(Park)
1. Distinctive characteristics of early developmental stages
(a) Eggs
Not yet recorded from the mangroves
(b) Early juveniles and juveniles
Siganus canaliculatus 1
A distinct black blotch is present on the shoulder but without numerous small spots all over the body. The dorsal fin has a forward projecting spine. The second spine among the heteracanth spines of the dorsal is the longest (not so in adults). Pigmented patches are scattered all over the body and pigment lines the dorsal fin like broken lines and above the anal fin as a streak.
2. Distinguishing characteristics of early juveniles and juveniles in similar species occurring in the mangroves
(a) Siganus javus
Wavy white lines are present on the lower half of the body. There are no spots on the shoulder, and the forward projecting spine is hidden under the skin in the nape.
3. Distinguishing characteristics of juveniles in similar species occurring in the adjacent coral and coastal seas
(a) Siganus stellatus
The blotch on the shoulder is brown in colour and persists even in adults. In addition, numerous small brown spots are present ail over the body (in early juveniles of S. canaliculatus a black blotch is present which disappears in adults. Moreover, small spots are absent all over the body).
4. Salient biological characteristics
(a) Maximum size
330 mm total length in coastal waters. The maximum length recorded in the shallow mangroves (of about 1 m in depth) was 131 mm in total length, beyond which this species moves to the adjacent seas with a relatively greater depth (Jeyaseelan, 1981).
(b) Food and feeding
A herbivore, feeding on algal forms attached to corals and rocks and on seaweeds and sea grasses (Fischer and Bianchi, 1984). In the mangrove waters, the juveniles browse on sea grasses, algal weeds and the adhered epiphytic pennate diatoms in the lower reaches (after Jeyaseelan, 1981).
(c) Reproductive biology
It breeds during the post northeast monsoon period (January-March) in the Southeast Indian coastal seas (Krishnamurthy and Jeyaseelan, 1981). In Singapore and Philippine waters, it breeds during January-April with peak spawning during January, February and March. A second minor spawning season is also reported during July to September/October. In the north pacific island of Palau it breeds between February and June with a peak during April-May with a second spawning during October-December. This species lays eggs on the bottom in shallow waters in and around corals. In the north pacific island, it breeds in the surf zone of the outer coral reef. Spawning is reported to occur from the 4th to 7th day after the new moon (5-7 days after the new moon in some regions) during spawning seasons.
The females and males come in shoals to the shallow tidal flats during rising tides. Spawning occurs after midnight until dawn. Females nudge the abdomen of the male to release the milt. Soon after the males response, the females release the eggs. Absolute fecundity varies from 0.3-0.4 million. Surf waves seem to facilitate the mixing of eggs and sperms. The eggs are small, transparent, colourless, spherical, demersal, adhesive, and possess oil globules (Lam, 1974).
(d) Growth
In the Philippine waters, it grows up to 80 mm in three months, 100 mm in four and a half months and over 140 mm in 7-8 months. In Singapore waters, it attains 120 g in a 9-11 month period (Lam, 1974).
5. Salient ecological information
This species tolerates a wide range of salinity and temperature fluctuations. It is reported to withstand direct transfer from seawater to brackish water of around 12 ppt salinity. It can be acclimatized to live in low saline water (around 5 ppt salinity). Young ones occur in the algal weed and sea-grass beds located in the lower reaches of the mangroves – where the turbidity is relatively lower. This species mostly occurs with S. javus in the South Indian mangroves and is represented sporadically in catch sampling.
6. Capture fisheries
Excepting certain oceanic islands of the Pacific, it does not constitute a major capture fishery, probably due to its behavioural characteristics and specialized habitat (coral and rocky reefs). In coral and and rocky reefs, traps are used to capture this species (after Fischer and Bianchi, 1984). Drag-nets and cast-nets are used to capture undersized fish (juveniles) from the algal and sea grass beds of the mangroves subject to neritic influence (Jeyaseelan, 1981).
7. Culture fisheries
A suitable species for cage culture but its economic feasibility depends on location, specific demand, and cost. It is primarily a herbivore, but accepts diverse types of supplemental feed. Complete feed with 25% protein could also be used for intensive farming. It responds to induced breeding by HCG. Artificial fertilization by ‘stripping methods’ is also successful when adults with ripe gonads are used. It is also a candidate species for brackish water aquaculture and aquariculture with water salinity not lower than 18 ppt. When macroalgae like Enteromorpha spp. pose problems in moderate saline aquaculture systems (especially in shrimp farming practices), this species could be used with appropriate management skill to control the aquatic weeds biologically (instead of using algecides like chelated copper to avoid heavy metal pollution in the long run because of the binding of micro elements in particulate matter). On the contrary, this species causes a menace to seaweed cultivation. According to recent FAO culture fisheries statistics, during 1990 Singapore produced 7 tonnes of S. canaliculatus while Saudi Arabia produced one ton through mariculture (FAO, 1992).