Monacanthid fishes are generally inhabiting both temperate and tropical shallow waters, there are about 95 species are known worldwide (Nelson, 1984). The bluefin leatherjacket Thamnaconus modestus (Günther, 1877) belonging to the family of Monacanthidae, is an important commercial bottom-dwelling marine filefish which distributes in the northwestern Pacific Ocean (Su and Li, 2002). As a traditional commercial fishery species, bluefin leatherjacket was widely fished in China, Korea and Japan in the 1990s (Xu et al., 1992). However, due to over fishing, bluefin leatherjacket has become an endanger species in 1990s(Qian, 1998; Chen and Zhan, 2000). In the past two decades, a series of studies have been conducted, including stocks and distributions identification (Xu et al., 1992; Ding, 1994; Chen and Zhan, 2000), ecology factors (Maekawa, 1989), cryopreservation of sperm (Kang et al., 2004), and genetics (Xu et al., 2010). Although several studies have been done on the morphological development and growth evaluation of bluefin leatherjacket (Zhao and Chen, 1980 ; Chen and Zheng, 1984), most of these studies were conducted under field survey. Results from these studies are not consistent due to the limitation of the sampling scales. Up to present, little information is available on the developmental characters of bluefin leatherjacket at early stage. Furthermore, artificial breeding of this species is still under trial scales.

Figure 1 Spawning frequency and egg numbers produced by T. modestus

Like most species from the family of Monacanthidae, the eggs of bluefin leatherjacket were subjected to adhesive egg. The adhesiveness of eggs from Monacanthidae is very strong (Zhao et al., 1985). In nature, this type of eggs was normally adhered to the substrate such as reef, algae and cay. There are a large number of viscin threads distributing on the egg surface of bluefin leatherjacket (Zhao et al., 1985; Safran and Omori, 1990). Previous studies indicated that by using viscin threads the fertilized eggs of bluefin leatherjacket can stick on the substrates such as shell, coral reef, large algae, and rocks in the natural environment (Qian, 1998; Su and Li, 2002). Results from our previous investigation indicate that the plastic plate and black soft plastic film attracted more eggs in the artificial breeding environment, and the viscin threads of eggs were not well stuck to the rough surface materials (Guan et al., 2012). Therefore, plastic corrugated plates were used to collect the eggs in the present study.

Table 1 Embryonic development and accumulative temperature of Thamnaconus modestus

 

Figure 2 Embryonic development of T. modestus. Stages and development information refer to Table 1

Growth and morphological characteristics of the larvae during the first five days are presented in Table 2. The mean total length (TL) of newly hatched larvae was (2.08±0.09) mm with an ellipsoid yolk-sac of (0.5331±0.0366) mm long and (0.2593±0.0393) mm wide. One oil globule with the diameter of (0.20±0.01) mm was presented at the anterior end of the yolk-sac. After hatching, fish larvae had un-pigmented eyes, and the mouth did not open. Melanophores were presented on the snout tip, the trunk and on the tissue surrounding the inner side of the yolk-sac. Xanthophores were presented on the trunk, abdomen and around the head. The mouth opened on 3dph, while first was started between 3 and 4 dph. Yolk sac was completed absorbed on 2 dph, while oil globule was absorbed completely on 4 dph (104.4 d·℃) when larval size reached to (2.54±0.06) mm (TL, Table 2).

Table 2 Yolk-sac and oil globule volumes in larvae of T. modestus (means ± SD)

Where Y is total length in millimeters and x is days post hatching (Figure 3). In this study, two heavy mortality periods were observed, which were found between 5 dph~10 dph and 30 dph~60 dph. After 70 days rearing, 0.33 million juveniles (mean total length>40 mm) were obtained and the ratio of seedling/ newly was (5.62±0.83) %.

Figure 3 Growth of T. modestus during the first 50 days

From the physical aspect, sufficient nutrient supply depends on the efficiency of the digestive system. During the early development stage, lack of sufficient digestive capabilities is common in this phase and larvae mainly depend on pinocytosis and intracellular digestion and absorption (Ma et al., 2012b). Previous study indicates that the nutrition is presumably the predominant factor affecting the growth of fish in early life stage (Yin, 1995). In species such as yellowtail kingfish (Seriola lalandi), growth is normally found accelerated after the digestive system is fully functional (Ma, unpublished data). In the present study, bluefin leatherjacket grew slowly in the original developmental stage (0 dph~16 dph), the specific growth rate was only about 4.7%/day. The growth turning point was found around 20 dph, when the growth was accelerated. This may indicate that a more efficient digestive system was formed around this stage.

The eggs were incubated in the larval rearing tanks (concrete, 28.8 m³) at an initial density of 3.0×10⁵ eggs/tank. Constant aeration was supplied to each tank. The salinity and temperature were maintained at 30‰, 21℃, respectively. After hatching, the plastic corrugated plates with un-hatched eggs were removed from the rearing tanks, and the yolk sac larvae were reared in the same tank.

2.3 Larval rearing

The rearing density of fish larvae was maintained at 6000 larvae/m³~8000 larvae/m³, and a total of 19 tanks were used in this study. The rearing tanks were supplied with sand-filtered seawater. Larvae were reared with a 24 h-light photoperiod, and the illumination was provided by fluorescent lamps suspended over the rearing tank, and light intensity was 1500-2000 lux (measured on the surface of water). The salinity and dissolved oxygen were maintained at 30‰, 5 mg/L ~7.0 mg/L respectively. The water temperature was gradually increased from 21.0℃ (0 dph) to 26.0℃ (16 dph), and then maintained at 26℃ until the experiment was finished (Figure 4). Water exchanging in the rearing tanks was started on 4 dph, the exchange rate was gradually increased from 10% (4 dph) to 100% (20 dph). On 20 dph, all the juveniles were collected and transferred to new rearing tanks (concrete, 28.8 m³) with a stock density of 2000 fish/m³.

Figure 4 Feeding scheme and rearing temperature for T. modestus during the first 45 days post hatching

Zhao C., Zhang R., Lu H., Lian C., Zang Z., and Jiang Y., 1985, Fish egg and larvae of China coastal sea, Shanghai scientific and technical publishers, Shanghai, pp.206