(Cuvier)
1. Distinctive characteristics of early developmental stages
(a) Eggs and larvae
Not yet recorded from the mangroves.
(b) Early juveniles (description based on around 30 mm T.L. specimens)
Pomadasys kaakan 1
The pores and pit in the chin are yet to become conspicuous at this stage, and the tip of the upper jaw is not completely hidden when the mouth is closed. The hind margin of the suborbital is not exposed. The posterior tip of the opercular edge just above the pectoral origin is highly pointed.
There are 7-11 distinct dark vertical bars covering the upper half of the body without interruption (when the fish attains 100-130 mm in length the uninterrupted bars become interrupted due to the transformation of the vertical pigment in rows which is uniformly distributed. When the specimens attain 14-20 cm in length, the vertical bars, made up of blotches, become faded. As the fish grow old, the vertical bars disappear.
The dorsal fin has 12 spines and 13-15 soft rays. Among the spines in the dorsal fin, the fourth, fifth and sixth spines are longer and stronger than the others (with a gradual reduction in length and strength from the 4th to the 12th spine).
The anal fin has 3 spines and 7-8 soft rays. Among the anal spines, the second spine is longer and stronger than the first and the third one.
2. Distinguishing characteristics of early developmental stages in similar species occurring in the mangroves
(a) Juveniles of Pomadasys commersoni
A dark spot is present at the edge of the operculum. Dark pigmentation lines the membrane between the spines and rays of the dorsal fin base. Over half the area of the dorsal side possesses back spots. Vertical bars are absent. (This species is reported from the Philippines and the African mangroves and not from any other mangrove from its distributional range.)
(b) Early juveniles of Pomadasys maculatus
Irregular blotches are present in the body. The spinous dorsal fin has a distinct blotch. The opercular edge at the hind-most part above the pectoral origin is not pointed (highly pointed in P. kaakan ). This species is reported from the mangroves of the Philippines, Singapore and Thailand. Although it occurs in the inshore waters of the Southeast Indian coast, it does not seem to enter the adjacent mangrove waters.
3. Distinguishing characteristics of early developmental stages in similar species occurring in the mangroves
(a) Gerreidae
Certain species of Gerreidae (Gerres abbreviates, G. filamentosus and G. oyena ) not only possess vertical bands (similar to P. kaakan ) at early developmental stages, but also have a protractile mouth (a protractile mouth is absent in the species of the Haemulidae family).
4. Distinguishing characteristics of juveniles of similar species occurring in the adjacent coastal/coral environment
(a) Pomadasys argyreus
There are no vertical bars on the body. The opercular edge has a black blotch but there are no blotches on the body and dorsal fin membranes.
(b) Pomadasys furcatus
Vertical bars are absent on the body. On the other hand, six horizontal bars are present.
(c) Pomadasys olivaceum
Vertical bars are absent on the body. The opercular edge has a black blotch lined anteriorly with yellow pigmentation. The second and third anal spines are almost equal in length and strength, but are much longer and stronger than the first spine. The anal fin has 11 or 12 soft rays (only 7 or 8 soft rays in P. kaakan ).
5. Salient biological characteristics (after Jeyaseelan, 1981; Fischer and Whitehead, 1974; Deshmukh, 1973; Nammalwar, 1974; Ramalingam, 1988; Krishnamurthy and Jeyaseelan, 1981)
Pomadasys kaakan grows to a maximum size of 800 mm in length. This species attains 220-275 mm, 340-400 mm, 460-525 mm, 520-600 mm and 560-645 mm at the end of the 1st, 2nd, 3rd, 4th, and 5th years respectively in different locations with varying levels of fertility conditions.
In male and female specimens, six stages of maturity conditions are recognized. Males mature for the first time at a size of 392 mm (50% occurrence in mature fish) while the corresponding figure for the females is interestingly low (358 mm). The females mature before the males and the sex ratio (male:female) was reported to be 1 : 1.32 from the Southeast Indian coast. All the males and females are mature beyond 666 mm in length (100% occurrence in mature fish) (Ramalingam, 1988).
Ova in the ripe gonads show a maximum size of 1.07 mm in diameter. Fecundity studies (with limited specimens) recorded over 15,000 ova per individual female. Females with a ripe gonad occur in the commercial catch all year round with short intervals (e.g. females with a ripe gonad were recorded during February, May, August and November from the Southeast coast of India). Spawning peaks were reported during May and November from the Southwestern part of the Bay of Bengal. It is principally a benthic carnivore and the adults of this species feed on penaeid shrimps, hermit crabs (mostly Avarus ), crabs, fish and stomatopods in the marine environment (Ramalingam, 1988). The juveniles feed on sergestids, amphipods, small crabs, hermit crabs, amphipods and small fish in the estuarine environment. In the Southeast Indian mangrove waters, the juveniles of this species feed on tanaids, gammarids, small penaeid shrimps, hermit crabs, polychaetes, mysids, sergestid shrimps, and pistol crabs in descending order of importance.
6. Salient ecological information
The early juveniles and juveniles enter the estuaries and mangroves from the sea and spend part of their life cycle in the brackish water. Specimens measuring as small as 15 mm enter the mangroves and after completing the nursery phase they leave the mangroves at a size of 120-190 mm in total length, depending upon the depth of the ecosystem (if the depth of the waterway is less, the departure of this species to the marine habitat is early and migration is delayed if the depth of the channel is relatively greater).
The spatial distribution of this species (through the movement of the upper-sized group within the nursery phase from the mangroves to the adjacent inshore waters and relatively deeper brackish water biotopes) is strongly influenced by the parameters which alter the depth, including the lunar phase and seasonal fluctuations in tidal amplitude. The juveniles occur in mangrove waters with salinity as low as 5 ppt and turbidity as high as 400 g/m^3 seston.
Peniculus scombri Gnanamuthu, a copepod parasite, was found to infest the juveniles of Pomadasys kaakan , the latter being a new host from the little known mangrove environment (Jeyaseelan, 1981). This species makes a sound when taken from the water and hence is known as ‘grunter’ or ‘trumpeter’. It is widely distributed thorughout the Indo-Pacific, from the Red Sea to South Africa being the western limit, and from Taiwan to the northern part of Australia being the eastern limit.
7. Capture fisheries
The early juveniles and juveniles are victims of destructive fishing in the brackish water ecosystems including the estuaries, back waters, brackish water lakes and lagoons and the mangrove waters (which serve as nursery grounds) by indiscriminate undersized fishing (specimens as small as 20 mm in length) through cast-nets, stake-nets and drag-nets in the Southeast Indian region (Jeyaseelan, 1981; Rollet, 1981). The loss thus caused to the coastal fishery through recruitment failure of this carnivorous fish species is alarming because of its potential to grow to as large as 800 mm in length in the inshore waters (Jeyaseelan and Krishnamurthy, 1980; Krishnamurthy et al., 1981; Jeyaseelan et al., 1991).
The adults are usually caught by lines using catamarans in the Coromandel coast. This species is also caught by bottom trawls, bottom long lines and gill-nets (Fischer and Bianchi, 1984).
8. Culture fisheries
Within a decade, this species may attract the aquaculturist in the Southeast Asian countries because of the following facts:
1. The ready availability of the fish hatchlings of this species in many unpolluted brackish water systems of the Indo-Pacific region;
2. the wide range of salinity and turbidity tolerance of young fish, including the hardiness of the hatchlings for handling;
3. early identification of hatchlings from natural waters (besides the scope for successful hatchery production through modern bio-technological means);
4. faster growth rate and attainment of a marketable size in a reasonably short span of time;
5. excellent taste (readily preferred by the South Indian fish eating community) and a ready market;
6. suitability of this species for high density stocking in cages both in brackish waters and coastal/open waters;
7. ready applicability of intensive and semi-intensive culture techniques developed through rigorous R&D for seabass and other selected carnivorous fish species (including semi-moist feed, cage culture techniques etc.);
8. aquaculture as a viable alternative to replace the destructive fishery and as a rehabilitation programme for the affected fishermen (in the case where conservation efforts are made); and
9. the rapid spoilage of land-locked brackish water nursery grounds in a diverse manner under the umbrella of development and the consequent likelihood of degradation of the coastal fishery and the ultimate resultant thrust to sustain fish production through culture under the pretext of conservation of valuable resources.
However, when attempting (through pilot projects) to promote this fish as a candidate species for aquaculture, certain constraints of the following kind could be anticipated in the near future:
1. The exorbitant cost of production of this carnivorous fish based on principles of the food chain;
2. the poor purchasing power of the local people;
3. levels of acceptability of the fish on the international market;
4. the uncertainly over the availability of trash fish and fishmeal because of population explosions and the demand for animal protein and severe competition for such ingredients to meet human requirements, and
5. the critical evaluation of the meaning behind the real production of carnivorous fish by taking into account the loss incurred at each trophic level and the input-output ratio of energy, nutritional value, protein etc.