The region of Tokyo Bay, inside an imaginary line drawn between Point Honmoku and Cape Futtsu, is characterized by a piled configuration, water depth under 30 m, simple sea-bottom layout, and smooth coastal lines. Reclamation operations have been carried out in some areas of the bay where the water depth is less than 5 m. Since 1912 when the first project was conducted along the waterfront of Kawasaki, approximately 290 km2 have been reclaimed. This area represents more than 20 percent of the total area of Tokyo Bay (Figure 8).
To the south of Nakanose, the water is deep and provides an entrance to the bay. This entrance is characterized by a crooked channel 6 km wide and 50 m deep; off the coast of Kurihama, the depth increases to about 200 m.
When Tokyo Bay is represented by the area north of an imaginary line extended between Point Kannon and Cape Futtsu, the following specifications characterize the bay. It has a surface area of about 1 000 km2, an average water depth of about 18 m and a capacity of about 17 900 million m3. In addition 7 000 km2 of river basin area belong to Tokyo Bay, and about 10 000 million m3 of fresh water run into the bay each year.
The climate around the bay area is mild throughout the year, as indicated by the mean temperatures and humidities for the four seasons: spring (April) 13.5°C and 66 percent, summer (July) 25.2°C and 79 percent, autumn (October) 16.9°C and 77 percent and winter (January) 4.1°C and 57 percent. Extremes in air temperature are represented by those for the Yokohama area which records a minimum of about 6°C in February and a maximum of about 29°C in August.
Precipitation in the bay area is high in the summer season and low in the winter. The typhoon season during September and October brings considerable rainfall as does the seasonal rainy period from June to July. The total number of rainy days is from 100 to 120 each year; the annual precipitation is about 1 600 mm.
Wind conditions above the bay are characterized by a seasonal northerly wind which is predominant in the winter, and a seasonal southerly wind which is due to the Ogasawara high pressure area in the summer.
In general the residual current in the bay is righthanded and the surface water tends to flow out of the bay while the bottom layer of water tends to flow into it (Figure 9 - 1 and 2).
The velocity of the tidal current is rather high in the central area of the bay and low in the coastal regions. A northeasterly current is predominant at high tide, while a southwesterly one prevails at low tide (Figure 10, 1 and 2).
A conspicuous hydrographic feature is the formation of a distinct vertical density stratification during the summer season (Figure 11).
Ever since consolidated works were first initiated in Tokyo Bay in 1887, reclamation projects along the coastal regions have been increasing. Unfortunately, in the process of land reclamation, some coastal areas which were excellent fishing grounds right back to the Edo period have gradually been eradicated.
Water pollution first became conspicuous in 1925 and since then has increased in degree and severity. Pollution problems stemming from urbanization frequently occurred even during the prewar period from 1933 to 1936 and caused enough concern to warrant the instigation and establishment of a water pollution control commission. This commission was delegated authority to implement counter-measures for the prevention of water pollution-related damage. Pollution in those days, however, was far less a problem than it is today.
During the period from 1944 to 1948, when the war and its subsequent repercussions were severely felt, the water systems of the Koto district and that of the lower Sumida River became clean enough to be inhabitable by fish and to be suitable for recreational fishing. This was attributed to a decrease in the discharge load of municipal waste water due to destruction of industrial plants as well as residential dwellings. However, serious aggravation of the quality of coastal water accompanied the rapid postwar reconstruction of the Tokyo metropolitan area. Two factors were largely taken as responsible for this condition - the high rate of migration of rural inhabitants into the metropolis and the development of many types of industries - and the problem has been increasing precipitously ever since 1950. Legal disputes focussing on water pollution have been frequent since 1956. Particularly noteworthy was the year 1958, when water pollution of rivers in urban areas was seriously aggravated by a water shortage in the metropolitan area. The result was an outbreak of an offensively odorous gas, particularly in those areas bordering the banks and coastal region of the Sumida River. Not only did the gas give off a highly disagreeable smell, but it also caused corrosion of many structures including culturally precious national treasures of the well-known temples located in the Asakusa area as well as metallic objects including household appliances.
Concurrent with this turn of events, an increased flow of traffic into Tokyo Bay by domestic and foreign vessels as the result of the high tempo of postwar economic growth brought with it deep anxiety over the effects on the local fisheries such as laver culture (Table 9).
A rise in the number of outbreaks of red tide in the bay has been witnessed since 1950. Large scale outbreaks involving practically the entire bay area first occurred in 1954 (Figure 12). The frequency of outbreaks has been highest in the summer season and has tended to increase from year to year during this season (Figure 13). It has been demonstrated that some of the outbreaks have been due to diatoms, but that in more recent years, damage due to flagellates has been on the increase (Figure 14). Typical red tide organisms are given in Figure 15.
In this section discussion focusses on a description of the results of a comprehensive water quality survey undertaken jointly by the Tokyo metropolitan government and those of the Chiba and Kanagawa prefectures during the period between 1970 and 1972.
The water quality of Tokyo Bay is characterized by considerable seasonal variation. As shown by changes in transparency, which is the simplest index of water quality, the water of the bay has been found to be relatively clear during the season of active circulation showing a value of about 3 to 7 m. During the season of reduced circulation, the water has an olive colour, and transparency is less than 1 m. Seasonal variation in transparency is shown in Figure 16, while yearly fluctuation is illustrated in Figure 17.
The distribution of total organic carbon (TOC) of the surface water during the circulating season in February is depicted in Figure 18 which illustrates the following features: TOC values of 1.5 mg C/1 at the entrance to the bay, 2 mg C/1 at the head, and 3 mg C/1 along the coastal areas from Kawasaki to Tokyo. These values reflect the background influx load, while in August, when circulation is less, they are generally two to three times higher than those given for February, e.g. 3 mg/C/1 at the entrance to the bay and greater than 6 mg C/1 at the head. Chemical oxygen demand (COD) attains a maximum during the stratifying season and declines during the circulating season (Figure 19). Dissolved oxygen (DO) is about 95 percent saturation in all layers during the circulating season; in the season of reduced circulation it is about 200 percent in surface water and lower in the bottom layer.
Measurement of chlorophyll a demonstrates a high concentration of about 70 μg/l during the season of reduced circulation in the surface layer. Phytoplankton were observed to reduce transparency, cause high values in TOC and COD, and to create conditions of supersaturation in DO. This phenomenon typically accompanies water pollution and is attributed to eutrophication. Concentrations of nutrient salts such as total nitrates and phosphates, which induce secondary pollution due to organic matter, are 1.5 ppm and 0.2 ppm, respectively, within the bay; similar values at the entrance to the bay are 0.5 ppm total nitrates and 0.09 ppm total phosphates (Figure 20, 1 and 2). These values are considerably greater than the standard values of 0.2 ppm nitrogen and 0.015 ppm phosphorus, which represent indices for the prevention of red tide outbreaks. They are indicative of conditions best described as super-eutrophication.
A survey of bottom deposits in the bay was conducted during the three-year period from 1971 to 1973. The “mud content ratio” was employed as an indirect index of pollution, since the amount of organic matter increases in the transition of bottom sediments from sandy to clayish composition. The mud content ratio is under 50 percent in all areas of the bay with the exception of the entrance and the off-shore coast of Funabashi which is located at the bay head, and is less than 40 percent in the entire area of the bay head bordered by imaginary lines drawn between the port of Tokyo and Makuhari, and between Point Honmoku and Point Banshu (Figure 21 - 1).
Ignition loss was 5 to 10 percent for both the entrance and area of the bay head, while values between 10 and 16 percent were found for other areas (Figure 21 - 2). Seasonal variations in both mud content ratio and ignition loss were not clearly recognized.
As for sulfide concentration, a restored condition was recognized in October; high concentrations which exceeded 0.5 mg S/g were found in the western half of the bay between the mouth of the Ara River and Point Honmoku, while values of 0.2 to 0.3 mg S/g were detected in other areas including the entrance to the bay (Figure 21 - 3).
These results indicated that pollution of bottom deposits due to organic matter was at a rather high level. It was especially conspicuous during the summer season when the water of the bottom layer was characterized by an oxygen-deficient condition, and practically all areas of the bay were brought into a restored condition. This phenomenon resulted in an expansion of lifeless benthic areas.
Regarding heavy metals, the horizontal distribution of total mercury (T-Hg), cadmium (Cd), chromium (Cr), lead (Pb), and arsenic (As) are shown in Figure 22. T-Hg was detected at concentrations ranging between 0.5 and 1.0 ppm in the western part of the bay from the port of Tokyo to the bay entrance, and in that part of the bay from Yokohama to Kisarazu. However, other areas were not found to be excessively contaminated at high concentrations of T-Hg. The concentration of Cd was 1 to 3 ppm in the western region of the bay head from the mouth of the Ara River to Point Honmoku, and this relatively high level was found to extend out into the area adjacent to Chiba prefecture. The concentration of Cr was greater than 100 ppm in the western part of the bay head, and a high concentration also extended across the bay into the area adjacent to Chiba. Lead was found distributed in all parts of the bay except for the western region at concentrations under 100 ppm. A high concentration of As was recognized in the coastal regions of the bay head from the port of Tokyo to Ichihara. High concentrations exceeding 15 ppm were recognized at the mouth of the Ara River as well as in the port of Tokyo. Mean concentrations of the five heavy metals detected in sediments sampled from all areas of the bay over the three year survey period are shown in Figure 23.
Results of the survey showed that ignition loss (as an indicator of organic substances in sediments) decreased from 25 percent in the surface deposits to 15 percent in lower layers to a depth of 10 cm; no change in ignition loss was recognized in sediments from layers beneath the upper 10 cm. Concentrations of T-Hg, Cr, Pb and Cd were high in sediment down to a depth of 15 cm, indicative of recent heavy metal pollution.
Changes in fishery yield during the past several decades are shown in Figure 24 - 1, 2, 3, 4 and 5. Some general tendencies are described in the following paragraphs:
In the vicinity of the Haneda-Tama River (Station A), the yield of clam has been decreasing since 1948, and the species has become practically non-existent since 1957. In the area surrounding the mouth of the Ara River (Station B), the yield increased up until 1952 but then decreased significantly from 1957. The yield of short-necked clam at station A has been increasing since 1948, and reached a maximum at station B in 1952. The species is found in the vicinity of station B at a fairly high density. Oyster yield was practically constant up to 1960, then declined significantly. According to a survey taken in 1960 a large population of ark shell was found inhabiting the bay area, even in the vicinity of the mouth of the Edo River. Up to 1962, the yield was almost constant at a level of approximately 20 000 tons.
The yield was 300 tons in 1956 but has declined significantly since 1962 and fell to nearly five tons in 1969.
General fluctuations have characterized the yield of these invertebrates. It decreased remarkably from 1961 and crustaceans had almost totally disappeared by 1970. However, as of 1975, they have reappeared around the mouth of the Edo River. As for prawns, over 100 tons were produced up to 1965, but production has decreased since 1967 and has completely ceased at the present time.
The yield of sardine has tended to decrease since 1958, when tonnage was 3 500. Open ocean species including skipjack, round herring, red sea bream and spanish mackerel practically disappeared in the period between 1961 and 1964; at the same time, fishes of the inland sea including flatfish, gray mullet and goby have maintained nearly constant levels, while the yield of common sea bass has increased.
The yield has fluctuated yearly but is generally on the increase.
The environmental pathway of various pollutants into areas of the Seto Inland Sea has been found to be extremely complicated as shown in the diagram given in Figure 25. Within this region, there are many shoaling beaches and inland bays supporting a population of more than 20 million inhabitants as well as providing a convenient location for the operation of littoral industries. The establishment of industries in this region has been encouraged since 1955. This has resulted in the growth of a heavy chemicals industrial complex during the period of precipitous economic development. Accompanying this period of technological progress, water pollution has increased in the region of the Seto Inland Sea due to the heavy influx of discharged industrial wastes.
Table 10 provides some indication of the trends in water quality which have occurred during the past several decades by comparing the results of recent water quality surveys with that conducted in 1952. As an index of the general state of water pollution, transparency is seen to have decreased over the years, while the total amount of nitrogen has increased. These indices are indicative of the higher level of water pollution in the Seto Inland Sea during recent years.
Regarding pollution counter-measures, strict effluent standards have been established for waters off the littoral industrial zone and along the waterfront areas of large cities largely as a result of the “Water Pollution Control Law” which was enforced in June 1971; moreover, monitoring and surveillance systems have been firmly established. Favourable results stemming from such legislation are presented in Table 11 from which it can be seen that the water quality in the major industrial areas improved following the enforcement of strict effluent regulations.
Prior to the “Water Pollution Control Law” the “Seto Inland Sea Conservation Law” was enacted on 2 November 1973 in an attempt to bring about a halt to further aggravation of the marine pollution situation. The first interim objective has been to reduce the pollution load (measured in terms of chemical oxygen demand) by 50 percent of the 1972 value; the projected date for realization of this goal has been set for November 1976. The prefectures concerned bordering the Seto Inland Sea have been required to establish and/or reinforce effluent standards to achieve this goal. It is hopefully anticipated that the water quality around the waterfronts of the industrial zones will improve once implementation of these counter-measures has been fully realized.
The results of monitoring and surveillance which have provided the groundwork for enactment of environmental quality standards are presented in Table 12. The results indicate that the frequency at which toxic substances have been detected to exceed the standard values have decreased to a level of 0.23 percent in 1973, reflecting annual improvement on a nation-wide basis. In the Seto Inland Sea, the incidences at which pollution guidelines were exceeded have been almost constant and at lower levels than the nation-wide averages during the same years; values were 0.22 percent in 1971, 0.21 percent in 1972 and 0.22 percent in 1973.
Regarding the recent rate of achievement for environmental quality standards in rivers, sea areas, lakes and marshes around the Seto Inland Sea, BOD levels in rivers decreased to 58 percent in 1973 from 64 percent in 1972, while COD levels in the sea areas increased to 83 percent in 1973 from 70 percent in 1971, and were followed by a decrease to 80 percent in 1973. In lakes and marshes COD values have been gradually increasing from 1971, and high achievement rates on the order of 82 percent were attained in 1973, indicating improvement of environmental quality. This may have been partly due to the fact that the amount of water discharged into rivers decreased in 1973 due to the occurrence of a nation-wide water shortage. Moreover, this apparent improvement may have been related to the fact that relatively high rates of achievement have been obtained for lakes and marshes, more of which are formerly listed as water bodies each year. In general, the percentages of water bodies which met the environmental quality standards have recently been high for rivers and sea areas, and low for lakes and marshes.
Table 12 (b) shows the comparison between annual mean values and those of the environmental quality standards for COD and BOD values in each type of water body. As for rivers, the number which met the environmental quality standards rose, while for sea areas the percentages went up to a level of 80 percent in 1972, increasing from 60 percent in 1971 and remaining almost unchanged at 79 percent in 1973. Furthermore, two lakes which had been newly categorized as water bodies in 1973 met the environmental quality standards, and this resulted in an increase in the rate from 0 percent in 1971 to 33 percent in 1973.
A comprehensive water quality survey project was implemented in the Seto Inland Sea area in 1972 in an attempt to clarify the pollution conditions and the processes which have created the situation. Figure 26 shows the horizontal distribution of surface COD in 1972, and Table 13 indicates the changes in the distribution of COD in the Seto Inland Sea area. The percentage of those areas where COD value was under 1 ppm decreased from 1972, while those exceeding 1 ppm increased. In particular, a high percentage (14.1 percent) was seen for bodies of water where values exceed 3.1 ppm. Such areas were not suitable for fisheries. This suggests that still greater efforts are needed to prevent pollution in such areas.
Thus it is apparent that the water quality as represented by COD value has gradually improved at some of the waterfronts of industrial areas in the Seto Inland Sea. However, it is still difficult to predict any improvement on the basis of surveys for environmental quality standards as described above, because of complications such as increases in survey stations, survey frequency, and yearly changes in water quality.
Data reflecting the number of red tide outbreaks in the Seto Inland Sea is presented in Figure 27. It is evident that the number of cases which have actually resulted in damage to fisheries has tended to decrease since 1971, while the total number of cases is still on the increase. This possibly indicates that eutrophication is still advancing in some areas of the Seto Inland Sea, though the increase in the number of red tide cases reported may be the result of better reporting via a red tide information monitoring network. The accumulation of nutrient salts by bacteria in water bodies tends to accelerate eutrophication which in turn induces secondary pollution due to organic matter caused by abnormal outbreaks of red tide plankters. Table 14 shows the classification of such plankters which were identified in red tide outbreaks during 1974.
The data presented in Table 15 reflect the present state of PCB pollution in coastal waters of the Seto Inland Sea, and demonstrate that these waters are some of the most highly polluted in Japan. The permissible limits for PCBs in fish and shellfish, effluent water and bottom deposits which have been established by the Environmental Agency are less than 3 ppm, less than 0.01 ppm and less than 100 ppm, respectively. Those areas which exceeded these limits included Osaka Bay, Harima Sound, the waterfront at Iwakuni, Beppu Bay and Aita Bay. In those areas where fish were found contaminated with PCBs exceeding the limit of 3 ppm, legislative action prohibited the sale of polluted fish and moreover established a directive whereby industries causing pollution problems would be held liable for the purchase of polluted fish from fishermen. In addition, provisions have been enforced whereby bottom deposits containing PCBs at concentrations greater than 100 ppm become free from pollution through dredging or reclamation operations.
In the coastal waters of the Seto Inland Sea those areas found to be most heavily polluted with mercury included the sea fronts at Tokuyama, Niihama and Mizushima. A survey revealed that fish and shellfish contained mercury at concentrations exceeding the provisional control limits. It was disclosed that two plants engaged in the production of sodium hydroxide (caustic soda) were located in Tokuyama Bay (the Tokuyama Soda and Toyo Soda Companies). The results of a mercury pollution survey of fish and shellfish indicated that mantis shrimp which were caught in Tokuyama Bay contained 3.32 ppm total mercury. A comprehensive survey of mercury pollution was implemented in 1973 by the Environmental Agency, and the pollution sources of mercury and its environmental levels in the Seto Inland Sea are presented in Table 16. In the survey, it was discovered that five species of fish including common sea bass, black porgy, kelp greenling, surfperch, and rockfish which were caught in Tokuyama Bay exceeded the provisional standards. As a consequence, fishery operations in the bay have been extensively curtailed through legislative control.
Counter-measure operations for mercury polluted sludges have been classified into two categories according to level of contamination. Sludges contaminated at levels ranging from 16 to 32 ppm are used in land reclamation projects, while those ranging from 15 to 20 ppm are dredged.
In Hiroshima Prefecture periodic monitoring of heavy metals and bacterial examination of oysters - a popular product of the prefecture - have been carried out since 1950. The programme was initiated to protect public health. Additionally, routine analyses of zinc, chromium, cadmium and mercury in water, bottom sediments and oysters have been conducted since 1968 during the harvesting season in November in all areas of the prefecture where oysters are cultured. The average values of heavy metals detected in oysters from 1968 to 1972 are shown in Table 17. The survey revealed that oysters collected along the eastern coastal region were contaminated with heavy metals at unusually high levels - from 2 to 5 ppm. Announcement of these levels through the news media created considerable concern among consumers regarding the potential health hazard of pollutant metals, and the oyster fishery suffered enormous damage and financial loss.
Pollution of bottom sediments in the Seto Inland Sea is considerable. The environmental water quality standards for fishing grounds with respect to sludge are as follows (on a dry weight basis): less than 20 milligrams/gram in COD, less than 0.2 mg/g in sulfides, less than 0.1 percent in n-hexane extracts, and “not detectable” in soluble toxic substances. Marine areas where COD value of bottom deposits exceeded the standard of 20 mg/g included the greater part of Osaka Bay, the east coast and central area of Harima Sound, the central area of Bisan Seto, the entire Hiuchi Sound, Bingo Sound, Hiroshima Bay, the southern part of Suo Sound and the entire Beppu Bay and Uwa Sea. Figure 29 shows values for COD, total sulfide and ignition loss as determined by the Southwest Seas Regional Fisheries Laboratory in October 1972.
Accompanying an increase in the volume of oil transported in the Seto Inland Sea, marine pollution created by unlawful dumping of oil by ocean going vessels, discharge of land-stored petroleum products and leakage or spillage resulting from maritime accidents involving tankers have been steadily increasing and have brought about a serious problem. The number of cases of marine oil pollution has increased yearly from 1969 to 1973 (Figure 30). Examination of the different sea regions which comprise the Inland Sea reveals that the number of cases of marine oil pollution is nearly proportional to the intensity of vessel traffic (Figure 31). The number of cases in regions adjacent to littoral industrial zones was relatively low in 1974. It is also noteworthy that vessels other than tankers showed a rather high incidence of involvement in oil pollution (Figure 32).
On 18 December 1974 an accident at the Mizushima Oil Refinery of the Mitsubishi Co. Ltd. resulted in leakage of about 43 000 kl of type “C” heavy oil from a storage tank. An estimated 7 500 to 9 500 kl inadvertently escaped into the neighbouring waters. The oil slick was widely dispersed in Mizushima Sound from the western part of Harima Sound to Kii Channel, and resulted in extensive damage and loss to local fisheries. Since the accident occurred in a semi-enclosed body of water protected from the open ocean, the oil slicks were eventually washed ashore causing extensive damage to nearshore fisheries such as seaweed (laver) and yellowtail culture. Furthermore, vertical dispersion of oil into the water and deposition on the bottom caused concern over detrimental effects on the benthic ecosystem. (Supplementary Figure III).
As can be seen from the preceeding discussion, water pollution in the Seto Inland Sea has come to be more of a serious environmental problem due to the effects of secondary factors including pollution stemming from organic matter and accidental oil spillage.
The seaweed or kelp beds in the Seto Inland Sea play an important role in marine production; disappearance of such areas of productivity would inevitably result in the extinction of many species of living resources endemic to or those cultivated in the area.
Seaweed beds in the Seto Inland Sea have been decreasing in area yearly. The area of the eel grass (“amamo”, Zostera) beds classified according to echo-sounding records prior to 1965, and the rate of reduction up to 1965, are shown in Figure 33. Similar data showing the area of Zostera zones in the different regions of the Seto Inland Sea in 1965 together with area reduction rates between 1965 and 1971 are presented in Figure 34. The decline in the area of these beds has been primarily attributed to reclamation operations for the creation of industrial zones, and secondarily to water pollution caused by sludge during such reclamation work or industrial waste water discharge following reclamation. The magnitude of land reclamation in terms of the area of land reclaimed annually from 1955 to 1970 is shown in Figure 35. The area of land reclaimed in the period 1955 to 1969 in the Seto Inland Sea according to prefectures bordering the Sea is given in Table 18, while Table 19 gives data on the area of land which has been reclaimed in different zones or regions in the Sea up to December 1971. It can be readily seen from these tables that approximately 18 000 ha of land were reclaimed from 1955 to 1969, and the projected plan as of 1 December 1971 involved a total of about 70 000 ha. The total land area involved in these reclamation projects accounts for about 35 percent of the total shallow water fishing grounds in the Seto Inland Sea.
Another aspect of the land reclamation operations is that they are generally carried out in coastal areas and result in the deformation of natural coastlines. The present state of coastline utilization as surveyed by the Environmental Agency in 1974 is presented in Table 20. Of a total 6 500 km of coastline, 19 percent of the coastal shoreline is on the mainland while 26 percent surrounds islands. This indicates that most of the coastline areas intended for reclamation work were completed in 1974.
It is quite obvious that deformation of coastlines eliminated some areas of scenic beauty, and in less aesthetic terms the loss of beaches due to embankments has reduced the capacity to decompose organic matter by inhibiting oxygen dissolution and self-purification processes.
Based on such considerations, it can therefore be seen that the effects of reclamation on the marine environment and marine resources are of tremendous impact and significance. Thus, it goes without saying that extremely careful and prudent evaluation and planning should be given to any proposal concerning land reclamation.
Among the thousand or so species of living organisms which inhabit the Seto Inland Sea, those of importance to fisheries are the following: about thirty species of fish such as red sea bream, black porgy, Spanish mackerel, anchovy, sand lance, conger eel, gray mullet, rockfish, lizard fish and flatfish; ten species of crustaceans such as prawn and swimming crab; sixteen species of molluscs such as common cuttlefish, Pacific octopus, short-necked clam, and cockle; and “wakame” seaweed.
The fisheries of the Seto Inland Sea can be divided broadly into two types - natural resource and mariculture. The fishery yield in 1972 is shown in Table 21; that for marine fisheries was 25 tons/year/km2. Adding that derived from mariculture production, the total is about 35 tons/year/km2. These statistics reflect very high productivity of the Seto Inland Sea in comparison to other marine regions.
In general and simple terms, a high level of water pollution results in decreased yield. When relatively small scale bodies of water such as bay heads, lakes and marshes are badly polluted, living organisms disappear. However, in a relatively large body of water such as the Seto Inland Sea, the increase of water pollution is a more gradual process, although there may be cases of localized rapid advancement. Bodies of water are classified into two categories, oligotrophic and eutrophic, according to concentrations of nutrient salts such as nitrogen and phosphorus. In polluted areas, eutrophication proceed due to the influx of industrial and residential waste waters. During the incipient stages of eutrophication, basic productivity increases; and as it advances, carnivorous fish decrease in number while herbivorous ones increase. This condition is suitable for the culture of laver, though productivity is occasionally reduced since this condition is also suitable for outbreaks of red tides.
Fisheries production in the Seto Inland Sea has shown a steady increase ever since the late 1920s, and annual yield on the basis of catch statistics reveals steep increases with the exception of a period of stagnation from the mid-1950s (Figure 36). In this regard, however, careful consideration should be given to changes in production composition. For example, catches of red sea bream, sharp-toothed eel, shrimp and swimming crab have suffered drastic declines since 1953 (Figures 37, 38 and 39). Important shoals of these fishes and crustaceans are located in the Seto Inland Sea with the exception of conger eel. A loss of excellent fishing grounds in the shallow areas of the Inland Sea has resulted from water pollution. The decline in populations has been caused by the influx of industrial and resident waste water, sludge sedimentation, and land reclamation.
A tendency has been observed towards increased catches of anchovy, sand lance, flat-fish, hairtail and conger eel among fish species; cuttlefish, sea cucumber and short-necked clam among molluscs; and “wakame” seaweed (Figures 37 and 38). This may perhaps be due primarily to increases in nutrients and foods which accompany eutrophication in the Inland Sea. The above-mentioned species are among those which are tolerant of marine pollution, since increases in productivity have been witnessed during times of recent polluted conditions.
Mariculture relies on the productivity of a given marine region as well as on artificial rearing facilities with adequate and sufficient feeding, and it is obvious that yields are ultimately determined by environmental conditions as well as culture techniques. Two examples can be given. The production of laver quadrupled between 1955 and 1970. This was accomplished through the development of culture techniques including artificial spat collection and a floating system together with expansion of culture grounds further offshore to escape the adverse effects of nearshore eutrophication. Similarly, the yield of cultured yellowtail quintupled during the same period, but in recent years has been damaged significantly by large-scale outbreaks of red tide. Programmes and centres for artificial fisheries propagation are considered promising from the standpoint of increasing natural populations of aquatic animals. This method consists of stocking the shallow waters of the Inland Sea with cultured fingerlings, but this technique has also been affected by the reduction of suitable culture grounds.
The total fishery yield in the Seto Inland Sea has not shown any tendency to decline, and has indeed increased, but nevertheless fishery operations in this maritime region have not escaped damage from water pollution and land reclamation. Changes in composition of resources, destruction of fishing grounds, massive fish kills due to red tide, and finally pollution of fishery resources by a variety of industrial pollutants have occurred. This illustrates that at higher levels of pollution, or in relation to certain stocks, the unfavourable aspects outweigh any favourable ones, e.g. due to moderate eutrophication. The balance of factors ultimately determines the success or failure of fisheries production. It can be predicted that if the degree of water pollution should worsen in the Seto Inland Sea during the ensuing years, the diversity of fisheries will be damaged.
Counter-measures for Seto Inland Sea pollution include the prompt establishment of environmental quality standards, reinforcement of effluent regulations, the rapid development of sewage systems, and expansion of waste oil treatment facilities. The main points are outlined as follows.
Seventy-two water areas have been classified by water body category for the application of environmental quality standards. Stricter effluent standards were also established to match the environmental quality standards. On 1 February 1974 the Director-General of the Environmental Agency established an apportionment of pollutant load limits among the eleven prefectural governments which border the Inland Sea based upon the “Seto Inland Sea Conservation Law”. This action was taken pursuant to an interim directive whose objective was to reduce the pollution load by 50 percent in terms of chemical oxygen demand (COD). The eleven participating prefectures were required to strengthen or reinforce effluent standards in such a manner that by November 1976 each prefecture could meet the pollution load which it had been assigned. This first interim measure was completed by the end of 1974.
The “Water Pollution Control Law” provides for a notification system when specified facilities are constructed or remodelled. In the eleven participating prefectures, however, it is necessary to procure the consent and approval of the governor in accordance with the “Seto Inland Sea Conservation Law” as well as assessing the effects upon the environment prior to such action. When an application is submitted for the construction or remodelling of specified facilities, the documents must be open to public inspection at which time opinions can be expressed. This provides a way for local residents to voice an opinion.
On 18 June 1974 the Environmental Agency called for careful, judicious and prudent consideration on the part of each prefectural governors' office in the issuance of permits for reclamation projects. This policy was based on the “Seto Inland Sea Conservation Law” and took into account general opinion that such projects would further aggravate the pollution situation in the Seto Inland Sea. It includes:
Measures for the conservation of sea areas, natural environment, and marine resources;
Designation of those sea areas where reclamation should be avoided (Figure 40);
Designation of those sea areas where reclamation should be avoided, except under specific conditions.
Pollution control programmes have been established in order to promote counter-measures. To date, eleven regions have had their respective programmes approved and these include Mizushima (first, and in the process of revising), Osaka (second), eastern part of Hyogo, Kitakyushu, Ohita (third), southern Harima, Ohtake, Iwakuni (fourth), and Kobe, Bingo, Sunan, Toyo (fifth). Among other regions, Wakayama, Okayama, Bizen, Hiroshima-Kure, Shimonoseki-Ube, and Kagawa are currently preparing programmes, and Tokushima is in the process of implementing a fact-finding survey.
A total of 130 billion yen was invested in 1974 as part of the “Five-year Sewage Plan” for the development of public sewage facilities with special designation of the eleven prefectures bordering the Seto Inland Sea. This amount represents about 28 percent of a total allocation of 470 billion yen under the Plan, and emphasizes the high priority placed on the expansion of facilities for sewage treatment.
Facilities for the treatment of oily ballast, tank-cleaning water, and bilge have been operating since 1967 at 34 sites in 20 ports located in the Seto Inland Sea.
The number of “specified” industries according to the “Water Pollution Control Law” located in the eleven prefectures and all the government ordinance cities was 30 492 as of the end of 1973. Spot inspections were made at 12 969 sites, administrative guidance was given in 9 096 cases, directives to improve designated facilities were issued in 392 cases, and provisional measures prohibiting the discharge of waste water from specified facilities were ordered in 58 cases.
Since the interplay of geographical features, tidal currents and pollution pattern makes it difficult to understand environmental pollution in the Seto Inland Sea, the Environmental Agency has had to conduct water quality and bottom deposit surveys every year since 1972. Development of a red tide forecasting technique, prevention of fishery damage due to red tide, and integrated surveys of regions in which red tide frequently occurs have highlighted the progress of activities undertaken from 1971 to 1973 by and between the Environmental Agency, the Fisheries Agency and the Maritime Safety Agency. Furthermore, another comprehensive survey was undertaken in 1974 to evaluate and predict the effect of the accidental heavy oil leakage in the Mizushima industrial complex upon the environment. This integrated survey was conducted through the joint cooperation of the Environmental Agency, the Fisheries Agency, the Maritime Safety Agency, the Ministry of Construction, the Ministry of Public Welfare and the Ministry of International Trade and Industry.
The Maritime Safety Agency has continued to bolster its pollution prevention capacity by expanding its surveillance and law enforcement staff as well as by reinforcement of its fleet of patrol boats and aircraft in the region of the Seto Inland Sea.
Finally, recent activities include restriction of the ocean dumping of excrement since 1 April 1972, dredging projects involving mercury and PCB-contaminated sludge in waterfront areas, and solidification of a monitoring surveillance system for water quality.
