(Forsskål, 1775)
1. Distinctive characteristics of early developmental stages
(a) Eggs and pro-larvae
Not yet recorded from the mangrove waters. However, fully or partially transparent eggs of 0.8 mm diameter were reported from certain estuaries of the West coast of India.
(b) Post-larvae
Sillago sihama 1
In post-larvae measuring 6.5 mm length, the spinous first dorsal fin has not yet formed and a thick fin fold is visible between the posterior part of the head and the anterior part of the second dorsal fin. A total of 20 and 21 soft rays develop in the second dorsal and anal fins respectively. The caudal fin has around 18 countable rays which are 3 to 4 segmented. As the 6 mm larva grows to 20 mm, the pre-anal myomere count increases from 10 to 14, while, the post-anal myomere count decreases from 24 to 20 due to the gradual shifting of the position of the anus.
Pigmented spots (each one below each ray) line the base of the anal fin at the larval stages, as well in certain young stages. The post-larvae measuring between 8-13 mm often have 2 pigmented spots at the anterior portion of the caudal peduncle, between the posterior ends of the dorsal and anal fins. In certain post-larval stages, two rows of chromatophores (one along the mid-dorsal line behind the dorsal fin and the other mid-ventrally behind the anal fin) were reported, which requires further confirmation.
(c) Early juveniles
Sillago sihama 2
Specimens measuring 22-25 mm in total length have a relatively shorter snout than in adults. A spine is present on the operculum. There is no pigmented spot on the pectoral base. Blotches are absent on the body. A small pigmented patch is at times present on the dorsal profile above the operculum. Eyes are fairly big without adipose tissue.
The first dorsal fin has 11 spines and the second spine is not filamentous. The second dorsal fin has 1 spine and 21 soft rays. Rows of blackish spots are absent on the second dorsal fin. The pelvic fin spine is long – about half the length of the longest pelvic ray. The first pelvic fin ray is not thickened. The anal fin has 2 spines and 21-22 soft rays.
2. Distinguishing characteristics of early developmental stages in similar species occurring in the brackish and inshore waters:
(a) Early juveniles and juveniles of Sillago maculata
The body and dorsal fins (both first and second dorsals) possess blotches. In the second dorsal, the blotches form horizontal bands (there are no blotches on the body and fins of S. sihama ).
(b) Juveniles of Sillago punctatus
Small spots are present above and below the lateral line (restricted to Australian waters).
(c) Early juveniles and juveniles of Sillago vincenti
The second dorsal fin possesses 5 to 7 rows of black spots (rows of black spots are absent in S. sihama ).
(d) Early juveniles and juveniles of Sillago (sillaginopodys) chondropus
The pelvic fin has a minute spine. The first ray of the pelvic fin is thickened (the pelvic fin spine is long and the first ray of the pelvic fin is not thickened in S. sihama ).
(e) Early juveniles and juveniles of sillaginopsis panijus
The second spine of the first dorsal fin is filamentous. Eyes are relatively small and covered with adipose tissue (not so in S. sihama ). (S. panijus is often restricted to the Sunderbans and estuarine mangroves of Orissa.)
3. Salient biological characteristics (after Palekar, 1957; Radhakrishnan, 1957; Bal and Rao, 1990; Jeyaseelan, 1981)
(a) Maximum size
It grows to a maximum size of 312 mm in total length.
(b) Age and growth
It grows to 155 mm, 205 mm, 245 mm and 275 mm in length at the end of first, second, third and fourth years respectively. This species lives for a maximum period of 6 to 7 years.
(c) Food and feeding
1) Food in the mangrove environment
In the shallow mangrove waters of South India, the juveniles and adults of this species feed on polychaetes, tanaids, gammarids, shrimp juveniles and Planktonic mysids.
2) Food in a brackish water lake
In Pulicat lake (located on the Southeast coast of India) the fish fry up to 4 cm size feed on planktonic copepods, and the fingerlings feed on polychaetes, copepods and other crustaceans, whilst adults prefer polychaetes. Under laboratory conditions, the fry prefer micro-crustaceans, tubificid worms and mosquitoes and chironomids larvae.
3) Food in the marine environment
i) Food of post-larvae and juveniles
The post-larvae and juveniles of this species are pelagic and feed mainly on planktonic crustaceans such as mysids, lucifers and copepods in the polychaetes; bivalves and fish are also taken by these life history stages.
ii) Food of adults
The adults of this species feed on benthos such as polychaetes, benthic crustaceans (including prawns, shrimps, small crabs, amphipods, pistol crabs and benthic copepods), juvenile molluscs, gobioids and leiognathids in the inshore waters. At times, seaweeds (Sargassum spp. and Enhalus sp.) were also reported as food from the East coast of India.
4) Seasonal change in food items and their preference by adult fishes
In the Indian waters in general, the adults feed on polychaetes throughout the year. Lamellibranchs form a major food source during August-October while amphipods (Ampelisca sp.) form the bulk of the food during June-August.
5) Feeding intensity
In adult fish, low feeding activity was reported during the pre-spawning season, with moderate feeding activity during the breeding season and active feeding during the post-spawning period.
(d) Reproductive biology
1) Size at first maturity
Geographic variations in size at first maturity is common in this species. It matures for the first time at a size of 130-140 mm in length on the Southeast coast of India, while on the West coast of India (the brackish waters of the South Karnataka coast) the corresponding size is 151-191 mm in length. Nutrition and environmental variables seem to alter the size at first maturity to a greater extent in certain brackish water systems. The males mature at a relatively smaller size than females.
2) Spawning season
Sillago sihama is reported to spawn only once a year, but in different seasons in various geographic locations. It is reported to spawn for a brief period in certain areas while it has an extended spawning period in some regions. In the South Indian region off Rameswaram Island, it spawns between August and February; on the Kerala coast, it spawns between November and February; in the brackish waters of South Kanara, this species breeds between August and April; while in North Kanara it is reported to spawn for a brief period between August and October. Over four years of observation on the occurrence of gravid females and the recruitment of early juveniles of this species in the mangroves of the Southeast coast of India, a variation in the spawning periodicity is revealed from year to year depending on the southwest and northeast monsoonal influence and intensity (including monsoonal failure). Spawning occurred in all months (spread over different years) except in March-May and October-November in this southwestern part of the Bay of Bengal.
3) Fecundity
The gonads of female fish of Sillago sihama possess from 14,000-166,000 eggs depending upon size and other fertility-related factors.
4) Maturity stages and description of ripe ova
Five maturity stages (immature, maturing, mature, ripe and spent) are reported. The ripe ova measure between 0.42-0.8 mm in diameter. Ova are fully or partially transparent and have a single or at times up to 3 oil globules.
4. Salient ecological information (after Jeyaseelan, 1981; Fischer and Bianchi, 1984; Bal and Rao, 1990; Anon, 1962; Kaliyamurthy, 1984; Radhakrishnan, 1957; Palekar, 1957)
(a) Habitat
This species occurs in the inshore waters, estuaries and mangroves. It breeds in coastal waters. It is also reported to breed in the estuaries of the West coast of India. Post-larvae, juveniles and adults frequently occur in brackish waters including the mangroves.
(b) Geographic distribution
It is distributed throughout the Indian Ocean, the Western Central Pacific and up to the northern coasts of Australia.
(c) Behaviour
Shoals of this species are common on the Coromandel coast during October. It burrows in the sand when alarmed and also when procuring its favourite prey organisms, which are mostly benthos. It is therefore also called the ‘Sand borer’. The adult fish has a sharp downward-pointing protractile mouth to capture the burrowing prey organisms such as polychaetes and benthic crustaceans. The adult fish are demersal, while the early juveniles and juveniles feed preferably on planktonic crustaceans.
5. Capture fisheries
This species is easily accessible to exploitation by handlines in the mouth of estuaries and mangroves especially during the early part of the rising tides. Shrimps and polychaetes (especially Marphysa spp.) are used as bait for handlines in the mangrove waters. It is caught in all types of nets, especially cast-net, bottom set gill-net, bottom trawls and shore seines. In certain brackish water lakes of India, it forms a major share of perch among the fishery constituting species.
6. Culture fisheries (after Jeyaseelan, 1981; Bal and Rao, 1990; Anon, 1962; Kaliyamurthy, 1984)
This species is considered as one of the potential fin fish for brackish water and coastal aquaculture. The ‘seed’ availability in brackish water and the excellent taste and attainment of marketable sizes in a short span of time (4-5 months duration) are some of the positive points for promoting this fish as a candidate species for aquaculture. However, the ‘seed’ production techniques, growout techniques, production cost of the fish, the demand in the local and international markets, and acceptable rates for producers and consumers, are all to be carefully and realistically assessed and standardized through pilot projects for this carnivorous/benthos feeding fish before proceeding with large scale ventures.
7. Utilization (after Anon, 1962)
The finest eating fish in many Indo-Pacific countries, this species is considered as nutritious for nursing mothers. It is often sold in fresh conditions because of ready demand.