Coral reef communities around Okinawa have been suffering various disturbances at least since 1972 (Nishihira & Yamazato, 1974). Nishihira (1987) reviewed the disturbance agents of the coral reefs and other coastal Okinawan environments, and listed 3 major agents: (1) Crown-of-thorns Starfish, (2) climatic stresses, and (3) human interferences.
Starfish predation is generally blamed for coral reef destruction in the Archipelago. In the past the GOJ (Government of Japan) has spent vast sums of money to "control" Crown-of-thorns (Acanthaster planci) infestations by removing the adult animals (Yamaguchi, 1986). However, not only have these control efforts failed, they have increased the problem in two ways. They divert money that could otherwise be used in basic research to the causes of the Crown-of-thorns (COTS) population increases, and they divert attention to what may be the real cause of the coral reef crisis: man's activities on land (Muzik, 1985).
Birkeland (1982, 1989) contends that outbreaks of Crown-of-thorns can frequently be predicted 3 years following heavy rains exceeding intensities of 30cm in 24 hours. The heavy rains increase the amount of nutrients in the water, therefore increasing the amount of plankton necessary for survival of the COTS larvae. The larval COTS, under conditions of increased plankton, have a much higher probability of survival. With remarkably high numbers
of eggs being produced (65 million per large female), even a slight increase in survival of
the larval COTS can result in high numbers of adults in a given area.
Among the human caused disturbances, both sedimentation and the suspension of reddish clay may be the ones having the most important effect (Nishihira, 1987; Yamazato, 1987). Runoff from land can be easily seen after every rainfall, as murky red water extending from shore to reef. Sometimes shortly following heavy precipitation, there are spectacular fingers of red clay in obvious contrast to the blue water, sometimes extending hundreds of meters offshore. This red clay precipitates out onto the living corals (Sakai & Nishihira, 1991), smothers them, and prevents their required physiological interactions. These deposits are harmful to the intertidal community, and to corals, and coral-associated plants and animals. If the clay does not precipitate out, but remains suspended, the reduced sunlight adversely effects the coral's ability to survive.
Where does the red clay runoff come from? It is a result of erosion on land, from extensive deforestation for dams, extensive leveling of land for new pineapple and sugar cane fields and from construction. The cutting down of dense northern Okinawan forests is threatening the habitat of many endemic animals and plants (e.g., an Okinawan bird, a rail, locally called the "Yambaru Quina", Rallus okinawae) (WWFJ, 1984, Red Data Book, 1991). The increased agricultural practices simultaneously increase the loss of valuable topsoil, so that more and more artificial fertilizers must be used, and more chemicals pour into the sea. This runoff is increased and speeded up by the widespread practice of straightening rivers, and lining them with cement. Therefore, river-associated ecosystems are being destroyed as well.
Throughout 1991 the pollution of Okinawa seashore waters caused by top red soil flowing into the ocean was a matter of concern to the island inhabitants. It is believed the problem was and is caused by the great number of private and public seashore tourist projects undertaken near the ocean (Sanchez, 1993). During and after construction, followed by precipitation, the top soil flows into the ocean. The twin scourges of both development (resulting in runoff) and the Crown-of-thorns Starfish, have wreaked havoc with the coral reefs of the Okinawa islands.
Although the environmental pressures foisted on the coral reef environment by recreational uses at this writing are not as severe as either starfish predation or runoff, it is envisioned to be a problem that can only intensify in the upcoming years. The normally warm Okinawan subtropical waters act as a natural magnet, attracting a myriad of people for recreational purposes. Among others, these recreational pursuits include SCUBA diving, snorkeling, shell and fish collecting, underwater photography, boating, surfing, and beach camping.
The great number of divers on Okinawa poses a potential environmental problem: stepping on the live coral as the heavily encumbered diver walks through intertidal waters on the way to his/her favorite dive site. Once the coral is broken it is frequently overgrown with a light-inhibiting growth of algae and the area frequently degrades quite rapidly.
Many divers become enamored with collecting shelled mollusks. Their indiscriminate search for "shells" sometimes results in busted and destroyed corals, as well as a serious perturbation of the local reef environment, e.g., busting and overturning coral heads, and overturning rock and coral rubble without replacing it. This abhorrent practice destroys eggs,larvae,and disrupts the habitat of countless reef-associated organisms.
The high amount of SCUBA activity enjoyed by Americans living on Okinawa is reflected in the number of military SCUBA facilities on the island:
In addition to the above SCUBA and Snorkeling activities, recreational boating is becoming more and more popular. There are a series of environmental problems associated with boating: anti-fouling paints leaching into harbor waters, anchors being dragged over live corals, the discharge of human wastes, oil, fuel, and plastics into the water.
All of the preceding recreational problems are collectively placing undue stress upon the fragile Okinawan marine environment.