(Bloch)
1. Distinctive characteristics of early developmental stages
(a) Eggs (not yet reported from the shallow mangrove waters) (after Dunstan, 1959; Russell, 1986; Garrett, 1987; Maneewongsa and Watanabe, 1984)
Lates calcarifer is a broadcast spawner of pelagic eggs. The fertilized eggs float, whereas the unfertilized eggs sink. Fertilized eggs are spherical, measuring 0.68-0.89 mm in diameter. A large oil globule measuring about 0.2 mm in diameter is present. At 26-28°C water temperature, the eggs hatch within 17-18 hours, whereas at 29-31°C, the incubation period is restricted to 12-15 hours
(b) Larvae (after Bensam, 1983; Kungvankij, 1987a; Ghosh, 1973; Mukhopadhyay and Verghese, 1979; Moore, 1980; Moore, 1982; Russell, 1987)
Lates calcarifer 1 (after Bensam, 1983)
The pro-larvae measure about 1.5 mm in length immediately after hatching. In 4 mm larvae, two large branching chromatophores are present behind the eye and 3 large chromatophores in the middle region of the lateral sides. In 5 mm larvae, the caudal region is oval in shape and the caudal rays have three segments. The upper jaw possesses minute teeth. Angular spines are short and are two in number in the preoperculum. Eight pre-anal myomeres and 12 post-anal myomeres are countable.
Chromatophores are in a dense patch on the dorsal aspect of the gut and a large chromatophore is present in the interior part of the dorsal fin. In general, the larvae measuring 5 to 10 mm are dark brown with scattered yellow spots. In post-larva measuring 7.5 mm length, the median finfold connecting the dorsal, caudal and anal fins, disappear, while the spines and rays are discernible. The dorsal fin has 8 spines and 11 rays, the anal fin has 3 spines and 7 rays and the caudal fin has 23 rays. Chromatophores develop in dorsal and ventral profile.
The post-larva, measuring 8.5 mm in length, has 3 transverse bands and scattered black blotches on the head. The caudal peduncle has a dorsal and central ocelli. Larvae of 10 mm in size have 14 rays in the pectoral fin (instead of 17 in adults). The preoperculum margin is serrated with 4 angular spines. Vertical bands of chromatophores are seen. From 12 mm onwards the young ones develop scales. The dorsal profile is straight instead of showing a deep depression as in adults. At 12.5 mm length, the larvae possess 5 angular spines in the serrated margin of the preoperculum. Three distinct bands develop at this stage.
(c) Early juveniles and Juveniles
Lates calcarifer 2
Early juveniles are darkly pigmented. Specimens measuring 20 mm in length and above (even up to 200 mm in certain cases) possess a brown band or stripe running between the snout and the dorsal fin. The lateral sides of early juveniles and juveniles are yellowish brown in colour with 3 to 5 dark grey vertical bands of chromatophores. A lateral line is traceable although not as far as the caudal fin tip. Scales start showing cutenoid features. The larval preoperculum spines are reduced as a serration and only one strong spine exists. The opercular spine is yet to develop in early juveniles of less than 30 mm in length.
2. Distinguishing characteristics of early developmental stages in similar species occurring in the mangrove and adjacent marine environment .
(a) Early juveniles and juveniles of Psammoperca waigensis
The lower edge of the preoperculum is not serrated, as is the case with L. calcarifer .
(b) Juveniles of Lates japonicus
Restricted to Japanese waters (Katayama and Taki, 1984).
3. Salient biological characteristics
(a) Maximum size
In natural waters, it grows to a maximum length of 2 metres.
(b) Growth
In natural waters, growth varies according to the productivity of the water. In general, it grows to about 0.5 kg in one year, 2.5 kg in 3 years, 4 kg in 4 years, 7 kg in 6 years and 12 kg in 8 years. In the Indian mangroves and brackish waters, it attains 30 cm, 50 cm, 70 cm and 80 cm at the end of the 1st, 2nd, 3rd and 4th years respectively. The southwest monsoon recruits grow faster than the post northeast monsoon recruits on the East coast of India.
In a culture system, It reaches 0.3 kg in 4 months and 0.5 kg in 5 to 6 months. Depending upon surplus food availability and lesser stocking density in certain culture systems, it can reach up to 3 kg in 1 year and 5 kg in 2 years.
(c) Reproduction and life history in natural waters (after Dunstan, 1959; Moore, 1982; Davis, 1982; Russell and Garrett, 1983; Russell and Garrett, 1985; Tattanon and Maneewongsa, 1982; Anon, 1984)
During the spawning season, tidal-based monthly cycles of spawning (almost a quarter moon spawning period) was reported in this species. Spawning takes place either in the lower reaches of estuaries and mangroves near the river mouth (if the depth is sufficient) or in the coastal waters adjacent to the river mouth, where the salinity ranges between 28 ppt and 36 ppt.
In Thailand, it is reported to breed in low saline waters with a muddy bottom and dense mangrove vegetation. The eggs and pro-larvae which are highly sensitive to fluctuations in salinity and temperature often remain around the breeding ground in accordance with a feeble tidal effect. The post-larvae enter predator-free and prey-rich supra-littoral wet-land swamps or mangroves and tidal pools during the spring tides, where they attain faster growth and better survival.
The young fish move upstream to the river and remain for 3-4 years until they grow to a size of 60-70 cm T.L. and 2.6-4.2 kg in weight. During this stage, they mature as males and at the end of the next dry season, they move down stream and spawn. A few studies from some locations reveal that the gonads of this species do not develop in adult fish dwelling in fresh water and land locked lagoons. Each male participates several times in spawning. Then at the age of 6-8 years, when the fish attain 7-12 kg in weight and 85 cm-100 cm in total length, sex reversal takes place.
The males become females and hence are called protandrous hermaphrodites. Synchronous hermaphrodites are also encountered, but rarely. Primary females (the immature fish at times develop into females without becoming males) also occur, scarcely, with 2 year old specimens measuring as little as 420 mm in total length. The fecundity ranges from 2.7 million eggs to 45.7 million eggs per fish. The smaller females are reported to be total spawners while the larger females are multiple spawners. Certain studies reveal that though the individual smaller females do not have an extensive spawning period, the continuous migration of mature fish (differential arrival) into the spawning ground leads to an extended breeding season.
(d) Spawning season and hatchling availability in natural waters
It spawns during April to September in the mangroves of Thailand, Sunderban and the Southeast Indian coast. In Chilka Lake, it spawns from May to September and then from January to March. In northern parts of Australia, it spawns between August and February while in Queensland it is reported to spawn from October to March.
Subsequent to spawning, the ‘seeds’ start to occur in the brackish water impoundments and depressions inundated by high tides, including the mangrove waters. However, the hatchling availability from natural ground is erratic and inconsistent, fluctuating widely, from year to year, depending on the environmental variables in the dynamic system and the varying levels of success of recruits.
(e) Food and feeding in natural waters
This species is a predacious column feeder and is cannibalistic when food is scarce. The juveniles of this species feed on penaeid shrimps, palaemonids, fish and sergestids in descending order of importance in the Southeast Indian mangroves. The adults of this species are voracious carnivores feeding on mugilids, clupeoids, crustaceans and molluscs in coastal waters.
4. Salient ecological information
(a) Habitat
In the mangrove waters of Thailand, the fry of this species occur in abundance in a salinity range of 1 to 20 ppt. The juveniles dwell in coastal lagoons, mangroves, brackish waters and move up to a mixo-oligohaline salinity condition regime. In general, young fish find shelter in the littoral waters. In certain regions, the fry which ascend upstream remain in fresh water and descend down to brackish and coastal waters during the breeding season. Adults occur in the deeper estuaries and coastal waters of up to 40 m depth. In certain locations, this species is reported to prefer slow moving clear waters of 6 m depth.
(b) Geographic distribution
This species is widely distributed in the coastal, brackish and fresh waters of the Eastern Indian Ocean and Western Central Pacific. In the Western Indian Ocean, it is reported from the Arabian seas, the west coast of India and Sri Lanka. Nowadays, it has been introduced in a few tropical countries for aquaculture and sport fishing.
(c) Behaviour
Carnivorous and at times cannibalistic in its feeding behaviour, it feeds on nekton, usually in the water column. It is reported to be caught in bottom trawls and bottom gill-nets besides traps and hand lines. The are large numbers of this species which congregate off the river mouths in certain locations during the winter.
5. Evolutionary significance
The family to which this species belongs has been denoted as Latidae by certain taxonomists and as Centropomidae by others. Recently the species of the Ambassis genus belonging to the same Centropomidae family were separated and kept under a separate family called ‘Ambassidae’ – thus removing the long existing confusion from the ‘eco-evolution’ point of view. The members of the Centropomidae family derived out of limited speciation and are quite large in size, have better longevity and a wider geographic distribution. On the other hand the members of the family Ambassidae are noted for rich speciation (with numerous species in a particular location), relatively less longevity and restricted geographic distribution.
The species Lates calcarifer is widely distributed in the tropical and sub-tropical Indo-Pacific region and seems to have a diverse population and race which are well isolated due to their peculiar distinct and differential reproductive characteristics. The members of the species which are distributed in the temperate region of the Japanese coast are further subjected to reproductive isolation and give rise to another new species of the Lates called L. japonicus . In African waters, the genus Lates is represented by L. niloticus .
6. Capture fisheries
In Southeast Asian countries, the bulk of the catch is obtained in winter, especially from the river mouths. In the brackishwater systems of India it forms a fishery. The adults of this species are attracted by the shade, and in Southeast Indian mangroves and estuaries, a small catamaran and a floating object (to provide shade) are used to lure and capture this fish by spear fishing. In coastal waters up to a depth of 40 metres this species is reported to be caught by bottom trawls and bottom gill-nets. Hand lines and traps are also used to capture this fish.
Besides commercial capture fisheries, this species offers scope for recreational fisheries, as practiced in Australia and Papua-New Guinea. It is a potential sport fish not only for warm, fresh, brackish and coastal waters, but also for cold waters. This species thus provides scope for recreational fisheries in picnic spots and resorts to attract foreign and domestic tourists.
Heavy fishing pressures could reduce the age at which sex reversal takes place (from males to females) in this species. The introduction of this species to newer areas for sport fishing etc., should be discouraged. (When Lates niloticus was introduced in a Ugandan lake it resulted in the degradation of a highly productive multi-species fishery to that constituted by selected species (3 spp.) with a poor catch in the long run) (FAO., 1990).
7. Aquaculture
(a) General information
It is a candidate species for marine, brackish water and even fresh water carnivorous fish culture because of its adaptability to a wide fluctuation in salinity. Thailand is the leading country in the culture of this species. It is a powerful predator causing a menace to ill-managed coastal shrimp ponds. The hardy grown up hatchlings of seabass could be cultured as a rotation crop in shrimp farms during the off season, with extremes in environmental variables. It grows to a large size and has a faster growth rate in the early part of its life cycle prior to maturity. It is also used in the culture systems to control weed fish. In the Konkan region of India it was introduced in rice-fish integrated systems. It is also reported to be an useful species in pen culture.
(b) Hatchling Production (after Kungvankij, 1987b; Moore, 1982; Whitehead, 1972; Tattanon and Maneewongsa, 1982; Maneewongsa et al., 1984; Maneewong, 1987; Kungvankij and Suthemechaikul, 1986)
1) Stripping method
Females with a ripe gonad were stripped and the ova were fertilized by the milt taken from the testes of males. The fertilized eggs hatched out in 12 hours and the larvae were reared.
2) Hormonal inducement
The broodstock of Lates calcarifer were induced by IVG administration 2 to 3 days prior to the full moon. In certain cases, LHRH ethylamide and mammalian LHRH were used to accelerate ovulation and the maturation of the ovary in seabass. Induced spawning was also effected by inserting LHRH analogue incorporated cholesterol pellets into the perivisceral cavity of mature males and females.
3) Spawning in captivity through environmental manipulation
Parental stock, obtained from nature as well the cultured stock, were successfully made to spawn through environmental manipulation. In a 100t tank with aeration facilities, 12 pairs of broodstock with ripe gonads were released and a 50-60% water exchange was done daily. The feeding rate of the broodstock was reduced from 5% to 1% body weight per day immediately after transferring to a breeding tank. The salinity of the water was raised gradually from brackish to near seawater range (30 to 32 ppt).
On full moon or new moon days, the water depth in the spawning tank was reduced to 30 cm at noon and the tank with brood stock was exposed to sunlight for 2 to 3 hours so that the water temperature could rise to 31°-32°C. Filtered sea water was flushed to increase the water level and to bring the water temperature down suddenly from 31°-32°C to 27°-28°C so as to simulate natural conditions similar to those existing in the mouth of brackish water systems during full moon/new moon high tides. During late evening hours (6-10 pm) the fish spawn. If no spawning occurred, the treatment was repeated up to 3 days from the full moon/new moon .
4) Larval rearing
In larval rearing tanks, the stocking density of pro-larvae was maintained at 100,000 /m^3. For the first 3 days the yolk-bearing larvae do not require any food. The system was well aerated and care was taken to maintain good water quality. Chlorella was added on the first day itself – not as live food – but to maintain the water quality. Yolk absorbed larvae were fed with rotifers (preferably Brachionus plicatilis ) at a density of 5,000-10,000 rotifers/litre for a week. Then the larval density was reduced from 100,000 to 40,000/m^3 and nauplii of Artemia were provided for 10 days.
For another 10 to 20 days the baby fish were fed with grown up Artemia . One week after feeding with Artemia , the larvae were graded through plastic trays (with holes of assorted size) periodically to avoid cannibalism. The hatching rate varied between 40% and 85%. The survival rate between the pro-larvae and post-larvae was 35%; from post-larvae to 1 cm fry it was 60%, and from fry to 2.5 cm fingerlings it was 45%. The overall mortality rate from pro-larvae to 2.5 cm fingerlings was 80-85% based on the state-of-the-art technology available at present. Further improvements are needed through R and D to increase the survival rate at the larval stage of this species.
(c) Culture techniques in ponds
1) Monoculture
This species is cultured extensively in many asian countries. The recommended depth of the pond is usually 2 m and since this species is cannibalistic, stingent size grading is needed in monoculture. Monoculture of 50 to 100 g size seabass resulted in 2.5 t per hectare fish production in six months duration under semi-intensive culture techniques by providing compounded feed, whereas trash fish alone as food resulted.
2) Polyculture
This species is generally not fit for polyculture due to piscivorous feeding habits. However, prey fish having a prolific breeding potential (such as Tilapia ) are cultured along with Lates calcarifer . In certain culture experiments, Megalops cyprinoides (another carnivorous fish) was used along with seabass as a polyculture species.
Appropriate stocking density, species composition, and the stages of Tilapia and seabass, result in a better provision of live food for seabass and checks the high density of Tilapia obtainable out of prolific breeding, allowing the leftover surviving Tilapia to grow to a more marketable size. In seabass/Tilapia polyculture, besides ideal management, the three following factors need to be considered for better production and profitability. They include: (a) a proper input of Tilapia hatchlings/broodstock for early and sustained reproduction, (b) a balanced combination of Tilapia and seabass, and (c) the appropriate hatchling size of the seabass and broodstock/young Tilapia with a proper understanding of the predator-prey relationship.
When Tilapia broodstock are introduced along with grown seabass fingerlings into pond systems, the sex ratio of the Tilapia is maintained at 1 : 1 and the total Tilapia/seabass ratio is maintained at 1 : 1.5 - 1 : 3. The stocking density of both Tilapia and seabass is maintained between 5000 and 8000 per hectare.
When young Tilapia are introduced along with hatchlings of Lates calcarifer , the male/ female sex ratio of the Tilapia is maintained at 1 : 3, the seabass/ Tilapia ratio at 1 : 15 and the total stocking density at 10,700 per hectare. The aggregated culture production from these two species is less than 1 tonnes/ha/year, however, with least expenditure on food for the carnivorous seabass. Under polyculture conditions, the seabass was cultured along with the Penaeus monodon with a 3-5 juveniles/m^2 and 4-30 post-larvae/m^2 stocking density respectively by providing adequate feed to avoid predation, and the results showed varying levels of success or failure in certain Southeast Asian countries.
(d) Cage Culture
The Lates calcarifer is one of the most ideal species for cage culture in the Indo-Pacific region. In Thailand, Malaysia, Singapore and Indonesia, it is cultured in floating cages made of nylon net with a 6-50 mm mesh size and with varying levels of density (10-500/m3 or 40/1000/m^2). The height and depth of the cages varies from 2-6 m, the maximum for rotating cages where there is severe fouling. The hatchling size also varies widely from 25 g to 200 g per individual fish in accordance with the rearing and growout stage. Under normal conditions and a moderate stocking density of 10-15/m^2 and the ideal hatchling size of 100-200 g per individual fish, the extrapolated net production is attained of 20-30 tonnes/ha/year. However, under excellent management and ideal stocking densities through intensification (120/m^2 or 60/m^3) a maximum of 150 kg/m^2 or 75 kg/m^3 seabass production is possible per year through cage culture (from two 6 monthly crops each using normal and stunted hatchlings respectively for 1st and 2nd crops) from appropriate unpolluted sites with conducive environmental conditions and optimum tidal flushing.
(e) Feeds used for culture
In tidal-fed culture ponds, trash fish are provided as food for this species during high tides. Cage culture of this species in Thailand by providing trash fish alone showed the FCR value of 1 : 7 to 1 : 10. Pelleted feed containing trash fish and rice bran are also provided to this species in growout culture systems. In semi-intensive culture systems, it is fed with slaughterhouse waste, poultry waste, squid waste, shrimp waste and trash fish at the rate of 5-10% body weight per day on an as-fed basis. In poorly maintained culture systems, the hatchlings of seabass make their entry and cause a menace by predating on cultured shrimps.
Nowadays, semi-moist feed is provided, especially for carnivorous fish species (including seabass). Nutritionally balanced and complete feed with not less than 55% protein, 12% lipids and 7.5% ash content is used for intensive culture of seabass, especially in cages. During the nursery phase, relatively more protein and lipids are provided through diet than for the growout stage.
If the produce appreciates and assures a remunerative price in local or international markets, the following feed formulation could bring better FCR under ideal management and stringent cost-effective strategies. The ingredients and their proportions (which may readily be altered or adjusted according to performance, local availability, quality of ingredients, cost effectiveness of ingredients and cumulative nutritional status including EAA and EFA profile) include: 25% fish meal, 10% meat and bonemeal (solvent extracted), 10% poultry by-product meal, 10% soya flour (antinutritional factors removed through appropriate processing), 10% defatted rice bran or wheat bran, 8% wheatflour or cornflour, 5% ground nut cake, 5% tapioca, 5% flourcod liver oil (or squid, sardine or any fish oil in order of preference and to match the product cost), 3% safflower or sunflower oil (or soya, crude oil, corn oil or rice bran oil), 2.98% vitamins and mineral mixture, 2% yeast (spent grain), 2% dicalcium phosphate, 2% leaf meal and 0.02% BHT (or any cost effective antioxidant).
8. Marketing and utilization
Lates calcarifer is a highly valued and delicately flavoured fish which grows to a relatively large size in a short span of time.