(Cuvier)
1. Distinctive characters of early developmental stages (after Delsman, 1926b)
(a) Eggs
Not yet recorded from the mangroves.
Hilsa ilisha egg
The pelagic eggs with 1.0 to 1.1 mm diameter possess double membrane. Yolk is segmented. A single yellowish oil globule is present in the yolk. Perivitelline space is large.
(b) Larvae
Hilsa ilisha 1; Hilsa ilisha 2; Hilsa kelee 3
Small oil globule is present in the yolk sac of prolarva. There are 37 pre-anal and 6 post anal myomeres. second day rudiments of pectoral fins appear between 3rd and 4th myomere. Black pigments are present on the dorsal side of the anterior part of gut and few patches at the end of the tail. In 8.5 - 18 mm size postlarvae, the anus is situated at 36th myomere whereas in 19.5 mm larvae it is located at 32nd myomere. Postlarvae occur from April to June in lower reaches of Godavari mangrove estuary. (Babu Rao, 1976)
2. Distinguishing characters of early developmental stages of similar species occurring in the adjacent freshwater environment (after Jones and Menon, 1951; Pillay and Rasa Jr, 1963)
(a) Eggs of Hilsa ilisha (a migratory fish which spawns in fresh water)
Hilsa kelee egg
Egg diameter varies from 2.1 mm to 2.3 mm (only half the size in the case of Hilsa kelee ). Segmented yolk of the egg has many oil globules of assorted size, which unite in due course to form a large and conspicuous globule. (Only one small oil globule is present in the case of Hilsa kelee ). Eggs are demersal in still water but buoyed and drifted by slight currents due to closer density with water. (In the case of H. kelee the eggs are pelagic).
(b) Larvae of Hilsa ilisha
Hilsa ilisha 1; Hilsa ilisha 2
The widths of primordial marginal fins in the dorsal and ventral sides of about 5 mm larvae are very less. (In the case of H. kelee the widths are quite large). In the prolarvae, segment yolks disappear gradually (segmented yolk is clearly visible in prolarval stage in the case of Hilsa kelee ). Though the eggs of Hilsa ilisha are very big, the larvae growth rate is very slow when compared to that of H. kelee . (Hilsa kelee reaches 4.5 mm larva in just 2 days whereas the larger species (Max. size 60 cm) of H. ilisha interestingly takes 7 to 10 days to reach similar size of 4.5 mm in fresh water.)
(c) Juveniles of Hilsa ilisha
Frontoparietal striae absent on top of head (It is present in H. kelee ). Gill rakers on inner arches are not curved (curved in H. kelee ). Scales net perforated (In H. kelee the scales are perforated). Juveniles occur rarely in mangroves, that too during North East monsoon (December) season. (Juveniles of H. kelee occur in abundance in the south Indian mangroves during south west monsoon season (June and July).
3. Salient biological characteristics
(a) Size and growth
This species reaches 17 cm by the end of one year. It grows to a maximum length of 30 cm.
(b) Ford and feeding habits
Juveniles of 14 mm to 105 mm feed on planktonic centric diatoms, detritus, planktonic pennate diatoms, tintinnids and dinoflagellates in descending order of importance in the mangroves of south east india. (Jeyaseelan, 1981)
(c) Reproductive Biology
This species spawns once in a year during April/May in coastal waters of south east India (Jeyaseelan, 1981) and during February - March in Godavari estuary (Babu Rao, 1976). It spawns during during November-December in Kerala coasts. The eggs were collected from waters of salinity ranging from 27.2% to 33.4% from the Java sea. (Delsman, 1926b) Ova diameter ranges from 0.75 mm to 0.8 mm. Fecundity ranged between 15,700 and 31,400 for specimens of 17.6 to 20.0 cm length. Small schools of adults were reported from the lower reaches of mangrove estuary and spawning could take place either in the lower reaches of estuary or in the nearby coastal waters. (Babu Rao, 1976)
4. Salient ecological information
(a) Habitat
Eggs have net yet been recorded from the mangroves. Larvae occasionally occur in lower reaches of mangroves and estuaries. Juveniles of 35 mm to 100 mm size enter in large shoals into the lower reaches of mangroves and are subjected to destructive under sized fishing in south east Indian mangroves during south east monsoonal period (June - July). Adults constitute considerable portion to "lesser sardine fishery" from the coastal regions of India.
(b) Geographic distribution
Persian gulf, East cast of Africa, India and south East Asia.
(c) Behaviour
It is a pelagic shoaling fish.