DDT, one of the residual organochlorine pesticides, has been still used in China and perhaps in developing countries of the equatorial hemisheric zone. Therefore, wild and cultured yellowtails were selected as the indices of DDT residue levels of the marine fishes, considered a main source of the residue in the general population of Japan.
Total DDT level of wild one, large type of fishes distributed from South China Sea to nearly north end of Hokkaido in latitude, increased severalf old during two years from Feb.-Mar. 1979 to Mar. 1981. Consequently, the levels of total DDT in wild yellowtail were higher than those in cultured one in 1981. It is noteworthy that especially three wild fishes catched from Nagasaski area were not only high in the levels, but low in the ratio of pp-DDE plus pp-DDD per total DDT. It is suspected that they were going forward to North with the Japan Current. Close relations were recognized among DDT residues, PCB patterns and sea areas catched, and marked differences of the residue levels were observed on account of the areas.
Furthermore, slight increase was also recognized about the level of cultured type in 1981 compared to the samples in 1979.
On the other hand, the level of dieldrin was higher in the cultured one than the wild as well as in the two years before.

It is a well-known fact that the environment has been globally contaminated with the persistent organochlorine insecticides and PCBs, and that all over the world the general populations have been polluted with them in spite of their minute quantity for some decades. These chemicals except PCBs are mainly neuropoison, and according to De Vlieger (1968), DDT and Dieldrin are translocated to human brain, and these concentrations of the tissue are greater than those of the blood. The adverse and ill effects of these residues on human central nervous system can be determined incompletely. In addition, their production and use have not yet been prohibited in some countries. Therefore, according to the previous report (1981), the contents of these compounds have been surveyed about wild yellowtails migrating around the Islands of Japan, and cultured ones fed on a sort of sardine catched near the Islands.
In the case of DDT, the content of wild samples catched in 1983, showed not only low level of total DDT, but the increase of its metabolites, compared with those of two years before: i. e., the mean concentration decreased to about one-half (from 0.38 to 0.17 ppm), and the ratio of its meabolites per total DDT inversely increased from 41 to 57%. In addition, the same tendency was also observed in those of cultured samples.
With regard to PCBs content, however, the fish of wild type showed the same level (0.10 ppm) as that of cultured type in 1983, and their peak patterns were observed similar. The PCBs contents have been almost unchanged in these two types since 1980 over a period of 3 years.