1962 Volume 28 Issue 11 Pages 1064-1069
The present report consists of two parts; (1) the introduction of eggs fertilized earlier in a different hatchery, and (2) the selective breeding to accelerate maturation.
(1) 34, 670 eyed eggs were brought to the Oizumi Trout Hatchery, Tokyo University of Fisheries, from the Samegai Trout Hatchery, Shiga Prefecture, on 25 Decomber 1951. The Samegai strain spawned in earlier months than that of the Oizumi at that time.
The first spanwing of the Samegai strain in the Oizumi Hatchery was in December 1953. The percentage of females stripped in the Oizumi against total females including both introduced and original stocks of the same age was shown in Fig. 1, where the percentage being compared in a 10-day period through out the spawning years from 1953 to 1956. The Samegai strain spawned 15-30 days earlier than that of the Oizumi, although it showed a tendency to retard slightly in spawning period with the advance in age.
(2) With an object finding the fully ripe fish which were permitted to strip eggs on earlier date, the two methods were undertaken; stepping up the date of sorting fish for ripeness (study after 1954), and taking eggs from the fish ascending a natural stream flowing into a brood pond in the hatchery (study after 1956). Then, selective breeding for advancing the period of maturation was carried out on these 2 types of early spawners.
The result arrived was that, the opening day of actual egg taking operation was advanced to the middle of November after 1954 from the end of December hitherto experienced (Table 1). The first spawning of the brood fish resulted from selected fish began in 1956, since then the peak of the spawning also advanced to the end of December from the middle of January (Fig. 2). The percentage of mature females against total females examined till the end of December (the peak of spawning) increased gradually from 1956 to 1961 (Table 2). Corollary is that the egg taking period gradually advanced, though the dates of peak did not much alter after 1956.
The relation between maturation of rainbow trout and temperature of water was discussed on the basis of the present study and available information. It stated that: 1. The character in spawning can not be held invariable in different water especially refering to the temperature. 2. The modification by selection of spawning period in one hatchery which requires a lengthy duration may not be highly evaluated in a practical sense than to maintain fishes of normal ripening reared in a number of hatcheries with varied temperatures. 3. Endocrinological treatment or light and/or temperature control will profit better than selective breeding in a sense that it saves the long time required for selective breeding.
