STUDIES ON THE MATURING PROCESS OF THE RAINBOW TROUT, SALMO GAIRDNERII IRIDEUS-I

Abstract

The maturation of the ovary of a one-year old fish was studied and the results obtained are as follows: 1. The one-year fish seasonally holds the ovaries which contain the egg-cells of the various stages of development from the oogonia to the ripened eggs. The course of the development of the egg-cells was described as occurring in eight stages. 2. The ovarian eggs show the development of the partial synchronism and the maturing ovaries contain two groups of oocytes, which are clearly distinguishable from each other in their development. The ovarian maturity of this species may be adequately represented by the stage of the more advanced oocytes. Maturity indices of this organism increase conspi-cuously after the ovaries have arrived at the primary yolk stage. 3. Based on the maturity factor, the distinction between the maturing fish and immature ones becomes clear by July or August. Thereafter, no fish which had an intermediate ovary ranging between the maturing and immature groups was found. In August the maturing group of fish has ovaries mostly in the primary yolk stage but sometimes that occur in the fatty drop stage, while the immature group yields ovaries that are younger than the yolk vesicle stage. 4. The maturing group of fish shows a much larger growth rate than the immature group, so that the difference between the body weight of the two groups becomes more distinct with the passing of time. As it comes near the spawning season, however, the growth rate of the maturing group decreases markedly and the fish tend to show a shrinkage in weight and even in size. 5. Methods of increasing the percentage of maturing fish were discussed.