바위털갯지렁이의 인공종묘생산과 대량사육시스템 개발

Abstract

The rockworm Marphysa sanguinea (Polychaeta: Eunicidae) is an important bait for fisheries and sports fishing. Moreover, the demand for these species has increased rapidly in recent years since these worms are considered as an important nutrient source for stimulating gonad maturation, spawning of fish and crustaceans in the hatcheries of aquaculture facilities. There is a little domestic production of rock worm and mostly they are being imported. Hence, artificial aquaculture technology is highly needed for this species. In order to achieve this technology, rock worm seeds must be supplied regularly for the aquaculture. In this study, different techniques such as analyzing reproductive cycles, environmental characteristics on larvae production, mass culture system, and biofloc system have been developed for the artificial seed production of rock worm.

1. Reproductive biology of M. sanguinea. Marphysa sanguinea is a dioecious worm. Based on the conceptual knowledge of sexual maturation, spawning cycle and physiological characteristics of M. sanguniea, we tried to develop artificial seed production techniques under the laboratory conditions. The rock worm has multiple stages in its reproductive biology such as multiplication, growing, mature and spawning stages, and also resting and degenerative stages during gonadal development processes. In the natural seawater flow-through system, gonads of female and male were matured from January to April. Spawning and spermatogenic spurt were observed from May to August. Then, resting stage was observed from September to November. Finally, gonadal development was observed in the following year. Investigation on the yearly egg production provided basic information for artificial seed production and reproductive parameters (i.e. weight, the number of segments, absolute fecundity and egg size). The approximate weight per individual of M. sanguniea was 4.31 g (production in 2006), 2.27 g (in 2007), 1.80 g (in 2008), 0.9 g (in 2009), 0.25 g (in 2010). The rate of segmentation had been increased by 20% (1 year), 27.8% (2 years), 2.3% (3 years) and 12.5% (4 years). Fecundity of M. sanguniea was 31,902 (in 2006), 15,037 (in 2007), 9,750 (in 2008), and 3,000-6,000 (in 2009). Egg size (275 ㎛) increased by 43% (in 2006), 15% (in 2007) and 7% (in 2008). In 2009, all released eggs were smaller than 245 ㎛. The worms which kept rearing for more than 2 years, had more than 200 chaetes. They had still ability for egg production. In order to become mature, mother worms were kept through semi-recirculation system and water temperature controlled for 100 days. After maturation, sperm and egg were collected and both were mixed for 10 minutes. Finally, eggs were fertilized and found in the size of 230~270 ㎛. Cell division began between 1 to 3 days. After 5 days, one part of larvae was developed to the tail and eye spot has appeared on another side. Each larva had two chaete. Since 6 days, the trochophore stage started to burrow inside where larvae spurted out to swim toward surface water. Rock worm eggs were cared until they became larvae. During that time, M. sanguniea had a primitive brooding behavior.

2. Environmental characteristics on larval release In this study, the relationship between environmental factors, such as sunshine duration, semilunar rhythm, humidity, sea water level, tidal time and the release of M. sanguinea larvae was investigated. During artificial seed production in a temperature-regulated culture system, there was an increase in the number of released larvae at tidal times between 2 and 9. Numbers of larvae released were significantly correlated with tidal rhythms. Water level also appeared to influence larval release, with increased numbers released during high sea water level. During seed production on natural sea water, larvae were released at 18℃. Larvae were largely produced at dawn hours while sunshine duration was slowly increasing. There was an increase in the number of released larvae at tide times 11 and 6 for 5 years. Larval releases showed a significant difference at different tide times in the rhythmic pattern from accumulated relative percentage. Similar patterns also observed even in the rearing tanks with flow-through system of natural sea water from May to August every year. These synchronized timing events in larval release must be a common set of environmental inputs responsible for a heritable component so that larvae are released into the water when conditions are optimal for transportation and survival. It was shown that there were strong positive relationship between larval production and tidal time. The observed relationship between larvae production of M. sanguinea and environmental factors could provide a valuable information for seed production.

3. Growth and survival on different mass culture system Mass production systems can make different results unlike lab-scale experiment. It is impossible to control all environmental factors. Therefore, mass production systems were compared with existing system through field scale seed production. Survival rate was 1 % (600 inds./tank) on multi stage cage system and the weight was 0.045±0.015 g. The system by heated seawater temperature showed 1.7 times higher growth at low temperature period. There was a significant difference in survival rates. The system by natural sea water was 1 % and system by heated seawater was 2%. Raceway system showed higher survival and growth rates than multistage system at low temperature period. After 1 year rearing, the weights were 0.10 ± 0.0214 g and 0.0948 ± 0.0071 g, respectively, with no significant difference between natural seawater and heated seawater. Survival was higher in heated seawater. However, survival rate was 0.12 %, which was very low. Comparative experiment results between Biofloc technology (BFT) circulation system and raceway and multi-cage system, survival rate of BFT circulation system was higher than flow-through system. As a result of breeding through biofloc system for 60 days, it showed 27 setigers and survival rate of 23.2%, showing faster growth and higher survival than the multi-cage system and raceway system. Therefore, the system that showed the highest survival and growth in the mass production system was the BFT circulation on raceway. Biofloc technology (BFT) has been used for the seed production of rock worms. Larvae were able to make its own niche/habitat and utilize them as food by using floc. BFT enables artificial rearing of closed systems through the purification of residual feed and excrement by heterotrophic microorganisms. Two months later, survival rate was 10.4 % when clean/stable water quality (nitrate less than 0.2 mg/L) was maintained. The possibility of seed production was shown. Results of seed production using BFT, survival rate of biofloc water was 18 %. Sand and oyster shells mixed with sand substrate were suitable for rearing substrate. On survival and growth, there was no significant difference in other salinity (32, 25, and 20 psu), and the temperature grew rapidly at 25°C. Stable water quality was maintained when amount of feeding was 0.02 g, and the survival rate was high. The initial stocking density, which showed the highest survival and growth rates at the rearing density of 10,000 inds./box. The number of worms finally harvested was 4,655±357 inds./box, which had the initial density of 50,000 inds./box. The seed production by BFT was more effective than the conventional method and proved the possibility of industrialization of mass seed production.