Nature in Akajima


Geographical features and environment


fig1 Kerama Islands lie 20 to 40 km west of Okinawa Island, and comprise some 30 islands of various sizes (Fig. 1).
From the distribution of Japanese cedar and Japanese cypress fossils in the rock stratum, it is thought that the Kerama Islands were part of a mountain range that extended to (northern) Okinawa and exceeded 1500m above sea level in the Pliocene (1.5 million years ago). Well developed coral reefs formed around Okinawa 600,000-200,000 years ago, and remain as uplifted Ryukyu limestone. This limestone layer in Kerama Islands, however, is about 80 m below sea level.
The Kerama Islands have subsided because of diastrophism. As a result, the sea spread out the Okinawa Island and Kerama Islands, and the peaks of the mountain range became the groups of small islands now known as Kerama Islands. The inland sea of Kerama consequently exhibits the geographical features of a subsided coast and presents a beautiful landscape (Kizaki 1992).
Akajima is located near the center of Kerama Islands (Fig. 1). The seawater temperature is lowest in February and March, but the monthly average never falls below 20OC, and is usually the highest, at 27.1 to 29.6 OC in July and August. The water temperature rose exceptionally high in August 1998 when the monthly average recorded was 30.4OC. It caused coral bleaching and catastrophic damage on coral reefs around Okinawa Island, however the percentage of dead colonies was merely 6.7 to 23.4% around Akajima (Iwao 2000). The reason for this lower mortality level is thought to be the presence of lower temperature water on the shelf around Kerama Islands (Nadaoka et al. 2001).

Typhoons are common, and are often experienced even in early summer. Summers have extreme oceanographic conditions with about five typhoons approaching every year, bringing high waves, wind and rain, followed by calm intermittent periods. From October to April, the northern monsoon brings strong north winds and large swells (Hayashibara 1995).


Hermatypic corals and biota

fig2 There are approximately 248 hermatypic coral species found on Akajima and the surrounding Kerama Islands (59 genera and 14 families) (Hayashibara 1995) - or 62% of the species recorded in Japan (Veron 1992). Considering their relatively small size the Kerama Islands are extremely rich in coral fauna (Hayashibara 1995). Figure 2 shows protected areas and survey localities using belt transects around Akajima Island. The northern coast of Zamami Island supports spurs and grooves and a tabular Acropora community, which used to be more extensive before the 1998 bleaching event and a more recent outbreak of crown-of-thorns starfish (Acanthaster planci). The southwestern coast of the island supports high diversity, and Agonoura Bay, in the southwest island, is said to have unique coral communities, but a satisfactory survey has not been conducted.
Kushibaru, on the northwestern coast of Akajima Island, also has extensive spur and groove systems and large tabulate Acropora colonies from reef edge to 4-6 m down the reef slope (Fig.3). Again, recent A. planci predation has reduced corals. Nishihama, on the eastern coast, supports divers coral assemblages on shallow slope. Maenohama, on the southern coast, supports artificial breakwaters about 100 m offshore; the sand bottom is scattered with sea grasses and rocks supporting massive faviids branching Montipora.
Mass coral spawning has been observed annually on Akajima. The spawning occurs in summer from May to September. Many Acropora and Montipora species spawn in May-June, Merulina in July, and some Favia in August-September. On the morning following the mass spawning, slicks may be observed on the sea surface. Although many eggs and embryos are washed ashore, some drift toward the west coast of Okinawa main island, and the larvae settles on the seabed. That is to say, the larvae from Kerama Islands enhance the coral communities around Okinawa main island.
fig3 Three hundred and sixty fish species, about 1,640 invertebrate species, including hermatypic corals, and   about 220 seaweed species have been recorded in the Kerama Islands to date, but many groups of organisms have not yet been surveyed (Ohba 1995; Iwao 2003) (Table 1). Green turtles, loggerheads, and hawksbill turtle lay eggs on the beaches in summer. Humpback whales (Megaptera novaeangliae) are watched very frequently in January to April, since they use the Kerama Islands as their breeding ground.
On the land, 620 species of flora have been recorded.
Many birds and butterflies are observed. The Kerama deer, a subspecies of the Japanese deer, live only in the Kerama Islands. The Kerama deer is smaller and darker than Japanese deer. The deer population on Akajima Island increased to 130 in 1995. They are fully protected on some islands as a nature treasure of Japan (Zamami Village History Compilation Committee 1989).

table1

History and present status of Akajima

Kerama Islands provided good moorings for ships that navigated between Okinawa and China during the Ryukyu Kingdom era from the middle of 1400s to the late 1800s, and for some 600 years the islanders had been employed as skillful navigators for the kingdom’s trading vessels. After Okinawa became part of Japan, the island prospered with bonito fishing and processing industry (“katsuobushi”, dried bonito) until the World War II. As is seen, the resident islanders have lived for centuries with the sea and shared in the blessings of the ocean. Swordfish, tuna, and skipjack are still being caught using hooks and lines; aggregations of spinefoot (“suku”) are caught using gill nets in summer. In addition, local residents collect shellfish and octopus by scouring the reef flat at ebb spring tide. Most of the catches are consumed locally.
The motive force of recent development has also been the bountiful nature of the islands. In Kerama Islands, considered one of the most beautiful coral reefs in the world, the tourist business such as recreational diving and lodging services has flourished, taking advantage of the diving boom, and this development is continuing even today. There are 43 diving businesses and even more for accommodation in Zamami Village which is spread over 3 main islands including Akajima. While fishing was the main source of income in the past, now more than 80% of the resident islanders are engaged in tourism. Owing to this, the population of Akajima increased from 214 to 348 during the 15 years from 1985 to 2000, despite the fact that many of the other islands in Okinawa have been troubled by depopulation.

Akajima Marine Science Laboratory

fig4 Akajima Marine Science Laboratory (AMSL) is a private institution which was established in 1988 under the auspices of the Science and Technology Agency, Japan. Since then, in cooperation with local residents and many visiting researchers, the laboratory has been actively researching coral reproduction, recruitment and coral settlement processes, which is highly applicable to conservation issues. Also technical and practical experiments concerning the restoration and remediation of coral reefs are being carried out (Omori and Fujiwara 2004). A number of scientists from Japan and abroad visits AMSL every year for research on the conservation and effective use of coral reef ecosystems. AMSL continues with education and awareness activities that involve the local school and the local community, by operating summer school and preparing pamphlets and newsletters.


Coral reef conservation in Akajima

Recently, many research reports have highlighted the Kerama Islands as the main source of coral larvae for Okinawa and surrounding islands. Therefore, preserving the reef around the Kerama Islands is a regional issue. Although heavily damaged by the crown-of-thorns starfish outbreak over the past two years, the Kerama Islands showed some resilience bleaching events in 1998 and 2001, the reefs are still rich in biodiversity and aesthetically appealing (Iwao 2004). There is a general consensus in the Kerama Islands that human disturbances need to be kept low. There are no large rivers, and pollution is low in the islands. The Zamami Fishery Cooperative and local recreational diving association are now actively engaged in coral reef conservation, which gives hope for Kerama Islands in future.


References

  • Hayashibara T (1995) Ecological studies on reef-building corals and their sexual reproduction around Akajima Island, Kerama Islands group. PhD thesis, Tokyo University Fisheries, 123 pp. (in Japanese)
  • Iwao K (2000) Study on the effect of geographical features on the cause of coral bleaching. In: Research and Development Bureau, Science and Technology Agency (ed.): Report on the Urgent Research on the Mechanism Elucidation of Coral Bleaching, pp.15-39 (in Japanese)
  • Iwao K (2003) Surveys of marine fauna around Kerama Islands by visiting scientists and staff members of AMSL. Midoriishi, 14: 38-41 (in Japanese)
  • Iwao K (2004) Kerama Islands. In: Tsuchiya M et al. (eds.): Coral Reefs of Japan. Ministry of the Environment and Japanese Coral Reef Society, Tokyo, pp.185-189
  • Kizaki K (1992) Geological history of the Kerama Islands. Midoriishi, 3: 1-2 (in Japanese) Nadaoka K, Nihei Y, Wakaki K, Kumano R, Kakuma S, Moromizato S, Omija T, Iwao K, Shimoike K, Taniguchi H, Nakano Y, Ikema T (2001) Regional variations of water temperature around Okinawan coasts and its relationship to offshore thermal environments and coral bleaching. Coral Reefs, 20: 373-384
  • Ohba H (1995) A list of seaweeds of Akajima Island and its vicinity in Kerama Islands, Okinawa Prefecture, Japan. Midoriishi, 6: 23-28 (in Japanese)
  • Omori M and Fujiwara S (eds.) (2003) Manual for the Restoration and Remediation of Coral Reefs. Bureau of Natural Environment, Ministry of the Environment, Tokyo, 84pp.
  • Veron JEN (1992) Conservation of biodiversity: a critical time for the hermatypic corals of Japan, Coral Reefs, 11: 13-21
  • Zamami Village History Compilation Committee (1989) History of Zamami Village. Vol. 1, Zamami Village Office, Okinawa, 710pp. (in Japanese)