(Bloch)
1. Distinctive characteristics of early developmental stages
(a) Eggs and larvae/alevins
Eggs are laid in oblong patches. The egg patches are attached to submerged objects and contain 1300-6000 eggs (an average of 2000 eggs) – representing the entire fecundity of the females. The smaller ones lay smaller egg patches, while the bigger ones with a higher fecundity lay bigger egg patches. The average area of the egg patch is around 200 cm^2.
The diameter of the eggs also shows a wide variation (between 1-2 mm) according to the size and quality of the broodstock. Capitalizing upon this behaviour (of eggs laid in patches) wooden planks and other artificial structures are kept inside the water near to the breeding grounds (especially in breeding ponds) to collect the eggs. Usually below the submerged objects, a cup-shaped nest is made by the females in the muddy bottom for convenience in depositing the eggs and for providing parental care to the eggs and larvae for about a month. The eggs hatch out in 3 to 5 days depending upon the climate/season of spawning.
(b) Early juveniles and juveniles
Etroplus suratensis 1
The early juveniles and juveniles have a black ocellus (lined with a pearly layer) in the soft dorsal fin between the 4th and 10th dorsal ray (adults possess no ocellus in the soft dorsal fin). The caudal fin is marginal.
2. Salient biological characteristics
It attains a maximum size of 450 mm in length in a 5 years period. Maximum weight recorded was 1.4 kg. When natural food is available in plenty, it reaches 210 mm, 300 mm, 360 mm and 410 mm respectively in the first 4 years. In the Southeast Indian mangrove waters of less than 1.5 m in depth, a maximum size of 214 mm T.L. was recorded.
This species attains maturity for the first time during the second year at a size of 150 mm in standard length. However, when subjected to overcrowding in culture systems, it matures even at a size of 105 mm in standard length.
The males are larger than females and hence distinguishing the sex is possible when the fish measure as small as 6-8 cm within a batch. In natural water, it prefers to breed around weed-rich zones having a depth of 0.6-1 m. Parental care is carried out by females. Prior to attaching a patchy egg mass on the submerged objects, the females clean the weeds around the attaching area. The eggs are laid in a single layer over a flat substrate. Fertilization is external (attached unfertilized eggs are fertilized by males). The females guard their young for up to 7 days.
Fry of less than 20 mm were reported to feed on the larvae of aquatic insects. The young fish feed on algal weeds, epiphytic diatoms and cyanophyceae in the mangrove waters. In addition to these food items, the gut contents of this species showed the presence of detritus, silt, benthic copepods, fish scales and nauplii of crustaceans from the mangroves. However, phytoplankton, zooplankton, worms, insect larvae and gastropods were also reported from the gut contents of the juveniles and adult stages of this species from brackish water/estuarine regions.
3. Salient ecological information
A brackish water fish, capable of acclimatizing even to fresh water (upon introduction in a South Indian dam far from the coast, it maintains its population although not flourishing). It is a hardy species and can withstand waters even with 40 ppt salinity. It breeds in brackish water as well in fresh water. Its tolerance to waters with high turbidity and poor dissolved oxygen are also remarkable. Phosphate levels in natural water is reported to influence the growth in this species (probably by governing the biomass of primary producers) which is noted for its short food chain. The young fish of this species (mainly Halophila ovalis and H. beccarii ) occur commonly in algal weed and sea-grass beds along the estuaries and mangroves of South India.
The pattern of hatchling availability of E. suratensis in brackish and fresh water systems in the coastal states of India is very interesting. The hatchlings are available in all months in Gujarat and Karnataka: except for July, August and October in Maharashtra, except December, January and February in Orissa; except June, July, August and September in Goa; except April, August, September and October in Kerala; except September, March, April and May in Andhra Pradesh and except August, September, October, November and February in West Bengal. In various situations, any one or more of the stages between eggs and fully grown fingerlings (even up to 10 cm in length) are considered as hatchlings and hence the variation in the occurrence of hatchlings is also enormous. Moreover, the movement of the hatchlings from brackish waters to the adjacent fresh waters (the lower reaches of freshwater rivers) and vice-versa could lead to a biased sampling/assessment.
Studies based on the recruitment of early juveniles and juveniles in the mangroves of the southeast coast of India (spread over 5 years), revealed the breeding of this fish all year round in all seasons except for February and July, apart from drastic year to year variations. This variation could be due to the complexities of various factors of the following, which include:
a) the influence of the southwest monsoon/northeast monsoon including extremes of failure and cyclonic flood;
b) rainfall in the catchment area and deltaic area and the consequent storage position of the reservoir in the upper reaches of the river system and the contribution of fresh water to the brackish water ecosystems, including the mangroves;
c) the uprooting of algal weeds and ideal structures from the breeding grounds by flash floods, and
d) sand bar formation in the mouth of brackish water systems affecting the tidal ebb and flow and ultimately the ideal conditions conducive for spawning, thus leading to recruitment failure.
A holistic approach is warranted to determine the hatchling availability and breeding seasons of such a complex species.
The pigmented spot present in the soft dorsal fin of early juvenile and juvenile stages alone in the species (the characteristic feature common in certain murrels and many cichlids) seems to aid in the orientation of these early stages during small-scale schooling – a behavioural aspect confined to the early part of the lifecycle.
4. Capture fisheries and utilization
Caught in cast-nets and gill-nets. Undersized destructive fishing by drag-nets in mangrove waters should be banned on the Southeast Indian coast. It is a good flavoured fish with an appreciable food value. It is reported to possess relatively more iron in its flesh and is considered a delicacy in the Southwest Indian region – hence its attractive price. To meet the demand in these regions, this species is cultured in brackish water ponds adjacent to backwaters and mangroves.
5. Culture fisheries
This brackish water species can be cultured in waters with widely varying salinity regimes (in other words, it is suitable for aquaculture in fresh water, brackish water and sea water). It has a shorter food chain, which is an added advantage from a production-cum-energy channeling point of view.
It is considered as one of the important species in brackish water polyculture, the other species being Mugil cephalus , Liza spp. and Chanos chanos . As such, there is no standardized technology for monoculture or semi-intensive culture of this species. It is cultured in a traditional style in the southwest part of India with no remarkable yield. The areas which needs rigorous R&D in the aquaculture of ‘pearlspots’ include the following:
a) hatchery production of hatchlings of this species which exhibit parental care,
b) manuring schedule for this principally herbivorous species,
c) feed formulation with appreciable FCR by incorporating protein-rich leafmeal as a major component,
d) production techniques for all male ‘pearlspots’ (to utilize the trait of males growing faster and bigger than females) and to avoid prolific reproduction, alteration in stocking density, over-crowding, stunted growth and undesirable channelization of energy for multiplication rather than for beneficial somatic growth, and
e) production techniques for sterile pearlspots and sterile interspecific hybrids of Etroplus if found promising in aquaculture.
Production of sterile Etroplus suratensis through the induction of triploidy (chromosome manipulation) may be somewhat difficult, unlike the ‘mouth brooding’ tilapines. This is because the parents are ‘guarders’ and timing the collection of adhered and fertilized egg patches from the environment for heat shock/cold shock/pressure shock/chemical shock may be a problem. Besides, artificial rearing of the eggs by eliminating the caring parent has not yet been demonstrated scientifically in this species (which is a prerequisite to attempting triploidy).
In this context, the production of all male Etroplus suratensis by administering synthetic androgenic hormones (like 17 alpha methyltestosterone, etc.) to early stages through feed or by dip treatment may be useful, but careful monitoring of residual effects/adverse effects in human beings, especially in the long run, is absolutely necessary.
If such sex reversal/sex alteration techniques are contemplated on a large scale, the traditional way of hatchling collection by capitalizing upon the behavioural trait of egg laying, will be drastically affected. Therefore, alternative ways of established and dependable hatchling production techniques (including hypophysation, use of HCG, LHRHA, etc.) should go hand in hand to avoid any mismatch and so as not to endanger the species in the long run.
For producing monosex interspecific hybrids by using E. suratensis as one of the male/female parents, only two species are available in the genus for such an attempt. Of these two species, one is very small (E. maculatus ) and of little use. The other one, (E. canarensis ) although rare, with restricted geographic distribution, could be of much use to working along these lines because of its better growth rate and size than the E. maculatus . However, such attempts, if any, should be restricted to protected environments without contaminating the valuable natural genetic resources.