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Natural spawning and larval rearing of silver bream, Rhabdosargus sarba (Forsskål), in captivity
Aquaculture, 120 ( 1994) 115- 122
Natural spawning and larval rearing of silver bream, Rhabdosargus sarba (Forssk%), in captivity Ming-Yih Leu* Husbandry Centre, National Museum of Marine Biology/Aquarium, Pingtung, Taiwan, ROC
(Accepted 2 1 September 1993)
Abstract Silver bream, Rhabdosargus sarba, was spawned naturally in captivity without the use of hormones or other treatment. Spawning occurred continuously from 24 December 1989 to 29 March 1990 over a period of 96 days, with a water temperature range of 13.8-23.5’ C. It is estimated that a female laid about 2 586 000 eggs during the spawning period. The hatched larvae were reared in 5-t rectangular concrete tanks and fed initially on rotifers, Brachionus plicatilis, followed by Artemia nauplii and finally weaned onto an artificial diet. Larval rearing trials for growth and survival to 45 days produced juveniles of 19.4 mm average total length, at a survival rate ranging from 7.3 to 43.2%. Few agonistic attacks were observed throughout the rearing period, despite high initial stocking densities. After 105 days the juveniles reached an average size of 6 1.6 mm fork length with a survival rate of 12.5%.
1. Introduction The silver bream, Rhabdosargus sarba (ForsskzU), is a widely distributed sparid species throughout the subtropical and tropical inshore waters and estuaries in the Indo-Pacific (Smith and Heemstra, 1986). Due to its high commercial value, the fish is considered to be a desirable species for aquaculture, especially in the Asian region. The juveniles used for culturing are usually collected from local coastal waters, but the supply of juveniles is not sufficient and this is becoming a major limiting factor in the development of R. sarba culture. R. sarba are serial spawners and spawn between December and March in the ‘Correspondence to: Preparatory Office, Kaohsiung Branch, National Museum of Marine Biology/ Aquarium, 5F-1, No. 111 Ming-Sheng 1st Road, Kaohsiung, Taiwan 800, ROC. 0044-8486 194 / $01.00 0 1994 Elsevier Science B.V. All rights reserved
ssDI0044-8486(93)E0218-X
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Pescadores ( 23 ’ 40’N; 119 o20’ E ) , when the temperature is between 15 and 26 ’ C (Lin et al., 1988). In Taiwan, spawning of R. surba has been induced following hormone induction (Lin et al., 1988 ). Natural spawning has also been observed (Lin et al., 1989,1990), and morphological development of eggs and early larvae has been described (Lin et al., 1988). The complete larval development of this species has also been described by Tsukashima and Kitajima ( 1982)) but no success has been reported so far on rearing of the larvae. This paper reports the results of experiments on natural spawning of captive R. sarba during the 1989-l 990 breeding season, as well as larval rearing systems. 2. Materials and methods R. surba broodstock were reared for 3 years from juveniles collected from the wild. Sixteen fishes (4 females and 12 males) that were used for spawning studies were stocked into a circular concrete tank ( 100-t capacity, 2.5 m water depth). The females weighed from 717 to 1236.5 g, and ranged in size from 31.7 to 34.5 cm fork length. The males weighed from 592.3 to 929.1 g, and ranged from 30.3 to 35.8 cm fork length. The spawning tank was equipped with a water inlet and outlet and an aeration system. The water quality in the spawning tank was maintained by changing 50-6096 of the tank water daily; temperature and salinity during the preconditioning and spawning period were ambient. The lighting schedule was ambient. Fish were fed on formulated diets containing 45% protein and artificial 4
diet w
20
0’ 0
” 5
” lo
15 Days
”
” 20
25
After
30
3s
’ 40
45
Hatching
Fig. 1. Growth of larval Rhabdosargus sarba in yotal length (mean + s.d. ) and food items given.
M-Y. Leu /Aquaculture lZO(1994) 115-122 Table 1 Composition
117
of the artificial diet
Ingredient
g/ 100 g diet
x-Carrageenan Oyster extract Egg yolk Shrimp extract Squid meal Concentrated fish solute Egg albumin EPA oil Soybean lecithin Soybean oil Yeast powder Mineral mixturea Vitamin mixtureb PapainC Total Cholesterold Egg lecithind
2.50 24.95 15.00 5.00 5.00 15.00 5.00 2.50 5.00 5.00 5.00 5.00 5.00 0.05 100.00 8.00 1.60
Proximate analysis (% of diet ) Moisture Protein Lipid Ash Nitrogen free extract
7.20 42.99 29.51 6.58 13.72
“Mineral mixture contained (% of mineral mixture): NaH,P04-2H20, calcium lactate, 1.964; seaweed powder, 65.746 (as carrier). bAccording to Halver ( 1957 ) . “Nakarai Chemicals Co. Ltd., Japan. dCoating materials.
30.305; ferric citrate, 1.435;
10% lipids, supplemented with raw oysters ( Crassostrea gigas) twice a week. The following diet formulated was used for the spawners: 72% white fish meal, 10% starch, 5% gluten flour, 5% minerals, 2% vitamins, 4% eicosapentaenoic acid oil, 1% choline chloride, 1% cellulose. In general, the fish were fed at a minimum of 3% of their body weight per day. Fertilized eggs were collected from the spawning tank with a fine dip net ( 100 ,um mesh size). Unfertilized eggs which settled to the bottom of the tank were removed by siphoning. The eggs were then transferred into nine 5-t rectangular concrete tanks for incubation and the larval rearing trials. The eggs were incubated in meshed baskets (350 pm) equipped with independent airlift systems which kept the eggs in continuous motion (Tandler and Helps, 1985 ). The use of these baskets made it possible to count accurately the hatched larvae before their introduction into the tanks. The rearing tanks were stocked with approximately 5 x 1O4hatched larvae ( 10 larvae-l- 1) each. The rearing protocol is summarized in Fig. 1. Briefly, rotifers, Brachionus pli-
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M-Y. Leu /Aquaculture 120 (1994) 115-122
catilis, served as the only food for the larvae for the first 20 days after hatching. The rotifers were added to the tanks at a density of 10 + 5 individuals-ml- l from cultures fed baker’s yeast, S’accharomyces cerevisiae, plus Nannochloropsis sp. at the rate of0.5-’ x lo6 cells~Brachionus-‘*day-l. As well as the rotifers, 50 l-day-’ of Nannochloropsis, at a density of about 30-50 x 1O6cells-ml-‘, were added as nourishment for the rotifers. Begining on day 20, the larvae were also offered newly hatched Artemia salina nauplii; from day 20 to day 30, the density of was maintained at 0.5-l individuals-ml- ‘; after day 30, the density of Artemia nauphi was increased to 1-2 individuals*ml-I. From day 30 onwards, an artificial diet (400-700 ,um particle size) containing 43% protein (Leu and Liou, 1992) was offered to the larvae. The composition of the artificial diet is presented in Table 1. By day 4 1, the larvae were fed the artificial diet only. Incubation and rearing salinity was uniformly 32 ppt, while temperature ranged from 19.2 to 22.3”C. The newly hatched larvae were reared in filtered seawater with very mild aeration ( 5- 10 ml-min- ’ ) . At the stage when the fish were fed Artemia, one third of the rearing water was changed once daily. When the larvae started feeding on the artificial diet, running water at a rate of about 10 1.min- ’ was applied to avoid water quality problems. After 45 days, all surviving juveniles were transferred to a 45-t rectangular concrete tank for further growth.
3. Results The broodstock spawned naturally 3 months after stocking. Spawning commenced on December 24th, 1989 and continued until March 29th, 1990. During this period, the water temperature in the spawning tank fluctuated between 13.8 and 23.5”C. Normally spawning occurred between 22.00 and mid-night. The spawning behavior was as follows: one female was closely followed by three or four males. Just before spawning occurred, the female turned laterally and hit the surface of the water. One male fish chased the female to the surface of the water where they released eggs and milt together. Daily spawning (Fig. 2) varied between 4500 and 77 225 eggs per female (average 26 937 eggs). This amounted to 2586 x lo3 eggs per female over this period. The total number of eggs collected was 10 344 x 103, of which 8658 x 1O3were fertilized (about 83.7%). The total number of newly-hatched larvae was 6277x 103. The hatching rate varied from 32.3% to 99.4% with an average of 72.5%. The fertilized eggs of R. sarba were spherical, non-adhesive and pelagic, and 0.95-l .04 mm in diameter (average 1.03 mm), with a colorless, transparent oil globule 0.22-0.24 mm in diameter. At a temperature of 19.2-22.3”C hatching time was 30-38 h after fertilization. Newly hatched larvae measured 2.25-2.48 mm in total length (average 2.38 mm) with 24 pairs of myotomes. Yolk-sac resorption was completed in 3-4 days. The morphological transition from the larval to the juvenile stage occurred during the 37th to 42nd day, between 13.5 and 17.2 mm in total length. Larvae grew
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M-Y. Leu /Aquaculture 120 (1994) 115-122
24
1
20 Feb/90
Dew89
20
l0
29
Mar/90
DArE
Fig. 2. Daily changes in hatching rates and number of eggs spawned by Rhabdosargus sarba during the 1989-1990 spawning season. No. of spawners=12 males and 4 females; water temperature= 13.8-235°C.
Table 2 The results of the rearing trials of larval Rhabdosargus sarba for 45 days after hatching. Trial number
No. of newly hatched larvae
No. of surviving juveniles
1
50 000 50 000 50 000 50 000 50 000 50 000 50 000 50 000 50 000
3648 10053 19752 8219 21603 13764 6851 11714 5245
2 3 4 5 6 7 8 9 Mean
Survival (96) 7.3 20.1 39.5 16.4 43.2 27.5 13.7 23.4 10.5 22.4+ 12.4
Total length (mm) 15.2 22.8 17.1 19.3 23.5 21.5 17.4 21.7 16.1 19.4k3.1
relatively slowly throughout the larval stage, but more rapidly after transformation and during the juvenile stage (Fig. 1). During daylight hours the juveniles swam in large schools along the perimeter of the rearing tank in the upper and middle levels; at night, they swam individually. With accompanying morpholog-
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/Aquaculture
120 (1994) 115-122
ical changes from larvae to juveniles and with adequate feeding bigger fish seldom attacked smaller ones. In nine trials, the juveniles (n= 100 849) reached a mean total length of 19.42 3.1 mm after 45 days of rearing; the suvival rate ranged from 7.3 to 43.2% with an average of 22.4% (Table 2). These juveniles were transferred to a 45-t rectangular concrete tank, the stocking density was approximately 2200 fish*m-3 with a 2-month survival of 55.8% (56241 fish). Average fork length of 105-dayold tish was 6 1.6 mm, with a range of 48.2-7 I .8 mm. Therefore, total survival obtained through the rearing processes was about 12.5%.
4. Discussion R. sarba is one of the few marine fish that can easily spawn in captivity. This is an advantage for the mass production of R. sarba juveniles as it minimizes the mortality of the spawners through stress due to handling and stripping. During the spawning season, in the present study, female R. sarba spawned every night in the tank, which is in agreement with the observations of Lin et al. ( 1990) who found natural daily spawning by one 4-year-old female in December-January. Matsuura et al. ( 1988) also reported similar results for red sea bream (Pagrus major).
Larval growth in the present study (Fig. 1) corresponds well with the results reported by Tsukashima and Kitajima ( 1982). Rapid growth of juveniles after metamorphosis could well be associated with the changes in morphological characters. Agonistic behavior by R. sarba larvae and juveniles is rare. This might be attributed to the fact that juvenile R. sarba are true omnivores (Nakatsugawa, 198 1; Blaber, 1984). The lack of agonistic behavior would make possible the use of production-scale systems of this species which are based on relatively high stocking densities. In contrast, agonistic behavior including cannibalism was one of the causes of high mortalities during the course of initial rearing of several other sparid species, such as black porgy, Acanthopagrus schlegeli (Tang et al., 1979), silvery black porgy, A. cuvieri (Hussain et al., 198 1)) P. major (Fukuhara, 1985 ) and A. sivicolus (Tawada, 1986). In the present study, the use of rotifers as a first feed, followed by Artemia nauplii, and an artificial diet, has proven to be successful. Nannochloropsis or socalled “green water” is commonly added to larval rearing tanks for stabilizing the water quality, as well as a food source for the rotifers. For experimental scale systems, rearing larvae entirely on live food has advantages. However, if the scale of production of juveniles is to be increased to production-scale, the uncertainties and labor requirements associated with the capture of live food impose severe restrictions. In recent years, artificial diets have been introduced and found to be potential practical replacements for live food in early marine fish larvae. Leu and Liou ( 1992 ) successfully weaned 1O-day-old R. sarba larvae on an artificial diet despite some growth retardation and low survival. According to Tanaka ( 1973 ),
M-Y. Leu /Aquaculture 120(1994) 115-122
121
the presence of gastric glands and pyloric caeca leads to the beginning of the juvenile period. In R. sarba juveniles, pyloric caeca appear around day 40 (Tsukashima and Kitajima, 1982), indicating that the digestive system is already functional at this stage and is able to cope with artificial diets. In the present study, weaning was progressive, from day 30 post-hatching, the Artemia ration was gradually reduced until day 4 1. During this period, the artificial diet ration was gradually increased from 0 to 100% of the ration offered to fish. Based on the present study, we clearly showed that artificial diets could serve as a convenient and cheap food for R. sarba juveniles (bigger than 1.4 cm). At this stage this diet could serve as a complete replacement for natural foods, such as minced trash fish and shrimp, or a supply of zooplankton. In conclusion, R. sarba readily spawns in captivity without the use of hormones or other treatment. The current feeding protocol in the present study makes possible a mass production procedure with survival greater than 22% from larvae to juveniles. With further improvements in methods, rearing systems, and diet formulation, survival should improve. This species has the potential for higher stocking densities than the 10 larvae-l-’ which were used in the present study. The optimal density in the rearing tank has yet to be determined.
Acknowledgements
I am grateful to the staff of Taihsi Branch of Taiwan Fisheries Research Institute (TFRI) who provided helpful advice and assistance with system maintenance. This study has been financed in part by a grant made by the Council of Agriculture, Executive Yuan, Republic of China.
References Blaber, S.J.M., 1984. The diet, food selectivity and niche ofRhabdosargussarba (Teleostei: Sparidae) in Natal estuaries. S. Afr. J. Zool., 19: 241-246. Fukuhara, O., 1985. Functional morphology and behaviour of early life stages of red sea bream. Bull. Jpn. Sot. Sci. Fish., 51(5): 731-743. Halver, J.E., 1957. Nutrition of salmonoid fishes. III. Water-soluble vitamin requirements of chinook salmon. J. Nutr., 62: 225-243. Hussain, N., Akatsu, S. and El-Zahr, C., 198 1. Spawning, egg and early larval development, and growth of Acanthopagrus cuvieri (Sparidae). Aquaculture, 22: 125- 136. Leu, M.-Y. and Liou, C.-H., 1992. Substitution of live foods with a micro-coated diet in the feeding of larval silver bream, Sparus sarba (Forskal): note on swim bladder inflation. J. Fish. Sot. Taiwan, 19(l): 65-73. Lin, K.-J., Chang, R.-M., Liu, C.-Y., Fang, Y.-K., Chen, C.-L., Juang, C.-Y. and Twu, J.-Y., 1988. Natural spawning of goldline sea bream Sparus sarba in the artificial environment and embryonic development. Bull. Taiwan Fish. Res. Inst., 45: l-l 6 (in Chinese, with English abstract). Lin, K.-J., Chang, R.-M., Twu, J.-Y. and Liu, C.-Y., 1989. Experimental ofthe propagation of goldline sea bream, Sparus sarba-breeder, culture, natural spawning in the artificial environment of 3year-old breeder, and hatching of fertilized eggs. Bull. Taiwan Fish. Res. Inst., 47: 2 l-37 (in Chinese. with English abstract).
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Lin, K.-J., Twu, J.-Y. and Chen, C.-H., 1990. Studies of daily spawning and quality of eggs in one goldline sea bream, Sparus sarba. Bull. Taiwan Fish. Res. Inst., 48: 251-262 (in Chinese, with English abstract). Matsuura, S., Furuichi, M., Maruyama, K. and Matsuyama, M., 1988. Daily spawning and quality of eggs in one female red sea bream, Pagrus major. Suisanzoshoku, 36( 1): 33-39 (in Japanese, with English abstract ) . Nakatsugawa, T., 1981. Some ecological notes of silver bream, Rhabdosargus sarba (FORSKAL), appearing in Zostera zone of the Aso-Kai lagoon. Bull. Kyoto Ocea. Fish. Sci., 5: 17-22 (in Japanese, with English abstract ) . Smith, M.M. and Heemstra PC. (Editors), 1986. Smiths’ Sea Fishes. Springer-Verlag, Berlin, 1047 PP. Tanaka, M., 1973. Studies on the structure and function of the digestive system of teleost larvae. Thesis for the degree of Doctor of Agriculture, Kyoto University, 136 pp. Tandler, A. and Helps, S., 1985, The effects of photoperiod and water exchange rate on growth and survival of gilthead sea bream (Sparus aurata, Linnaeus; Sparidae) from hatching to metamorphosis in mass rearing systems. Aquaculture, 48: 7 l-82. Tang, H.-C., Twu, J.-Y. and Su, W.-C., 1979. Experiment on the artificial propagation of black porgy, Acanthopagrus schlegeli. II. Fertilization, hatching and larvae breeding. China Fish. Month., 322: 3- 10 (in Chinese, with English abstract ) . Tawada, S., 1986. Development of eggs, larvae and juveniles of the Acanthopagrus sivicolusAkazaki reared in the hatchery. Suisanzoshoku, 33(4): 189-196 (in Japanese). Tsukashima, Y. and Kitajima, C., 1982. Rearing and development of larval and juvenile silver bream, Sparus sarba. Bull. Nagasaki Pref. Inst. Fish., 8: 129-135 (in Japanese, with English abstract).
Natural spawning and larval rearing of silver bream, Rhabdosargus sarba (Forssk%), in captivity Ming-Yih Leu* Husbandry Centre, National Museum of Marine Biology/Aquarium, Pingtung, Taiwan, ROC
(Accepted 2 1 September 1993)
Abstract Silver bream, Rhabdosargus sarba, was spawned naturally in captivity without the use of hormones or other treatment. Spawning occurred continuously from 24 December 1989 to 29 March 1990 over a period of 96 days, with a water temperature range of 13.8-23.5’ C. It is estimated that a female laid about 2 586 000 eggs during the spawning period. The hatched larvae were reared in 5-t rectangular concrete tanks and fed initially on rotifers, Brachionus plicatilis, followed by Artemia nauplii and finally weaned onto an artificial diet. Larval rearing trials for growth and survival to 45 days produced juveniles of 19.4 mm average total length, at a survival rate ranging from 7.3 to 43.2%. Few agonistic attacks were observed throughout the rearing period, despite high initial stocking densities. After 105 days the juveniles reached an average size of 6 1.6 mm fork length with a survival rate of 12.5%.
1. Introduction The silver bream, Rhabdosargus sarba (ForsskzU), is a widely distributed sparid species throughout the subtropical and tropical inshore waters and estuaries in the Indo-Pacific (Smith and Heemstra, 1986). Due to its high commercial value, the fish is considered to be a desirable species for aquaculture, especially in the Asian region. The juveniles used for culturing are usually collected from local coastal waters, but the supply of juveniles is not sufficient and this is becoming a major limiting factor in the development of R. sarba culture. R. sarba are serial spawners and spawn between December and March in the ‘Correspondence to: Preparatory Office, Kaohsiung Branch, National Museum of Marine Biology/ Aquarium, 5F-1, No. 111 Ming-Sheng 1st Road, Kaohsiung, Taiwan 800, ROC. 0044-8486 194 / $01.00 0 1994 Elsevier Science B.V. All rights reserved
ssDI0044-8486(93)E0218-X
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Pescadores ( 23 ’ 40’N; 119 o20’ E ) , when the temperature is between 15 and 26 ’ C (Lin et al., 1988). In Taiwan, spawning of R. surba has been induced following hormone induction (Lin et al., 1988 ). Natural spawning has also been observed (Lin et al., 1989,1990), and morphological development of eggs and early larvae has been described (Lin et al., 1988). The complete larval development of this species has also been described by Tsukashima and Kitajima ( 1982)) but no success has been reported so far on rearing of the larvae. This paper reports the results of experiments on natural spawning of captive R. sarba during the 1989-l 990 breeding season, as well as larval rearing systems. 2. Materials and methods R. surba broodstock were reared for 3 years from juveniles collected from the wild. Sixteen fishes (4 females and 12 males) that were used for spawning studies were stocked into a circular concrete tank ( 100-t capacity, 2.5 m water depth). The females weighed from 717 to 1236.5 g, and ranged in size from 31.7 to 34.5 cm fork length. The males weighed from 592.3 to 929.1 g, and ranged from 30.3 to 35.8 cm fork length. The spawning tank was equipped with a water inlet and outlet and an aeration system. The water quality in the spawning tank was maintained by changing 50-6096 of the tank water daily; temperature and salinity during the preconditioning and spawning period were ambient. The lighting schedule was ambient. Fish were fed on formulated diets containing 45% protein and artificial 4
diet w
20
0’ 0
” 5
” lo
15 Days
”
” 20
25
After
30
3s
’ 40
45
Hatching
Fig. 1. Growth of larval Rhabdosargus sarba in yotal length (mean + s.d. ) and food items given.
M-Y. Leu /Aquaculture lZO(1994) 115-122 Table 1 Composition
117
of the artificial diet
Ingredient
g/ 100 g diet
x-Carrageenan Oyster extract Egg yolk Shrimp extract Squid meal Concentrated fish solute Egg albumin EPA oil Soybean lecithin Soybean oil Yeast powder Mineral mixturea Vitamin mixtureb PapainC Total Cholesterold Egg lecithind
2.50 24.95 15.00 5.00 5.00 15.00 5.00 2.50 5.00 5.00 5.00 5.00 5.00 0.05 100.00 8.00 1.60
Proximate analysis (% of diet ) Moisture Protein Lipid Ash Nitrogen free extract
7.20 42.99 29.51 6.58 13.72
“Mineral mixture contained (% of mineral mixture): NaH,P04-2H20, calcium lactate, 1.964; seaweed powder, 65.746 (as carrier). bAccording to Halver ( 1957 ) . “Nakarai Chemicals Co. Ltd., Japan. dCoating materials.
30.305; ferric citrate, 1.435;
10% lipids, supplemented with raw oysters ( Crassostrea gigas) twice a week. The following diet formulated was used for the spawners: 72% white fish meal, 10% starch, 5% gluten flour, 5% minerals, 2% vitamins, 4% eicosapentaenoic acid oil, 1% choline chloride, 1% cellulose. In general, the fish were fed at a minimum of 3% of their body weight per day. Fertilized eggs were collected from the spawning tank with a fine dip net ( 100 ,um mesh size). Unfertilized eggs which settled to the bottom of the tank were removed by siphoning. The eggs were then transferred into nine 5-t rectangular concrete tanks for incubation and the larval rearing trials. The eggs were incubated in meshed baskets (350 pm) equipped with independent airlift systems which kept the eggs in continuous motion (Tandler and Helps, 1985 ). The use of these baskets made it possible to count accurately the hatched larvae before their introduction into the tanks. The rearing tanks were stocked with approximately 5 x 1O4hatched larvae ( 10 larvae-l- 1) each. The rearing protocol is summarized in Fig. 1. Briefly, rotifers, Brachionus pli-
118
M-Y. Leu /Aquaculture 120 (1994) 115-122
catilis, served as the only food for the larvae for the first 20 days after hatching. The rotifers were added to the tanks at a density of 10 + 5 individuals-ml- l from cultures fed baker’s yeast, S’accharomyces cerevisiae, plus Nannochloropsis sp. at the rate of0.5-’ x lo6 cells~Brachionus-‘*day-l. As well as the rotifers, 50 l-day-’ of Nannochloropsis, at a density of about 30-50 x 1O6cells-ml-‘, were added as nourishment for the rotifers. Begining on day 20, the larvae were also offered newly hatched Artemia salina nauplii; from day 20 to day 30, the density of was maintained at 0.5-l individuals-ml- ‘; after day 30, the density of Artemia nauphi was increased to 1-2 individuals*ml-I. From day 30 onwards, an artificial diet (400-700 ,um particle size) containing 43% protein (Leu and Liou, 1992) was offered to the larvae. The composition of the artificial diet is presented in Table 1. By day 4 1, the larvae were fed the artificial diet only. Incubation and rearing salinity was uniformly 32 ppt, while temperature ranged from 19.2 to 22.3”C. The newly hatched larvae were reared in filtered seawater with very mild aeration ( 5- 10 ml-min- ’ ) . At the stage when the fish were fed Artemia, one third of the rearing water was changed once daily. When the larvae started feeding on the artificial diet, running water at a rate of about 10 1.min- ’ was applied to avoid water quality problems. After 45 days, all surviving juveniles were transferred to a 45-t rectangular concrete tank for further growth.
3. Results The broodstock spawned naturally 3 months after stocking. Spawning commenced on December 24th, 1989 and continued until March 29th, 1990. During this period, the water temperature in the spawning tank fluctuated between 13.8 and 23.5”C. Normally spawning occurred between 22.00 and mid-night. The spawning behavior was as follows: one female was closely followed by three or four males. Just before spawning occurred, the female turned laterally and hit the surface of the water. One male fish chased the female to the surface of the water where they released eggs and milt together. Daily spawning (Fig. 2) varied between 4500 and 77 225 eggs per female (average 26 937 eggs). This amounted to 2586 x lo3 eggs per female over this period. The total number of eggs collected was 10 344 x 103, of which 8658 x 1O3were fertilized (about 83.7%). The total number of newly-hatched larvae was 6277x 103. The hatching rate varied from 32.3% to 99.4% with an average of 72.5%. The fertilized eggs of R. sarba were spherical, non-adhesive and pelagic, and 0.95-l .04 mm in diameter (average 1.03 mm), with a colorless, transparent oil globule 0.22-0.24 mm in diameter. At a temperature of 19.2-22.3”C hatching time was 30-38 h after fertilization. Newly hatched larvae measured 2.25-2.48 mm in total length (average 2.38 mm) with 24 pairs of myotomes. Yolk-sac resorption was completed in 3-4 days. The morphological transition from the larval to the juvenile stage occurred during the 37th to 42nd day, between 13.5 and 17.2 mm in total length. Larvae grew
119
M-Y. Leu /Aquaculture 120 (1994) 115-122
24
1
20 Feb/90
Dew89
20
l0
29
Mar/90
DArE
Fig. 2. Daily changes in hatching rates and number of eggs spawned by Rhabdosargus sarba during the 1989-1990 spawning season. No. of spawners=12 males and 4 females; water temperature= 13.8-235°C.
Table 2 The results of the rearing trials of larval Rhabdosargus sarba for 45 days after hatching. Trial number
No. of newly hatched larvae
No. of surviving juveniles
1
50 000 50 000 50 000 50 000 50 000 50 000 50 000 50 000 50 000
3648 10053 19752 8219 21603 13764 6851 11714 5245
2 3 4 5 6 7 8 9 Mean
Survival (96) 7.3 20.1 39.5 16.4 43.2 27.5 13.7 23.4 10.5 22.4+ 12.4
Total length (mm) 15.2 22.8 17.1 19.3 23.5 21.5 17.4 21.7 16.1 19.4k3.1
relatively slowly throughout the larval stage, but more rapidly after transformation and during the juvenile stage (Fig. 1). During daylight hours the juveniles swam in large schools along the perimeter of the rearing tank in the upper and middle levels; at night, they swam individually. With accompanying morpholog-
120
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/Aquaculture
120 (1994) 115-122
ical changes from larvae to juveniles and with adequate feeding bigger fish seldom attacked smaller ones. In nine trials, the juveniles (n= 100 849) reached a mean total length of 19.42 3.1 mm after 45 days of rearing; the suvival rate ranged from 7.3 to 43.2% with an average of 22.4% (Table 2). These juveniles were transferred to a 45-t rectangular concrete tank, the stocking density was approximately 2200 fish*m-3 with a 2-month survival of 55.8% (56241 fish). Average fork length of 105-dayold tish was 6 1.6 mm, with a range of 48.2-7 I .8 mm. Therefore, total survival obtained through the rearing processes was about 12.5%.
4. Discussion R. sarba is one of the few marine fish that can easily spawn in captivity. This is an advantage for the mass production of R. sarba juveniles as it minimizes the mortality of the spawners through stress due to handling and stripping. During the spawning season, in the present study, female R. sarba spawned every night in the tank, which is in agreement with the observations of Lin et al. ( 1990) who found natural daily spawning by one 4-year-old female in December-January. Matsuura et al. ( 1988) also reported similar results for red sea bream (Pagrus major).
Larval growth in the present study (Fig. 1) corresponds well with the results reported by Tsukashima and Kitajima ( 1982). Rapid growth of juveniles after metamorphosis could well be associated with the changes in morphological characters. Agonistic behavior by R. sarba larvae and juveniles is rare. This might be attributed to the fact that juvenile R. sarba are true omnivores (Nakatsugawa, 198 1; Blaber, 1984). The lack of agonistic behavior would make possible the use of production-scale systems of this species which are based on relatively high stocking densities. In contrast, agonistic behavior including cannibalism was one of the causes of high mortalities during the course of initial rearing of several other sparid species, such as black porgy, Acanthopagrus schlegeli (Tang et al., 1979), silvery black porgy, A. cuvieri (Hussain et al., 198 1)) P. major (Fukuhara, 1985 ) and A. sivicolus (Tawada, 1986). In the present study, the use of rotifers as a first feed, followed by Artemia nauplii, and an artificial diet, has proven to be successful. Nannochloropsis or socalled “green water” is commonly added to larval rearing tanks for stabilizing the water quality, as well as a food source for the rotifers. For experimental scale systems, rearing larvae entirely on live food has advantages. However, if the scale of production of juveniles is to be increased to production-scale, the uncertainties and labor requirements associated with the capture of live food impose severe restrictions. In recent years, artificial diets have been introduced and found to be potential practical replacements for live food in early marine fish larvae. Leu and Liou ( 1992 ) successfully weaned 1O-day-old R. sarba larvae on an artificial diet despite some growth retardation and low survival. According to Tanaka ( 1973 ),
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the presence of gastric glands and pyloric caeca leads to the beginning of the juvenile period. In R. sarba juveniles, pyloric caeca appear around day 40 (Tsukashima and Kitajima, 1982), indicating that the digestive system is already functional at this stage and is able to cope with artificial diets. In the present study, weaning was progressive, from day 30 post-hatching, the Artemia ration was gradually reduced until day 4 1. During this period, the artificial diet ration was gradually increased from 0 to 100% of the ration offered to fish. Based on the present study, we clearly showed that artificial diets could serve as a convenient and cheap food for R. sarba juveniles (bigger than 1.4 cm). At this stage this diet could serve as a complete replacement for natural foods, such as minced trash fish and shrimp, or a supply of zooplankton. In conclusion, R. sarba readily spawns in captivity without the use of hormones or other treatment. The current feeding protocol in the present study makes possible a mass production procedure with survival greater than 22% from larvae to juveniles. With further improvements in methods, rearing systems, and diet formulation, survival should improve. This species has the potential for higher stocking densities than the 10 larvae-l-’ which were used in the present study. The optimal density in the rearing tank has yet to be determined.
Acknowledgements
I am grateful to the staff of Taihsi Branch of Taiwan Fisheries Research Institute (TFRI) who provided helpful advice and assistance with system maintenance. This study has been financed in part by a grant made by the Council of Agriculture, Executive Yuan, Republic of China.
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