Examination of some current trends in major fish resources has revealed an increase in the population of young mackerel (Scomber japonicus), a recovery in the population of sardine (“maiwashi”, Sardinops melanosticta), while resources of anchovy (“katakuchi iwashi”, Engraulis japonica) and round herring (“urume iwashi”, Etrumeus micropus) have been relatively constant in recent years. In the case of saury (“sanma”, Cololabis saira), the yield was less than the normal annual catch in 1974, but this may have been due to sea conditions during that year.
The catch yield of mackerel has been rapidly increasing from 1965, accounting for over a million tons in 1968 and 1969 and reaching a peak of 1.3 million tons in 1970; the annual catch decreased slightly in 1972 and 1973. As for sardines, the total yield has been increasing rapidly in the last few years from 0.02 million tons in 1970 to 0.35 million tons in 1974. The total yield of horse mackerel (Trachurus japonicus) has been gradually on the decline, decreasing from about half a million tons in 1965 and 1966 to 0.15 and 0.13 million tons in 1972 and 1973. The yield of common squid (“surumeika”, Todarodes pacificus) has fluctuated annually, giving a maximum of 0.668 million tons in 1968, and a minimum of 0.346 million tons in 1973. In the case of saury, production remained at a rather constant level from 1964 to 1967, ranging between 0.21 and 0.24 million tons, but declined to a minimum of 0.06 million tons in 1969. During the subsequent years, the yield slowly recovered attaining levels between 0.18 and 0.19 million tons and even reached a maximum of 0.41 million tons in 1973. However, this was followed by a significant decline and a value of 0.135 million tons was reported as of November 1974. Data reflecting annual yield according to classification of each major fishery are presented in Table 8.
In view of the varying trends described in the last paragraph, it is difficult to discern or assess any general effect of environmental pollution to fishery resources on the basis of nation-wide statistics. It is therefore apparent that in order to evaluate, assess or predict the potential effect of pollution damage to fisheries, it is necessary to collect data and information for certain designated bodies of water. It is clear that only through a carefully planned and concerted programme can we hope to realize our aim, i.e. a meaningful assessment of the relationship between environmental pollution and damage to local fisheries. The following pages of this report therefore focus on a discussion of the problem under consideration for two geographical regions - Tokyo Bay and the Seto Inland Sea.