Induced Spawning and Larval Rearing of Grouper (Epinephelus Salmoides Maxwell)

SEAFDEC   AQD
PHILIPPINES

NETWORK OF AQUACULTURE IN ASIA
BANGKOK, THAILAND

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INDUCED SPAWNING AND LARVAL REARING OF GROUPER
(EPINEPHELUS SALMOIDES MAXWELL) *

P. KUNGVANKIJ **

Network of Aquaculture Centres in Asia
Regional Lead Centre in The Philippines
Iloilo City, Philippines

L. B. TIRO, B.P. PUDADERA AND I.O. POTESTAS

SEAFDEC AQD
Iloilo, Philippines

Abstract

Broodfish of grouper (Epinephelus salmoides) were induced to spawn by hormonal induction. The hormones used for this experiment were HCG + pituitary gland (PG) and LRH-a. The results showed that at the dosages of 500 IU HCG + 3 mg of PG per kg of fish for first injection and 1,000 IU HCG + 3 mg of PG per kg of fish at the final injection at an interval of 24 hours, the treated fish spawned naturally in a spawning tank 12 hours after the final injection. At lower dosages of 500 IU HCG + 3 mg PG at 12-hour intervals or 500 IU HCG + 3 mg PG at 24-hour intervals or using 10 mg LRH-a at 12-hour intervals, the eggs can be artificially fertilized only by stripping.

* This paper was presented at the First Asian Fisheries Forum 24–30 May, 1986 Manila, Philippines.

** Present Address : FAO/UNDP Seafarming Development Project (INS/81/008), FAO, Jakarta, Indonesia.

The larval rearing experiment ws conducted in a 250-liter fiberglass tank at the stocking density of 2,500 larvae per tank. Feeds used in this experiment were : (a) Isochrysis + sea urchin eggs; (b) Isochrysis + Brachionus; and (c) Tetraselmis + Brachionus. The experiment results showed that newly hatched larvae fed with Isochrysis mixed with sea urchin eggs and then switched to Brachionus on day 10 gave the best survival rate (9% at day 30).

Introduction

The grouper (Epinephelus salmoides) is a popular marine food fish of high market value in Southeast Asia. It has been farmed commercially in marine cages and ponds in Thailand, Malaysia, Singapore, Hongkong and China. The fish can grow fast and attain market size of 600–800 g in about 6–8 months (Chua and Teng 1980; Chen 1979). The fry used for culture are usually obtained from the wild which means the supply is very erratic and inconsistent. Large-scale commercial production of grouper in marine cages or ponds is, therefore, hampered by shortages of fry. To ensure consistent and adequate supply of fry, efforts have been made to produce them under controlled conditions.

The grouper undergoes sex reversal starting out as a female when young and then becoming male when older and larger. Sex determination of 68 specimens of Epinephelus tauvina collected from the South China Sea showed the biological minimal size of females to be 450–500 mm, while male fish with ripe testes were above 700 mm in standard body length and more than 11 kg in body weight (Tan and Tan 1974). Red grouper (E. morio) become more significantly male at the weight of 9 kg and above (Moe 1969). In fact, male grouper are scarce and found only in deep seas. Breeding of grouper, therefore, depends on the availability of males. Females Epinephelus tauvina could be sexually transformed with methyl testosterone treatment. Females could also be induced to ovulate by application of human chorionic gonadotropin (HCG) and salmon pituitary extract (Chen 1979). However, the survival rate of fry was very low.

The purpose of this paper is to report the results of the experiments on induced spawning of grouper using different types of hormones and larval rearing systems.

Materials and Methods

The grouper (E. salmoides) used were caught in bamboo traps at depths of 15–10 m and were transported to the experimental station by boat. The size of fish ranged from 3 to 15 kg. They were conditioned in 60-t rectangular concrete tanks for 6–12 months at stocking density of ½ m³, fed daily with fresh trashfish at the rate of 5% body weight; 50–70% of the water in the tank was changed daily.

Egg maturation was determined through egg diameter measurement. The fish were first anaesthetized. The eggs were sampled a using a 2-mm plastic tube inserted through the genital opening. The maturation of the males was determined by pressing the abdomen for milt. Only females with mean egg diameter of 400 υg and males with running milt were selected for induced spawning by hormone injections. The induced fish were placed in separate cages installed in another holding tank.

The different types of hormone used, dosages and time intervals between the first and second injection are showed in Table 1. Twelve females with weights ranging from 3.6 to 6.5 kg and four males ranging from 10 to 16 kg were selected for this study. The female and male fish were injected intramuscularly with the assigned hormone and dosage as shown in Table 1. In cases where ovulation did not occur after the second injection, a third injection was given.

The fertilized eggs from natural spawning in the spawning tanks were collected with a fine dip net (100 micron mesh size). Other plank tonic organisms and detritus included in the collection were removed by screening. Unfertilized eggs which settled down to the bottom of the tanks were removed by siphoning. The eggs were placed in hatching containers and hatched out in about 15–20 hours.

The larvae from the hatching containers were collected and stocked in nine 250–1 fiberglass larval rearing tanks at a stocking rate of 2,500 larvae per tank. Three types of feed (Fig. 1) were tested in three replicates on a completely randomized design.

The larvae were reared indoor under intensive care conditions. Beginning on 1 day 3 the feed was introduced. During the larval rearing trial, 20– 30 % of the water was changed daily.

Sea urchins were collected from the sea and kept alive in tanks with flow-through seawater. The gonads were collected 1 hour before feeding time and immersed in filtered seawater. The eggs were removed by opening the gonad and collected by means of a screen net. Only eggs were used for feeding and they were given to the larvae twice daily at 8 a.m. after water management and at 4 p.m.

Results

The fish under treatment D (Table 1) spawned naturally in the spawning tank 12 hours after the final injection (Table 2). Fish under treatment B (Table 1) ovulated 12–15 hours after the final injection, but fertilization was completed only by artificial stripping. The fish in treatments A and C required a third injection for ovulation. The eggs were artificially fertilized 9–15 hours after the final injection.

Ovulation rate of the females treated with hormonal injections at 24-hour intervals was higher than those at 12-hour intervals (Table 2). This may be due to the handling which caused the fish stress.

The results of the larval rearing experiment showed that sea urchin eggs are a suitable feed for grouper larvae. The diameter of the sea urchin eggs was about 50 μg. The survival rate of fry fed with sea urchin eggs and Isochrysis from hatching to 20-days old was 9% and 2% for those fed with Isochrysis and Brachionus. None of the fry fed with Tetraselmis and Brachionus survived; all the larvae died after a culture period of six days (Table 3).

Discussion

Two interesting observations were noted during this study. Firstly, the grouper, E. salmoides can be spawned naturally in captivity after induced spawning by hormone manipulation. This is very advantageous for mass production of grouper fry as it minimizes the mortality of the spawners through stress due to handling and stripping. Secondly, sea urchin eggs have been shown to be a suitable food for the grouper larvae.

The most suitable time interval between the first and second injection is 24 hours. The shorter period might have caused the fish more stress damage. The dosages of hormones used 500 IU HCG + 3 mg PG/kg of fish, and double the quantities for the second injection, showed the best result in the experiment. However, the appearance of natural spawning in captivity after hormone manipulation showed the possibility of induced spawning by environmental manipulation which occurs in seabass Lates calcarifer. The key to successful spawning by environmental manipulation lies in the condition of the broodstock. Broodstock development should be the prime work for further studies.

The fish larvae which were fed with Tetraselmis and Brachionus died six days after stocking, despite the fact that Brachionus is one of the most valuable larval feeds. This might be attributed to the size of the Brachionus and activity of the Tetraselmis. It should be noted that there are two types of Brachionus, L type (100–150 μg) and S type (40–100 μg). Unfortunately, the predominant type in the hatchery was L type, and since it is larger than the larvae's mouth this may be the cause of their mortality. Studies on feeding grouper larvae with S type Brachionus should be done in the future.

References

Chen, F.Y. 1979. Progress and problems of netcage culture of grouper (Epinephelus tauvina F.) in Singapore. Proc. Annu. Meet. World Maricult. Soc. 10:260–271.

Chua, T.E. and S.K. Teng. 1980. Economic production of estuary grouper, Epinephelus salmoides Maxwell, reared in floating net cages. Aquaculture 20:187– 228.

Moe, M.A., jr. 1969. Biology of the red grouper, Epinephelus movio (Valenciennes) from the eastern Gulf of Mexico. Fla. Dep. Nat. Resour. Mar. Res. Lab. Prof. Pap. Ser. 10:1–95.

Simpson, K.L. G. Klein-Mac-Phee and A.D. Beck. 1982. Zooplankton as a food source, p.180–201. In G.D. Pruder, C.J. Langdon and D.E. Conklin (eds.) Proceedings of the second international conference on aquaculture nutrition: biochemical and physiological approaches to shellfish nutrition. Special Publication No.2. Louisiana State University, Division of Continuing Education, Baton Rouge, Louisiana.

Tan, S.M. and K.S. Tan. 1974. Biology of tropical grouper, Epinephelus tauvina (Forskal): a preliminary study on hermaphroditism in E. tauvina. Singapore J. Primary Ind. 2(2):123–133.

Table 1. Type of hormones, dosages and time intervals used for induced spawning of E. salmoides.

Table 2. Induced spawning of Epinephelus salmoides using different hormones.

* Insufficient mature male.
** Used running male only with one injection given during the last injection for the female.

Table 3. Survival of Epinephelus salmoides larvae at day 20. Tanks started with 2,500 fish and fed according to treatments in Fig. 1.

Fig 1 Feeding Scheme for larval rearing of Epinephelus Salmoides