The annual, land-locked salmonoid fish, the Koayu, Plecoglossus altivelis, reared in the outdoor hatchery ponds and exposed to short-(8 hours daily), long-photoperiodic regime (16 hours daily) and natural day-length (control) were examined to elucidate the activities of hypothalamo-hypophysial neurosecretory complex and a possible interrelationship between the hypothalamic nuclei and maturation of the fish. The period of experiment extended from July 2, 1966 to January 22, 1967. Five females and five males collected at monthly intervals were supplied to inspect the growth, maturity and histological feature of the neurosecretory system. High mortalities were encountered between the months September and November in the short photoperiodicity, October and December in the control, and December and January in the long photoperiodicity, respectively, in parallel with the highest level of gonadal maturation and of cyanophilic gonad stimulating cells. Untill each prespawning season, the perikarya and juxta-somal axons of the preopticonucleic cells heavy loaded with AF-stainable neurosecretory material have been observed in each experimental group.Early in each spawning season, fine droplets of material in smaller amount are scattered around the nuclei of the cells. After that, a notable depletion of the material from the perikarya and juxta-somal axons is detected. Recovery phase of the neurosecretion is seen in the female fish survived beyond each spawning time and tided over the winter. On the contrary, all of the males were deceased untill the next year. Changes in the amount of the storage material in the neurohypophysis appear to be parallel with those of neurons. During each breeding time, on the other hand, the highest activity is reached by the nucleus lateralis tuberis which were stained with acidic dyes. These results suggest that there is a role of the light rhythm upon the maturation of the Koayu via hypothalamic neurosecretory system, and the possibility of artificial control of the breeding time in the fish.
Teleosts ovary undergoes regular changes in many species, but the breeding season and spawning reflex are varied. The ovarian histomorphology and annual changes and the spawning periodicity of this catfish were studied on the total of 78 individuals collected monthly in one year period of 1963-64. By gross morphology and its seasonal changes, especially on the changes in cell structure in this species, nine separate stages were distinguished to categorise the developing oocytes. As to the annual ovarian changes, seven phases of ovarian cycle was described based on the percentage and duration of different stages of developing oocytes, average diameter of oocytes and nature of ovarian wall-Immature Virgin (October to February), Preparatory Virgin (March-April), Maturing Virgin (April-May), Prespawning Virgin (June), Spawning (late June through August), Depletion (late August-September), and Recoupment (October-November). The studies revealed a close correspondence among gonosomatic index, ova diameter and water temperature. Discussion was made on some interpretations as to the origin of new crops of oocytes in teleosts ovary. In Mystus the new oocytes were found to be formed from dividing oogonia, which were transformed from stroma cells; the fish lacked a well defined germinal epithelium. As to the spawning periodicities, of which examples in teleosts were introduced, the present species revealed to produce two or more batches of eggs during the same season (April to August or early September) by finding that only two types of ova are present in ripe ovaries, immature and mature, the latter containing oocytes in late yolk stage and mature stage, and by examining gonosomatic index, ova diameter and histological nature of ova. Finally, relative and total fecundity in bony fish were discussed, and the present study suggested that the unshed ripe oocytes undergoing atresia must be counted in computing the total fecundity. It was estimated that in Mystus nearly 93% oocytes finally ripen and are shed.