Shuhei Sawayama

Mapping Sargassum beds off the coast of Chon Buri Province, Thailand, using ALOS AVNIR-2 satellite imagery

Abstract Sargassum species grow on rocks and dead coral, forming dense seaweed beds in their growth seasons. Sargassum beds play a range of ecological roles in coastal waters, but their sustainability is threatened by pollution and reclamation within ASEAN countries with rapid economic growth. Before conservation initiatives can be implemented, it is necessary to establish their present distribution. These beds can be mapped using the noncommercial satellite, ALOS, which can provide multiband images using high spatial resolution optical sensors (sensitive to 10 m-2 plots), AVNIR-2. These images are of sufficient quality for examining coastal ecosystems and, in this study, for mapping the Sargassum beds in the waters off the coast of Sattahip, Chon Buri Province, Thailand. This coastal zone is a natural marine park reserve and protected from human activities such as fishery and reclamation. Biomass data were obtained in January, February, March, and December 2009 from quadrat sampling. The biomass of Sargassum aquifolium (Turner) C.Agardh at a bottom depth of 1 m and 1.5–2 m was determined to be 7.73 and 92.75 g dw m-2, while that of Sargassum oligocystum Montagne was found to be 44.05 and 87.97 g dw m-2, respectively. Ground truth data were obtained in February and October 2012 from serial images taken by manta tow. Supervised classification is a procedure for identifying spectrally similar areas on an image by identifying “training” sites of known targets and then extrapolating those spectral signatures to other areas of unknown targets. By applying this methodology, the Sargassum beds off Sattahip can be detected with an accuracy of about 70%. It is estimated that the error is caused by mixel effects of the bottom substrates in individual pixels, each of which covers an area of 10×10 m. Our results indicate that the images captured by ALOS AVNIR-2 are informative and useful for mapping the Sargassum beds in Southeast Asia.

Impact of the 2011 Tsunami on Seagrass and Seaweed Beds in Otsuchi Bay, Sanriku Coast, Japan

The Sanriku Coast facing the Pacific Ocean is an important area for fishing and aquaculture. The Sanriku Coast was hit hard by the catastrophic tsunami event produced by the Great East Japan Earthquake on 11 March 2011. The tsunami destroyed not only infrastructures of coastal fisheries and aquacultures but also coastal ecosystems, such as the seagrass and seaweed beds vital for sustainable fisheries. This study surveyed the seagrass and seaweed beds in Otsuchi Bay, Iwate prefecture in June 2011 and found that the seaweed beds around the mouth and middle of the bay were not impacted by the tsunami, but that the seagrass beds in the bay head had been destroyed. However, observations verified the growth of seagrass seedlings germinated from seeds produced in previous years. This response of the seagrass demonstrates its resilience to huge tsunami events occurring in an interval of several decades. Huge seawall construction plan against tsunami threatens their recovery. The recovery program in Sanriku Coast must be established on a holistic approach including social and ecological aspects between land and the sea because coastal areas are ecotone and also “sociotone” between them.

Did huge tsunami on 11 March 2011 impact seagrass bed distributions in Shizugawa Bay, Sanriku Coast, Japan?

Seagrass beds play important roles for coastal ecosystems as an ecosystem engineer and also as a habitat for fish and mollusks as spawning, nursery and feeding grounds, and provide us important ecological services. On 11 March 2011, huge tsunami hit Sanriku Coast, Japan, after the big earthquakes occurred in Northwestern Pacific Ocean. Seagrass beds were distributed on sandy or muddy bottom in Shizugawa Bay, Sanriku Coast. Thus, remote sensing research was conducted to evaluate impact of the tsunami on seagrass bed in Shizugawa Bay, Sanriku Coast. GeoEye-1 multi-band imageries taken on 4 November 2009 and 22 February 2012 were analyzed to map seagrass beds before and after the tsunami, respectively. Analysis of the former imagery showed seagrass beds were distributed in sheltered bottom against waves along the coast corresponding to seagrass distributions obtained through inquiry to fishermen and references on seagrass bed distributions before the tsunami. Analysis of the latter imagery indicated that seagrass bed distributions on 22 February 2012 were less than on 4 November 2009. Seagrass beds in the bay head disappeared while some seagrass beds remained behind the points along the north coast. This was verified by the field survey conducted in October 2011 and May and October 2012. Since the tsunami waves propagated into the bay along the longitudinal axis of the bay without crossing both sides of the bay, they produced only big sea-level changes during the propagation along the both sides from the center to the bay mouth. Their energy is concentrated the bay head and removes seagrass with sand and mud substrates. On the other hand, the tsunami higher than 12 m could not completely destroy seagrass beds due to topographic effect protecting seagrass from strong force by the tsunami. Thus, all seagrass weren’t destroyed completely in Shizugawa Bay even by the hit of the huge tsunami.

Mapping is a Key for Sustainable Development of Coastal Waters: Examples of Seagrass Beds and Aquaculture Facilities in Japan with Use of ALOS Images

Sound coastal ecosystems provide important ecological services such as food supply, nutrient cycling, and stabilizing effects of environments (Costanza et al., 1997). They are indispensable for sustainable development of coastal areas. However, human impacts such as fisheries or reclamation destroy coastal environments and ecosystems (e.g. Huitric et al., 2002). To conserve or restore sound coastal ecosystems, it is necessary to know present situation of coastal areas including coastal ecosystems such as seagrass beds, seaweed beds, coral reefs and tidal flats, and also human activities such as fisheries related facilities and reclamation. Diving and observation from the boat are usually used for checking bottom habitats. However, these methods are laborious and time consuming (Komatsu et al., 2002a). If a boat is employed to survey a broad area with many aquaculture facilities, it requires a long time to survey it due to obstruction of rafts against navigation. The survey with the boat is unsuccessful to localize many aquaculture facilities with GPS in the cases that the boat can’t easily approach to the aquaculture facilities due to ropes and rafts. Thus, it is desired to develop efficient mapping and monitoring systems of coastal areas.

Impact of huge tsunami in March 2011 on seaweed bed distributions in Shizugawa Bay, Sanriku Coast, revealed by remote sensing

Seaweed beds are very important for abalones and sea urchins as a habitat. In Sanriku Coast, these animals are target species of coastal fisheries. The huge tsunami hit Sanriku Coast facing Pacific Ocean on 11 March 2011. It is needed for fishermen to know present situation of seaweed beds and understand damages of the huge tsunami on natural environments to recover coastal fisheries. We selected Shizugawa Bay as a study site because abalone catch of Shizugawa Bay occupied the first position in Sanriku Coast. To evaluate impact of tsunami on seaweed beds, we compared high spatial resolution satellite image of Shizugawa Bay before the tsunami with that after the tsunami by remote sensing with ground surveys to know impact of the tsunami on seaweed beds. We used two multi-band imageries of commercial high-resolution satellite, Geoeye-1, which were taken on 4 November 2009 before the tsunami and on 22 February 2012 after the tsunami. Although divers observed the tsunami damaged a very small part of Eisenia bicyclis distributions on rock substrates at the bay head, it was not observed clearly by satellite image analysis. On the other hand, we found increase in seaweed beds after the tsunami from the image analysis. The tsunami broke concrete breakwaters, entrained a large amount of rocks and pebble from land to the sea, and disseminated them in the bay. Thus, hard substrates suitable for attachment of seaweeds were increased. Ground surveys revealed that seaweeds consisting of E. bicyclis, Sargassum and Laminaria species grew on these hard substrates on the sandy bottom.

Introduction of geospatial perspective to the ecology of fish-habitat relationships in Indonesian coral reefs: A remote sensing approach

Coral reef ecosystems worldwide are now being harmed by various stresses accompanying the degradation of fish habitats and thus knowledge of fish-habitat relationships is urgently required. Because conventional research methods were not practical for this purpose due to the lack of a geospatial perspective, we attempted to develop a research method integrating visual fish observation with a seabed habitat map and to expand knowledge to a two-dimensional scale. WorldView-2 satellite imagery of Spermonde Archipelago, Indonesia obtained in September 2012 was analyzed and classified into four typical substrates: live coral, dead coral, seagrass and sand. Overall classification accuracy of this map was 81.3% and considered precise enough for subsequent analyses. Three sub-areas (CC: continuous coral reef, BC: boundary of coral reef and FC: few live coral zone) around reef slopes were extracted from the map. Visual transect surveys for several fish species were conducted within each sub-area in June 2013. As a result, Mean density (Ind. / 300 m2) of Chaetodon octofasciatus, known as an obligate feeder of corals, was significantly higher at BC than at the others (p < 0.05), implying that this species’ density is strongly influenced by spatial configuration of its habitat, like the “edge effect.” This indicates that future conservation procedures for coral reef fishes should consider not only coral cover but also its spatial configuration. The present study also indicates that the introduction of a geospatial perspective derived from remote sensing has great potential to progress conventional ecological studies on coral reef fishes.

Marine habitat mapping: using ALOS AVNIR-2 satellite image for seagrass beds at Rabbit (Koh Tonsay) Island, Cambodia

The coastline of Cambodia stretches along the Gulf of Thailand including 69 islands. It supports rich diversity of marine species. Distributions of habitats including mangroves, coral reefs and seagrass beds along the pristine Cambodian coast still remains unknown compared to neighboring countries, Thailand and Vietnam. Cambodian seagrass beds form habitats with rich biodiversity and economical value through marine ecosystem services playing as a key role against climate change by reserving large amount of carbon. However, the general status of these seagrass habitats is poorly researched and documented. Satellite image of Advance Visible and Near Infrared Radiometer type 2 (ALOS AVNIR-2) with high resolution (10×10m) provides good information for seagrass habitat mapping. Study site was selected around Rabbit (Koh Tonsay) Island with area of 2 km2. The objectives of this study are (1) to know spatial distribution of seagrass beds around this island by ground survey and (2) map seagrass beds using the ALOS AVNIR-2 image with ground truthing data. Ground truth survey was conducted in June 2011. Surveys along three transect lines revealed 8 species of seagrasses belonging to Hydrocharitaceae (4 species) and Cymodoceaceae (4 species) around the island. We analyzed ALOS AVNIR-2 taken on 22 December 2009 to map distribution of seagrass beds around Koh Tonsay Island. Results showed that remote sensing using ALOS AVNIR-2 data provides a practical tool for mapping seagrasses beds around the island and information for future management and conservation of seagrass beds in Cambodia.

Detection of seagrass beds in Khunk Graben Bay, Thailand, using ALOS AVNI2 image

Coastal habitats having high productivity provide numerous ecological services such as foods, protection from strong waves through buffering effect, fixation of CO2 through photosynthesis, fostering biodiversity etc. However, increasing human impacts and climate change decrease or degrade coastal habitats. ASEAN region is developing most rapidly in the world. In the developing region, it is necessary to grasp present spatial distributions of habitats as a baseline data with standardized mapping methods. Remote sensing is one of the most effective methods for mapping. Japan Aerospace Exploration Agency (JAXA) provides non-commercial satellite images with ultra-high spatial resolution optical sensors (10 m), AVNIR2, similar to LANDSAT TM. Using ALOS AVNIR2 images it may be possible to make habitat map in the region. In Thailand, shrimp ponds cause degradation of coastal ecosystem through cutting mangroves and eutrophicated discharge from ponds. We examined capability of remote sesing with ALOS AVNIR2 to map seagrass beds in Khung Kraben Bay, Chanthaburi Province, Thailand, surrounded by shrimp ponds. We analyzed ALOS AVNIR2 taken on 25 January 2008. Ground truth survey was conducted in October 2010 using side scan sonar and scuba diving. The survey revealed that there were broad seagrass beds consisting of Enhalus acroides. We used a decision tree to detect seagrass beds in the bay with quite turbid seawater coupled with Depth-Invariant Index proposed by Lyzenga (1985) and bottom reflectances. We could succeed to detect seagrass beds. Thus it is concluded that ALOS AVNIR2 is practical to map seagrass beds in this region.

Spectral classifying base on color of live corals and dead corals covered with algae

Pigments in the host tissues of corals can make a significant contribution to their spectral signature and can affect their apparent color as perceived by a human observer. The aim of this study is classifying the spectral reflectance of corals base on different color. It is expected that they can be used as references in discriminating between live corals, dead coral covered with algae Spectral reflectance data was collected in three small islands, Spermonde Archipelago, Indonesia by using a hyperspectral radiometer underwater. First and second derivative analysis resolved the wavelength locations of dominant features contributing to reflectance in corals and support the distinct differences in spectra among colour existed. Spectral derivative analysis was used to determine the specific wavelength regions ideal for remote identification of substrate type. The analysis results shown that yellow, green, brown and violet live corals are spectrally separable from each other, but they are similar with dead coral covered with algae spectral.

Damage to seagrass and seaweed beds in Matsushima Bay, Japan, caused by the huge tsunami of the Great East Japan Earthquake on 11 March 2011

ABSTRACT Damage to seagrass and seaweed beds caused by the huge tsunami of the Great East Japan Earthquake on 11 March 2011 was investigated in Matsushima Bay, Miyagi Prefecture, Japan, by comparing satellite images taken before the tsunami in November 2009 and after the tsunami in May 2011. The results showed that the tsunami destroyed 220 ha of seagrass and seaweed beds, from 320 ha in 2009 to 100 ha just after the tsunami. Zostera marina beds on the sandy and muddy bottom were rather severely damaged compared with brown seaweed beds of Sargassum horneri on the rocky substrate. Substrates of the beds and directions of the tsunami seem to be responsible for the magnitude of the damage to the beds. Maps of the spatial distributions of the beds before and after the tsunami can provide information for the restoration of seagrass and seaweed beds as natural infrastructures of coastal fisheries resources.