Journal of Oceanography, Vol. 58, pp. 93 to 107, 2002 Review Progress in Seto Inland Sea Research HIDETAKA TAKEOKA* Center for Marine Environmental Studies, Ehime University, Bunkyo, Matsuyama 790-8577, Japan (Received 14 May 2001; in revised form 1 September 2001; accepted 5 September 2001) The Seto Inland Sea is a representative coastal sea in Japan with a complicated geom- Keywords: etry and thus a variety of marine environments. This sea is, at the same time, one of ⋅ Seto Inland Sea, the most industrialized areas in Japan, and its marine environment has been signifi- ⋅ anthropogenic cantly affected by the anthropogenic impacts over the last four decades. The wide impacts, ⋅ range of marine environments in this sea and the serious environmental issues result- heat bypass, ⋅ nutrient trap, ing from these impacts have attracted the attention of Japanese coastal oceanogra- ⋅ tide and tidal phers. It is believed that the nature and scope of these studies might be an example of current, the progress of Japanese coastal oceanography. The historical changes in the Seto ⋅ red tide, Inland Sea environment in the last four decades are briefly summarized, and the ⋅ oxygen-deficient progress in the studies of the Seto Inland Sea is reviewed with reference to historical water mass, changes. Some recent research topics and activities are also mentioned. ⋅ fronts. 1. Introduction studies might be a good example of the progress of Japa- The Seto Inland Sea in Japan is a semi-enclosed nese coastal oceanography. This paper therefore presents coastal sea surrounded by Honshu (the main island of a review of these studies. In Section 2, historical changes Japan), Shikoku and Kyushu Islands (Fig. 1). It has a in the marine environment of the Seto Inland Sea are dis- length of 500 km, an average depth of 30 m and contains cussed as a background to the present paper. The various approximately 600 islands. The sea is divided by islands individual studies are then discussed in Section 3. Ad- and peninsulas into wide basins, some of which are called vances in Seto Inland Sea studies are also reviewed in “nada” in Japanese, and these basins are connected by Section 3, classified according to subject. Recent research narrow channels called “seto”. This complicated geom- topics and activities are mentioned in Section 4. etry results in wide variations in the marine environment. The Seto Inland Sea region is also one of the most 2. Historical Changes in the Seto Inland Sea Envi- industrialized areas in Japan. After the New Industrial City ronment Law was enacted in 1963, many facilities for heavy in- Figure 2 shows a chronological table of events af- dustry were built in the coastal areas surrounding the sea. fecting the Seto Inland Sea over the last four decades, Urbanization of the coastal area also increased. At present, along with a description of completed studies concerning approximately 35 million people live within the Seto In- to the region and the annual number of red tide occur- land Sea watershed. This industrialization and urbaniza- rences in the region. The number of red tide incidents tion required substantial reclamation of land. The marine can be used as an indicator of eutrophication. During the environment of the Seto Inland Sea has been significantly 1970s, eutrophication advanced rapidly due to an increase affected by these impacts over the last four decades. in the volume of industrial and urban waste, resulting in The wide range of marine environments in the Seto a frequent occurrence of red tides and oxygen-deficient Inland Sea and the serious environmental issues result- water masses. These occurrences had a great impact on ing from anthropogenic development in the region have the marine environment. In 1970, a mass mortality of fish attracted the attentions of Japanese coastal oceanogra- occurred in Hiuchi-Nada due to an oxygen-deficient wa- phers. It is believed that the nature and scope of these ter mass. Large-scale red tides of Chattonella often oc- curred in Harima-Nada. The red tides that occurred in * E-mail address: [email protected] 1971 damaged the fish farming industry up to the value Copyright © The Oceanographic Society of Japan. of 7.1 billion yen. 93 Fig. 1. Map of the Seto Inland Sea. Lines in the inland sea denote the routes of the ferry boats (see Subsection 3.8). To solve these problems, the Environmental Agency 3. Progress in Seto Inland Sea Research of Japan enacted “The Interim Law for Conservation of the Environment of the Seto Inland Sea” in 1973 and “The 3.1 Basic descriptive studies Law Concerning Special Measures for Conservation of The Prefectural Fisheries Observatories around the the Environment of the Seto Inland Sea” in 1978. These Seto Inland Sea started monthly observations of water laws resulted in a COD reduction to some extent. In ad- temperature, salinity and transparency at 320 fixed sta- dition, the annual number of red tides gradually reduced tions in 1964 in a project supported by the Fisheries between the mid 1970s and the mid 1980s, but has re- Agency. The Maritime Safety Agency accumulated tide mained stable since then. and tidal current data. In the 1970s and 1980s, many de- Prior to 1973, land reclamation was intensive. In scriptive studies summarizing these data were completed. average, approximately 16 km2 was reclaimed annually Yanagi and Higuchi (1979) analyzed the historical tidal between 1965 and 1973. This was regulated by the In- current data measured by the Maritime Safety Agency, terim Law for Conservation of the Environment of the and produced a chart of residual current flow patterns. Seto Inland Sea, but still took place to some extent after Yanagi and Higuchi (1981) also produced charts of the the law was introduced. The total area of land reclaimed amplitude and phase lag of the M2 and K1 constituents of since 1965 is approximately 250 km2, which is approxi- tide and tidal current. Summarizing the historical mately 12% of the area with a depth of less than 10 m in hydrographic data obtained by fisheries observatories, the Seto Inland Sea. More than half of the marine forest Takeoka (1985) revealed the distribution of stratification that existed in the early 1960s has been lost by reclama- in the Seto Inland Sea. Takeoka (1987) also described the tion. transparency distribution and analyzed the seasonal and Another environmental issue that has recently be- spatial differences in the distribution. come acute is the dredging of sand and gravel from the Among the results of these studies, distributions of seabed. The great demand for building materials and the M2 tide and M2 tidal current are shown here, because the lack of suitable quarries in western Japan led to an in- tide and tidal current are the most basic and important crease in dredging activity. Sea-sand dredging increased factors characterizing the Seto Inland Sea and the M2 rapidly in the late 1960s, thereafter 2 × 107 m3 of sea- constituent is dominant over almost the whole Sea. Fig- sand has been dredged annually. However, some prefec- ures 3(a) and (b) indicate the distributions of the tidal tures surrounding the Seto Inland Sea prohibited dredg- range and the phase lag of the M2 tide, and Figs. 3(c) and ing due to public pressure. (d) the distributions of the amplitude and the phase lag of 94 H. Takeoka Fig. 2. Chronological table of the events related to the Seto Inland Sea and the themes and projects of the Seto Inland Sea studies. Progress in Seto Inland Sea Research 95 Fig. 3. Distributions of (a) tidal range, (b) phase lag of tide, (c) current amplitude and (d) phase lag of tidal current of M2 constituent in the Seto Inland Sea (after Yanagi and Higuchi, 1981). Areas where the current amplitude is larger than 70 cm s–1 are hatched in (c). the M2 tidal current (Yanagi and Higuchi, 1981). In the features can be seen from the tide and tidal current phase Pacific Ocean, south of the Seto Inland Sea, tidal waves distribution plots. The phase of the tide is almost equal in propagate from east to west at significant speeds due to the central region and differs spatially in the eastern and the great water depth. Hence the phase of the tides at the western regions. The phase difference between the tide mouths of the Kii and Bungo Channels (see Fig. 1) are and tidal current is approximately 90° in the central re- approximately equal (Fig. 3(b)). The tidal waves propa- gion and is much smaller in the eastern and western re- gate at a much lower speed in the Seto Inland Sea due to gions. As a result of such tidal features, the flood tidal the shallow water depth. The waves propagate from the currents are directed to the area into which the tidal waves two channels into the inland sea over a long period of assemble. Moreover, the transport volume of the tidal time and meet at the central part of the Seto Inland Sea current is larger in the outer regions, and almost vanishes between the Bisan Strait and Hiuchi-Nada where the phase in the assembling area. Therefore, except in the narrow lag of M2 tide is the largest. They are delayed by about straits, the amplitude of the tidal current is generally larger 150° from the mouths of the channels (Fig. 3(b)). Both in the outer regions and smaller in the eastern part of tidal waves entering from the channels are dissipated Hiuchi Nada (Fig. 3(c)). In the narrow straits the ampli- during propagation. Therefore, the amplitudes of the tidal tudes of the tidal currents are much larger than those in waves propagating eastward and westward are approxi- the basin interior.