Open Access

Toyotaka Tanoue

• 1. The migration of the spotted mackerel, Pneumatophorus tapeinacephalus distributing in the coastal sea of Japan was investigated in relation to the geographical distribution of the fishing grounds, seasonal change of fishing condition. sea conditions and fork length. Secondarily, some anatomical and histological observations were carried out on spotted mackerels caught in the coastal sea area around Kagoshima and its vicinity to clarify the sex differentiation and the seasonal cycle of the gonads. 2. Spotted mackerels are distributed throughout a wide sea area stretching from north of Formosa to the south of Japan Sea. including the Pacific coastal sea from Kyushu to Chiba Prefecture. The northern limit of the distribution area is assumed to be the sea areas off San-in and Chosi. 3. The schools of adult fish make a feeding migration to the circumference of Saishu Island and to the sea area off Ashizuri cape in summer. and these schools make a spawning migration toward the sea area around the Osumi Islands and the southern area of the East China Sea in winter. 4. In winter some schools of adult fish remain living in the sea area south of the Izu Islands. These schools belong to a group isolated incompletely from that of the East China Sea. as some of them are those which came from the East China Sea. 5. The larvae grow while they are being brought by the sea current or tide current. When they have reached 50~60mm. in total length. they aggregate in schools and approach the coast. In spring they swim in the coastal nursery grounds. 6. From summer to autumn, the schools of the young fish make a feeding migration to the sea off San-in and to the eastern coastal sea of Chiba Prefecture. In winter. they make a seasonal migration to the coastal sea of South Kyushu, the East China Sea and the southern sea area of the Izu Islands. 7. The range of vertical distribution of the larvae is supposed to be the layer from the surface to 40m. in depth. The vertical distribution of the adult fish is chiefly in the layer, 40-70m. in depth, during the period from late autumn to early spring. It becomes shallower in late spring and summer, the depth being about 20-40m. 8. The ranges of water temperature and salinity in the sea where the adult fish schools are distributed are 17.0-26.0°C and 34.0~34.8%0. respectively. 9. The spawning takes place during the period from the end of January to June in the southern part of the East China Sea and the sea areas around the Osumi Islands, off Ashizuri Cape and around the Izu Islands. These spawning grounds are sea areas where a comparatively rapid current is running towards a land shelf. 10. The ranges of the optimum water temperatures and salinities for the spawning are assumed to be 17-23°C and 34.0-34.8 0/00, respectively. 11. The primordial germ cells seem to migrate to the gonad by amoeboid movement from other places than the gonad. 12. The early indifferent gonad is very slender and suspended with a mesogonium, in the coelom. It is composed of peritoneal epithelium, stroma cells and primordial germ cells. 13. The formation of the gonocoel begins as a longitudinal depression on the surface of the gonad, facing the mesentery. This depression takes place in the gonad of the fish, about 60mm. in fork length, prior to the sex differentiation. 14. The sex differentiation occurs directly without a phase of a juvenile hermaphrodite. 15. The gonad in which the gonocoel is greatly enlarged becomes an ovary, while that in which the gonocoel is left narrow becomes a testis. 16. In the early ovary the layer containing oogonia is surrounded with stroma cells. The surface of the ovary is covered with cuboidal epithelium. 17. In the ovary of the fish, 100-130mm. in fork length, the wall of the ovocoel forms small protuberances, which become the lobes of the ovary. The oocytes are situated in these lobes. The yolk formation begins in the oocytes, 15.....,20.a in diameter, 18. The maturing process of eggs is clasified into the following 7 stages; the chromatin nucleolus, the peripheral nucleolus, the yolk vesicle, the early yolk globule, the late yolk globule, the migrating nucleus and the matured stage. Ovarian eggs at the migrating nucleus stage and the matured stage are observed in the fish, more than 300mm. in fork length. 19. The surface of the early testis is covered with peritoneal epithelium. The interior is filled up with the multiplied stroma cells and the spermatogonia scattered among them. In the testis of a somewhat later stage, a lot of branches are stretched out of the testocoel. Some of the spermatogonia are arranged directly beneath the peritoneal epithelium and the others are buried deep in the testis. The testis lacks a layer of stroma cells under the peritoneal epithelium. 20. In the testis of young male fish the spermatogonia increase in number and surround the small branches of testocoel; they form seminiferous tubules. The testocoel and its large branches become the rete apparatus constructed of collecting ducts. The maturation division appears in the testes of the fish more than 280mm. in fork length. 21. The sex ratio of the young fish is approximately 1 : 1. The ratio between the gonad length and the fork length shows an exponential increase. The gonads of adult fish are enlarged about 9-13 % of the original length during the spawning season. 22. During the months from July to November the oocytes in the ovaries of adult female :fish are at the chromatin nucleolus stage and the peripheral nucleolus stage. During the same season there are only spermatogonia in the testes of adult male fish. The gonads of adult fish begin to increase in size in December and become the largest in March and April. The increase in size of the ovary is chiefly due to the enlargement of ova on account of yolk deposition. The increase in size of the testis is due to accumulation of spermatozoa. 23. A few oogonia can be seen m the ovanes of adult female fish during and immediately after spawning. Numerous spermatogonia appear along the inner walls of the seminiferous tubules late in the spawning season.