| Budget Amount *help |
¥1,900,000 (Direct Cost: ¥1,900,000)
Fiscal Year 1992: ¥300,000 (Direct Cost: ¥300,000)
Fiscal Year 1991: ¥400,000 (Direct Cost: ¥400,000)
Fiscal Year 1990: ¥1,200,000 (Direct Cost: ¥1,200,000)
|
|---|
| Research Abstract |
In comparison with the coral communities of Sesoko Island of Okinawa (in coral reef area). some features of the coral communities in Amakusa have been made clear.
1) In spite of the long coast line, suitable shallow area for coral growth is narrower than that of coral reef area. However in several areas in Amakusa, coverages of corals sometimes reached 75%, and were not so much differed from that of Okinawa. However, total number of eggs spawned in Amakusa was quite fewer than in okinawa. 2) Presumably for this reason, density of larval settlement in Amakusa was quite low, only 1/10000 of Sesoko Is. 3) Effect of grazer such as Echinometra seemed as a distinct mortality factor of young corals. Density of this grazer is lower in Amakusa than Sesoko. Consequently, the discrepancy of both areas decreased relatively and the density of young corals in Sesoko Is. reached only 5 times of that in Amakusa. 4) During the growing season, the growth rate of corals in Sesoko Is. was 1.5 times higher
… More
than in amakusa. If the 5 times density is continuously maintained, rapid increment of coverage is expected in Sesoko Is. However, such rapid change in coverage have been not reported. This fact shows that some powerful predators such as Acanthaster and other influential mortality factors cause the reduction in the density. However, such influential factors lowering the density were raltively rare in Amakusa. According to the general survey on the distribution of corals by manta-method carried out in 1990, feeding scars and colonies partially died were rare in Amakusa. 5) It reported that most of corals in Amakusa died during the severe cold wave of 1962/63 winter. Diameter of table type corals, such as Acropora soliataryensis, sometimes reaches up to 2 m in Amakusa. If annual growth rate of 9 mm in diameter, measured in the present study, continues for 30 years, the maximum size of table type corals now present agree with the above report. This fact shows that scleractinian corals in Amakusa are growing under good conditions after the cold wave. Size frequency data of A. solitaryensis showed a successive pattern from small to large size. This successive size structure suggests a continuous settlement of young colonies. 6) In Amakusa where there is no powerful predator, effective mortality factors seem to be low temperature in winter, high temperature in summer and wave action during typhoon season. Many borers associated with corals also seems to contribute this reduction of corals during typhoon. Less
|
