Long-term changes in structure of fish communities on coral reefs infested by the coralfeeding starfish Acanthasterplanci were determined using 20 mZ visual transects. We censused a living coral reef consisting mainly of staghorn Acropora spp. at Saluyama Bay and a dead coral reef with low structural complexlty of coral branches (about 2 yr after A. planci infestation) at Amitori Bay, Iriomote Island (Ryukyu Islands, Japan) in September 1984. Two yr later, we recensused the dead coral reef, which had changed into a flat plain of unstructured coral rubble ('rubble reef'). Mean numbers of species and individuals per transect severely decreased in the following order of reef types: (1) living reef (1984). (2) dead reef (1984), and (3) rubble reef (1986). Correlated with these decreases in numbers were several patterns: (1) coral-polyp feeders completely &sappeared on both dead and rubble reefs, probably due to absence of food; (2) numbers of resident species and individuals decreased on the dead reef, perhaps due to decrease in living space or shelter associated with the reduction in structural complexity of coral branches; and (3) numbers of species and individuals of both resident and visitor fishes declined on the unstructured rubble reef, likely due to shortage of living space and food.
Abmmf-Autonomous Underwater Vehicles (AWs) are suitable for condition SUNV of artillcial sbocinres such as pillars and caissons in harbors. This paper describes a method to trace the structure's surface using a iight-section p m w g system. This proliling system determines the continuous shape of the target objects over a wide area by the light sectioning method The vehicle navigates referencing the principal shape of the stlpcture, tracing its surface while taking video images. This method dso enables threedimenslond mapping oftraced structures and the seabed The method is implemented using testbed A W "Tri-Dog 1" and vedied by tank tests. The AUV navigate robustly against Mvid objects such as small obstades and Uoating particles. Precise depth mapping of the tank botlom Is carried out.
I. JNTRODIJCITONAutonomous Underwater Vehicles (AWs) are suitable for ocean observation since they can swim k l y without the restriction imposed by an umbilical cable. It can be said &at A W s are increasingly finding practical applications worldwide [1][2][3][4][5]. Commercial obsemtion of the seabed, for investigation of underwater oilfields and resources, has already been executed by A W s using side scan sonar, mdtibeam sonar, sub-bottom profiler etc [6].There is a great demand for A W s not only for wide area survey but also for condition survey of artificial underwater structures such as pillars and caissons in harbors, dams, bodies of ships, and installations of deepsea oilfields. Such structures require periodic inspection. These task are typically carried out by human divers and Remotely Operated Vehicles (ROVs). However the use of buman divers carries an inherent risk and is not applicable at great depth. ROVs require an umbilical cable that restricts their movement. The demand for development of highly intelligent A W s that can undertake such missions is expected to increase greatly over the next few years.Intelligent behavioral control, sensing, navigation and motion control are essential to accomplish autonomous observation of underwater structures using A W s . In an underwater environment, it is especially difficult to detect obstacles (sensing) and to obtain precise position data (navigation). The authors have proposed the autonomous observation method using A W s that can overcome these challenges [7][8]. The method has been implemented by detecting the target objects, localizing them, and then approaching them by taking video images while closely tracing their shape. A laser ranging system consisting of laser pointing devices and a CCD camera was introduced for sensing. The system was shown to provide precise distance to the target object so that the vehicle could trace its shape vhile maintaining a constant distance and attitude to it. This method is hown as relative navigation. The method wa8 implemented using the testbed A W 'Tri-Dog 1" 191 and has been proved experimentally by tank tests. Sea trials were also carried out at Kamaishi port in Japan [IO].
The A W performed observation of breakwater caissons.D...
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