Habitat characters of juvenile mud crab, Scylla serrata

Megumi Minagawa*, Takeshi Hayashibara*, Motohiko Sano*, Motoya Tamaki*,
Kouki Fukuoka*, and Katsuyuki Hamasaki**
(*: Aquaculture and Resource Enhancement Section, Ishigaki Tropical Station,
**: Yaeyama Station, Japan Sea-Farming Association)
ABSTRACT
The mud crab, Scylla serrata is a promising species for aquaculture and resource enhancement in tropical regions of Japan (Fig. 1).  Habitat characters and the selectivity for particle size of the substratum were examined to develop techniques of releasing juveniles and increasing habitat of the mud crab.
INTRODUCTION
The mud crab is an important species for fisheries in the Indo-West Pacific and genus Scylla has 4 species.  In Japan the mud crab is distributed along the southern Pacific coast. S. serrata is dominant in Okinawa and S. paramamosain in the mainland Japan. Ecological studies of juvenile mud crabs are necessary to develop techniques for aquaculture and resource enhancement. We examined environments of juvenile habitat and selectivity against particle size of the substratum.
RESULTS
1)Juveniles were distributed in the tidal flat and mangrove areas at 0.03-0.73 /m2, whereas no juveniles were collected below the low tide line. Juveniles were observed more frequently in the tidal flat near the estuary where the salinity becomes low at low tide. They move to the upper stream when they grow.
2)There are no differences in selectivity for particle size of the substratum between wild and hatchery-reared juveniles. Juveniles selected a particle size of 0.063 mm in individuals smaller than 5mm carapace width and that between 0.063 and 0.125 mm in larger individuals.
3)Wild juveniles could burrow in large particle size substratum and took less time than hatchery-reared ones to do so. Individual differences in the time to burrow were larger in hatchery-reared juveniles than in the wild ones.
These results show that a suitable place to release juveniles is where the particle size of substratum is smaller than 0.125 mm. Tidal flats near the estuary are suitable place to raise hatchery-reared juveniles. The results of the present study are expected to help constructing aquaculture ponds and for the selection of juvenile habitats.
Fig. 1. Scylla serrata found in Okinawa. Large individuals exceed 3 kg in body weight.
Fig. 2. Photograph showing the investigation of juveniles of S. serrata in a 1×5m quadrat at the tidal flat in Ishigaki Island, Okinawa. 
Fig. 3. Juvenile density of S. serrata and environments of habitat in Ishigaki Island, Okinawa. Juveniles were collected in the places where the salinity becomes low at low tide. A: under low tide line, B: tidal flat near the low tide line, C and C’: tidal flat near the mangrove area, D: mangrove area the near tidal flat, E: mangrove area in further upstream than site D.