(Forsskål)
1. Distinctive characteristics of early developmental stages
(a) Eggs
No information from the mangroves (in general, Silverbellies breed only in a marine environment).
(b) Larvae
The head is large, the mouth is protrusive downwards. The preoperculum has serrations, one strong spine and a few small spines. The median occipital-cum-nape blade is serrated (which seems to be the future nuchal crest in adults). The mandibular region and lower portion of the operculum are mildly curved inwards to give a characteristic shape to the head. The body is laterally compressed. The rays of the dorsal and anal fin develop before the spines in early larvae (this observation needs further confirmation). In the late post-larvae of 9 mm in size the spines of the dorsal and anal fin (though developed late) become prominent with a second dorsal spine and second anal spine stronger and longer than the other spines (which are typical characteristic features of most of the juveniles and adults of Leiognathus spp.). The ventral portion of the body below the pectoral and anal fin base are strongly pigmented.
Leiognathus equulus 1
Leiognathus equulus 2 (Courtesy: Dr. P. Bensam, India)
2. Salient biological characteristics
(a) Maximum size (James, 1984)
Leiognathus equulus – 240 mm
(b) Food habit
In the marine environment this species is reported to feed on polychaetes, small crustaceans and small fish (James, 1984). In the mangrove environment, the juveniles of this species feed on benthic copepods, detritus, polychaetes, bivalve veligers, planktonic copepods, nematodes, benthic diatoms, gastropod veligers, zoeae of crabs, foraminiferans, cypris stages of barnacles and sponges (Jeyaseelan, 1981).
3. Salient ecological information
In general, leiognathids are demersal fishes. Species of the Leiognathus genus encountered in the mangroves feed both on benthic and planktonic organisms, while the Secutor species seem to feed only on planktonic organisms. The position of the protrusive mouth could be attributed to such feeding types in these juveniles. Among the above mentioned (5 spp.) leiognathids, besides the post-larvae, early juveniles and juveniles, the adults of L. equulus were also recorded from the mangrove waters. Species belonging to all the three genera of the Leiognathidae family occur in the mangroves (Gazza , Leiognathus and Secutor ) of which Leiognathus spp. dominated the other two genera, both in species richness and abundance.
4. Capture fisheries and utilization
Silver bellies constitute the bulk of the demersal fishery resources/trawl fishery/ trash fish fisheries. They are one among the main source of fish for the fish meal industry. The sun-dried fish reach the interior markets and cater for the animal protein needs of the rural poor. The low price of the commodity readily fits the rural economy. Moreover, silver bellies are noted for their medicinal value also – the rich bone content and lean flesh provide calcium and protein nutrition respectively.
Nowadays, the tremendously expanding shrimp farming industry creates a higher demand for trash fish for their fish meal/fish feed plant. As a result, silver-bellies which hitherto served as the major marine protein source to the poor inland rural community may not reach them any longer, but are diverted to the shrimp feed industry to earn foreign exchange at the cost of hampered human resources development.
The fixed stake-nets which are under operation in the mangrove waters of Southeast India indiscriminately catch undersized fish during the northeast monsoon and post- monsoon seasons. Since many species of the Leiognathidae family use mangroves as a nursery ground, safeguarding the nursery environment will improve the marine catch and largely benefit the demersal fishery resources.