is a legume, Leuceno lezmxephula. “Rice hulls were used instead of cellulose. dVitamin mix: Halver’s mixture. ‘Mineral mix: U.S.P. XII Salt mixture No. 2.

experiment on the effect of lipids were placed in separate tanks and fed diet E or G.

Spawning and larval rearing The body weights of the spawners immediately before spawning were 115-600 g (X= 314 g, n= 85) for males, and 180-670 g ( f= 410 g, n = 54) for females. Spawning was induced simply by introducing one female with l-2 males (depending on the size of the female) into a spawning tank on the day of the first quarter moon.

13(U)

Mar.

13(13)

14(14)

A

Apr.

Commercial pellet

13(12)

(

Feb.

1984

NS, No spawning was observed.

c3 c4 Dl D2 El E2

2nd group

Al A2 A3 A4 A5 A6 Bl B2 Cl c2 Fl F2 Gl G2

1st group

Female no.

/\

lO(10)

May.

Diet

lO(11)

\/

Diet

June

600) 7(9) 7(9) 800) 600) 901)

July

9(13) 7(11) 7(11) 7(11) 602) 903)

C,D,E

9U3)

B

Aug.

NS NS NS NS 904) 603)

Sep.

Dates of spawning of Siganus guttatus in captivity; days of lunar month are also given in parentheses

TABLE 2

> NS NS NS NS lO(16) 9U5)

703)

V

Oct.

503)

Nov.

503) 4G2) 301) 402) 402) 2GO)

C,F,G,

Dec.

3(13) 2(12) 303) 2U2) 202) 2W)

Jan.

1985

5(16) NS 506) 4(15)/ 4U5) died

Feb.

>

1(10)

201) NS died 23(9)

Mar.

The spawning tanks were cylindrical plastic containers (about 1 m diameter) with semi-transparent covers, containing 450 1 of water at a depth of 55 cm. Gentle aeration was maintained in each tank. Water was changed and bottom sediments were siphoned out daily. Several pieces of corrugated plastic sheets were placed on the bottom of the tank as the substrate for the adhesive demersal eggs of S. guttutus. This facilitated transfer of eggs into other tanks for incubation and subsequent larval rearing. The larval tanks were cylindrical plastic containers (about 1 m diameter) containing 500 1 of water at.a depth of 65 cm, and rectangular concrete tanks containing 5 m3 (2 x 3 m ) or 10 m3 (3 x 4 m) of water at a depth of 85-90 cm. These were situated outdoors under a roof-cover. Since the early larval stages were intolerant of handling, the number of eggs transferred from spawning tank to incubation/rearing tanks was adjusted so as to avoid any additional handling, such as reducing the density, during the first week of rearing. Spawning, incubation and larval rearing were done in natural seawater (31-34%0 S) or in diluted seawater at 20, 26 and 32%0 S ( t 1%0 ) and at ambient temperatures ( 25-30” C ) . Natural illumination at the water surface in the incubation/rearing tanks at 11.00 h ranged from 1000 to 5000 lux. RESULTS

Spawning and hatching Natural spawning occurred every month throughout the 14-month period from February 1984 to March 1985 (Table 2). All spawning dates fell within the,period between the first quarter and the full moon, i.e. on lunar days 9 to 16. Spawnings took place around midnight (22.00-02.00 h, n=5), dawn (02.00-06.00 h, n = 13) and during morning hours ( 06.00-10.00 h, n = 7)) but rarely in the afternoon (14.00-18.00 h, n= 1) or evening (18.00-22.00 h, n= 1). Females mated for the first time spawned without fail. In the experiment on repeated spawning by the same females for 4 consecutive months, of 12 females 5 spawned on 4 consecutive months, 5 spawned for only 2 consecutive-months, 1 died after spawning for 3 consecutive months, and 1 died after spawning for 2 consecutive months (Table 2). Females fed diets rich in cod liver oil (diet E) and diets containing a combination of cod liver oil, soybean oil and lecithin (diets F and G) tended tomspawn more repeatedly than those fed diets with little or no such lipid sources (diets C and D) . The estimated number of eggs spawned per female varied from 210 000 to 1 160 000; the average for 16 spawnings was 570 000 eggs/female. The rates of fertilization ranged from 0% (in 2 spawnings) to 100% (in 9 spawnings) , with a mean of 84.2% (n= 38). Within the spawning salinities of 26-32%0, no significant difference in fertilization rates was observed. Fertilized eggs were

264 TABLE 3 Percent size distribution of rotifers from a mass culture, when strained through plankton net, or when not strained ( n = 100 for each sample) Rotifer sample

Strained 80-pm mesh loo-pm mesh Not strained

Lorica length (and width) of rotifers (pm) I162(117)

I 178 (127)

I203 (142)

> 203 (142)

95 54 17

5 22 25

0 20 49

0 4 9

spherical, 0.537-0.593 mm in diameter (X= 0.564 mm, n= 30 eggs from each of 37 spawners). Hatching occurred 18-20 h after spawning at water temperatures of 26-28” C. Hatching rates ranged from 12.2% for 1 spawning to 100% in 3 spawnings, with a mean of 89.6% (n= 34). Newly hatched larvae were 1.62-2.10 mm (i= 1.39 mm, n= 30) in total length (TL) .

Initial feeding Larvae consumed their yolk and started to feed on day 2 after hatching. Nevertheless, high mortality occurred during the first few days in the early trials on larval rearing, particularly on days 3 to 4. This high mortality was considered to have been caused partly by an insufficient supply of rotifers of an adequate size for the S. guttatus larvae. The maximum size of rotifers in the guts of larvae on days 2,3, and 4 (n= 10 larvae per day) was 162,178, and 203 pm in lorica length, respectively. The rotifer culture at SEAFDEC AQD consisted of individuals of which only 17, 42, and 91% were smaller than these maximum sizes, respectively (Table 3). When the rotifers were strained through 80-pm and lOO-pm-mesh plankton nets, the percentage of small-sized individuals increased. When these small-sized rotifers were fed to larvae on days 2 to 4, rotifer intake and survival and growth of larvae increased in proportion to the percentage of rotifers less than 162 pm in size, made available by straining (Table 4). In all subsequent experiments involving larval rearing, larvae were fed on days 2 and 3 with rotifers strained through 80-pm-mesh plankton nets.

Salinity and tank size Eggs at the eye vesicle stage from spawning (3 females) at 32%0 S were transferred to a series of nine 800-ml volume beakers with water of 20,26, and 32%0 S (n= about 100 eggs per beaker). The hatching rates were high in all

90

100

100

Day 3

100

100

100

Day 4 5.9 (2-13) 3.6 (O-15) 0.9 (O-2)

Day 2 7.1 (3-12) 6.6 (l-10) 5.0 (O-9)

Day 3

No. of rotifers/larva (5, range)

11.0 (4-21) 10.6 (2-22) 8.0 (5-14)

Day 4 5.4 2.6 0.7 0.7 0.5 0.6

Day 5

Survival rate (%)

3.18kO.13 3.16 f0.18 3.13kO.13 3.llkO.14 3.05 Zbo.13 3.08kO.14

Day 5

TL of larvae (mm) (f? SD)

“Sample size was 10 larvae each for rotifer intake and 20 each for survival and growth. Sampling was made from a 500-l tank containing about 10 000 larvae at 26%0S. Rotifers provided at density of 20 individuals/ml. For survival rate and growth, data for two replicates are shown separately.

50

70

100~,ummesh

Rotifers not strained

100

Day 2

Larvae with rotifers in the gut (%)

Rotifer intake

Rotifers strained 80-,nm mesh

Feeding treatment

Rotifer intake, survival and growth in total length (TL) of Sigunus guttutus larvae fed with rotifers strained through plankton net or not strained during the initial feeding stage”

TABLE 4

266 TABLE 5 Mean survival rate (%, n=2) 7,14, and 21 Female no.

of Siganw g&tutus larvae at different salinity levels (20,26 and 32%0) at days

Survival rate ( % )

Initial stocking rata of larvae ( X lO”/ms)

20%0 Days

In 500-l tanks A4 A5 A6 In 5-m3 tanks Bl Gl

7

26%0 14

18-39 15 40 28” 20

21

7

32%0 14

20.6 6.6 0.3 9.9 12.5

a.7 7.9

9.6 5.6

21

7

14

25.5 3.6 1.0 38.0 21.4

23.0 16.5

10.4 9.3

21

6.0 5.9 0.5 36.4 25.6

32.9 18.3

30.1 12.5

“Only one trial at different salinity levels was done.

salinities, from 95.5% to lOO%, indicating that salinity levels and change in salinity within 20-32%0 did not affect normal incubation and hatching of S. guttutus eggs. In another set of experiments, the eggs of each of 5 females spawned at 32%0 S were separately incubated, hatched and reared at 20, 26, and 32%0 S. Again, there were no significant differences in survival of larvae to day 21 in these salinities [at 20%0: 8.42 7.6% (mean? SD); at 26%0: 9.9 + 10.1%; at 32%0: 6.3 -I-9.4%] (Table 5), Table 5 also shows that survival rates in 500-l tanks were lower than those in 5-m3 tanks, except for the first trial. In another trial in 500-l tanks, rearing was discontinued on day 5 due to low survival, O-1.3% in all treatments. Mass rearing of larvae Six trials of mass larval rearing to the juvenile stage were conducted with the following basic guidelines. (1) Spawning is carried out in 500-l plastic tanks at 32%0 S. (2) Eggs are transferred into 5- or lo-m3 concrete tanks for incubation and initial larval rearing at 26%0 S (except in one trial testing different salinities). (3) Water in the rearingtanks is gradually replaced (20-60%) by natural seawater and sediments on the tank bottoms are siphoned out every day from day 5 or later. ChbreZZu sp. is maintained at a density of about 5~ lo5 cells/ml during the rearing period. (4) Larvae are not transferred into other tanks until they are at least 21 days old. (5) Feeding regime is as outlined in Table 6. The results of the mass rearing of larvae are summarized in Table 7. The survival rates at day 21 were generally higher in the experimental runs where

TABLE 6 Feeding regime for Siganus g&tutus larvae during the 45-day rearing period Kind of food given

Feeding period (days)

Amount of food given

Botifer Brine shrimp (nauplii) Artificial diet” TP-2 (280 ,~n) TP-3 (560 /un) Minipellet (980 p)

l-30 15-38

5000-20 000 ind./l 200-1000 ind./l

23-33 27-44 40-44

1 g/m3, l-2 Xper day l-4 g/m”, 2-6Xper day 4 g/m3, 3 X per day

“commercial diet (Yeaster Co. Ltd.) containing 53-59% protein; particle size given in parentheses.

rotifers strained through an 80-m-mesh plankton net were fed to larvae on the first few days. Nevertheless, once the larvae survived the critical period during the first 2 weeks, mortality decreased markedly, even when the larvae were transferred (handled). The survival rate from day 21 to day 45 ranged from 43.8% to a high of 92.2%, with a mean of 65% (Table 7). Rotifers were provided at densities of 5-10 individuals/ml during the first 2 weeks; amounts were doubled on days 17-19 when food consumption by larvae increased rapidly. S. guttatus larvae accepted novel food readily. When Artemiu nauplii were first introduced on day 15, larval guts turned orange within a few minutes. When artificial diet (TP 2, Table 6) in flake form was first introduced on day 23, the larvae fed on it within a few minutes. The growth rate of larvae from female Al was high after day 7. Larvae had a TL ($2 SD, n= 10) of 2.88 2 0.88 mm on day 1, and 3.312 0.12 mm on day 7. TL increased to 4.77 20.35 mm on day 14, 8.632 1.33 mm on day 21, 14.45 + 1.59 mm on day 28, 19.53 2 4.71 mm on day 35, and 21.88 t 7.23 mm (n=45) on day 45. The eyes of the larvae started to become pigmented on day 1. The mouth and anus opened at about the same time. Yolk was completely absorbed and the mouth developed by day 2. The first elements of the dorsal and anal fins appeared on day 8. Flexion of the notochord tip was completed on day 13. On day 17, some postlarvae had begun the transition to the juvenile stage, and by day 21 most had done so. Some of the larger larvae chased the smaller ones, often biting their tails off. On day 29, some juveniles had turned brownish in body color; these formed a school and kept to the deeper portion of the tanks. On day 35, most of the juveniles had such a brown color, and often dived to the tank bottom when they sensed a disturbance. On day 45, the juveniles became heavily pigmented and soon developed the orange spots characteristic of the species.

Hatching rate (%)

A2

A3

Bl

B2

2

3

4

5

Mar. 14 1984 Apr. 14 1984 June 10 1984 Aug. 9 1984 Oct. 7 1984

1162.6

773.5

210.5

1028.5

378.3

83.5

96.6

95.0

12.2

93.9

5 5 5 5 5

12

12

12

12

144.0 144.0 144.0 144.0 144.0

689.7

200.0

99.1

355.2

9.6 10.4 13.4 30.1 4.5

11.3

18.0

6.4

5.9

Survival rate (%) at day 21

6.8

7.2

12

5.5

16.6

5.5

3.5

At day 45

43.8

60.8

48.1

92.2

85.9

59.1

Days 21 to 45

Survival rate (% )

12

12x2

12x2

12

12

Water volume (m’)

Rearing (days 21-45)

20.9

29.2

37.5

33.2

5.5

12.4

No. of juveniles produced (X103)

Production

21.7k4.3

24.Ok5.7

19.4k4.6

23.9k6.3

35.0 + 7.8

21.9 If:7.2

Total length of juveniles (mm, f&SD)

(day 45)

“Experiment Nos. 1 and 2: larvae were reared in the same 12-m3 tank from day 0 through day 45. Experiment Nos. 3 and 4: larvae were separated in two 12-m3 tanks on day 21. Experiment No, 5: larvae were held in five 5-m3 tanks till day 21 (for experiment on the effect of salinity; cf. Table 5) and stocked together in two 12-m3 tanks on day 21. Experiment Nos. 1 and 2, using 100~pm mesh to strain rotifers for initial feeding; Experiment No. 3,80- and lOOpm mesh Experiment Nos. 4 and 580-w mesh.

Al

No. of larvae stocked (x103)

Water volume (m’)

No. of eggs spawned (x10?

Female

Date

Rearing (days O-21 )

Spawning

1

Experiment no.

Results of mass rearing Siganus guttatw larvae”

TABLE 7

269 DISCUSSION

Bryan et al. (1975) were able to induced spawning of captive S. canaliculutus every month during a l-year period with the aid of hormones. In their work, natural spawnings occurred in April, June, July and August, and hormonal spawnings were successful during the “off season” months (September to March). S. uermiculatus is also known to spawn in captivity during several consecutive months: Popper et al. (1976) reported successful hormonal spawnings of this species from December to March, and Popper and Gundermann (1976) reported natural spawning in November. No repeated spawning by a single female has been reported before, although Bryan et al. (1975) observed that a female S. cunulicdutus was again ripe 1.5 months after her first spawning. The results of our study are remarkable in revealing that captive S. guttutus can spawn without hormonal treatment every month throughout the year, and that a single female can spawn repeatedly during at least 4 consecutive months. Existence of a lunar spawning rhythm is known in several siganid species. Spawnings have been reported to occur 4-7 days after the new moon in S. cunuliculatus (Manacop, 1937 ) ; on or around the first lunar quarter in S. uermicdutus (Popper et al., 1976; Popper and Gundermann, 1976) ; 3 days preceding the full moon in S. Zineutus (Bryan and Madraisau, 1977) ; and 2-4 days after the new moon in S. urgenteus (Burgan and Zseleczky, 1979). The spawning dates of siganids thus all fall within the days between the new moon and the full moon, centering around the first quarter. The spawning periodicity of S. guttutus revealed in this study is very similar to that of other siganids, with a tendency towards a slight delay compared with other species. This delay may have been caused by the practice of putting together the spawners for mating only on the day of the first quarter. The ability to spawn repeatedly all year round in captivity is certainly a great advantage of S. guttutus as a candidate for mariculture. This ability makes possible a constant supply of seed for grow-out farms on a year-round basis, and minimization of the number of broodstock to be maintained and of the facilities for housing them. The occurrence of fry of S. guttutus, S. cunulicdutus, and S. spinus in Philippine coastal waters nearly throughout the year ( Alcala, 1979)) together with the above-cited monthly spawnings of S. canaliculutus and S. vermiculutus, suggest that most, if not all, siganid fishes will spawn in captivity during many months of the year. Hormonal treatment may not be necessary if spawnings are attempted on the right days of the lunar month as suggested by Popper et al. (1976) and Popper and Gundermann (1976). The results of our study indicate that another factor for successful natural (and repeated) spawning of captive siganids is nutrition of the spawners. The lipid content of the broodstock diet appears to be important, although

270

the exact effect of lipids on gonadal maturation and on egg quality has yet to be elucidated. Previous trials in siganid larval rearing, dependent largely on plankton of mixed species for initial feeding, have resulted mostly in poor survival rates (e.g. Popper et al., 1976). Use of fertilized oyster eggs has not yielded consistently good results either (May et al., 1974; Kitajima et al., 1980; our preliminary experiments). Furthermore, procurement of a large quantity of oyster trochophores at the desired time is difficult in many areas. Due to its ease of mass culture and its nutritional suitability as food for fish larvae, the rotifer Brachionus plicatilis is considered to be the most suitable food organism for siganid larvae, at least for the time being. The most critical problem in the use of rotifers for siganid larval rearing is their size, as has been shown in this study. The work on S. fmcescens also exemplifies the importance of rotifer size to survival: while the use of small rotifers (L=ca. 180 ,um) yielded excellent survival (Shinhata and Shima, 1980)) mass mortality occurred in larvae fed large rotifers (L= 280 pm) (Kitajima et al., 1980). There are basically two ways of obtaining small rotifers. One is to select small-sized individuals by straining; this entails culture of large quantities of rotifers, hence, more tank space. The other way is to isolate and culture populations of small-sized rotifers. There are two strains of B. plicatilis being cultured in various parts of the world: the small or S-type and the large or L-type (Fukusho and Iwamoto, 1980,1981). In southeast Asia, the S-type is found in the Philippines, Indonesia, Singapore, Thailand and Vietnam (Fukusho and Okauchi, 1982). Although the genetic and size stabilities of these two types are yet to be investigated, it is possible to maintain generations of the so-called Stype. A combination of these two methods, i.e. straining of rotifer cultures composed of small individuals, is also practicable. Tsukashima et al. (1983) obtained good survival in larval rearing of S&go japonicus using this method. Next to the rotifers, the brine shrimp Artemia sp. is most important for larval rearing of marine fishes. It is often used for more advanced larval stages when the fast-growing larvae require far greater amounts of calories than rotifers can provide. Artificial diets may take the place of brine shrimp, starting on the third week of S. guttatus rearing, with good results. Thus, a sequential feeding with rotifers, brine shrimp and artificial diet is adequate for S. guttatus larvae, as had also been shown by Juario et al. (1985 ) . Salinity manipulation increased larval survival in Acanthopagrus sivicolus, for which 70% seawater was found optimal (Tawada and Fujimoto, 1977)) and in Sebastiscus marmoratus, for which 50% seawater was optimal (Morizane et al., 1983). However, there were no significant differences in survival of S. guttatus larvae between 20 and 32%0 S in the present study. Larvae of this species are tolerant to fairly low salinities (8-24%0; Duefias, SEAFDEC AQD, personal communication, 1985). S. guttatus seed production would thus not be

271

adversely affected during the rainy season or dry season, even when the hatchery tanks are outdoors. Popper et al. (1973), May et al. (1974) and Popper et al. (1976) obtained better results with large-volume tanks in rearing trials of S. riuzdutus, S. canaliculutus and S. vermiculatus. Similar results were obtained in repeated rearing trials with S. guttatus. The reasons why heavy mortality occurred in small tanks are not clear. In summary, our 14-month investigation shows that year-round spawning and seed production of S. guttutus is now practicable, with more or less satisfactory larval survival rates and production of substantial numbers of fry ready for stocking in commercial grow-out farms. However, there remain some aspects that deserve further investigation, including the effect of broodstock diet (e.g. lipid and vitamin contents) on the survival of larvae, especially in the initial feeding stages. ACKNOWLEDGEMENTS

We thank Dr. Akio Kanazawa of Kagoshima University, Japan, and Dr. Felicitas P. Pascual, SEAFDEC Aquaculture Department, for technical advice on the broodstock diets, and Ms. Teodora U. Bagarinao of SEAFDEC AQD for technical assistance. This study was conducted with partial financial support from the Japan International Cooperation Agency (JICA) .

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