Minoru Nomura

Endocrine profiles in the females of a twice-annually spawning strain of rainbow trout

Abstract Endocrine profiles of the reproductive phases in individual females were studied using forty-four 2.5-year-old fish for 1 year following their first ovulation in winter. Ovulation occurred from 24 April to 28 July and from 14 December to 26 January, in 24 individuals (68.6%) and 33 individuals (94.3%), respectively, out of 35 females which survived the whole experimental period. Ovulation occurred over a few months in the summer breeding season, whereas it was rather concentrated in the winter breeding season. In the fish that were successful ovulators in the summer, plasma estradiol-17β rapidly increased in January, attaining the highest values from March to May and then decreased before ovulation. Plasma gonadotropin (GtH) gradually increased from January followed by a second increase just before ovulation in summer. In the fish that failed to mature in summer, plasma estradiol and GtH showed little increase from January to February, and synchronously decreased in March. Lengthening daylength in spring probably inhibited GtH secretion in the once-spawners, but not in the twice-spawners which already had high plasma estradiol and GtH at the critical period between late February and late March. In almost all females, synchronous increases in plasma estradiol and GtH occurred in July, probably triggered by the shortening daylength. The highest values for estradiol were attained in October.

Endocrine profiles in the males of a twice-annually spawning strain of rainbow trout, Salmo gairdneri

The male reproductive cycles of a twice-annually spawning strain of rainbow trout were studied by monitoring the plasma gonadotropin (GtH) and steroid hormone levels in individual fish for more than a year using thirty-five 2.5-year-old mature males. Twenty-five males survived the whole experimental period and were divided into four groups according to the amount of milt and endocrine profiles. In the summer breeding season, milt amount was negligible in Group I and small in Group II with low plasma testosterone, 11-ketotestosterone (11-KT), and 17 alpha,20 beta-dihydroxy-4-pregnen-3-one (17 alpha,20 beta-diOH-P) levels, whereas both groups showed a large amount of milt and a distinct increase in steroid levels in the winter breeding season (November-January). Group III expelled a large amount of milt in both the summer and winter breeding seasons, and plasma testosterone, 11-KT, and 17 alpha,20 beta-diOH-P showed a clear peak in each breeding season. In Group IV, milt was expelled from December to July, and plasma steroid levels remained high until June before declining to the basal levels; however, these fish failed to mature in the ensuing winter breeding season. Plasma GtH levels in Groups III and IV were significantly higher than those in Groups I and II in the summer breeding season. These results clearly indicate that fish in Group III are twice-annual spawners.

Effect of temperature and photoperiod on sexual maturation and spawning of the common carp: III. Induction of spawning by manipulating photoperiod and temperature

Abstract Japanese common carp “Magoi”, Cyprinus carpio (Linnaeus), were maintained in cool water (16°C) combined with long (16L-8D) or short (12L-12D) photoperiods from November. After about 3 months at 12L-12D 16°C , four out of five fish spawned totally when transferred to 16L-8D 24°C , while fish kept under 16L-8D 16°C failed to spawn when transferred to 12L-12D 24°C . Similarly, all five fish from 16L-8D 16°C spawned immediately at 16L-8D 24°C , but those at 12L-12D 16°C failed to spawn when transferred to 12L-12D 24°C . After about 6 months carp held at 16L-8D 16°C spawned even at 12L-12D 24°C . It is presumed that this group was closer to final maturation and did not need the stimulative effect of a long photoperiod. When maintained at 16°C, carp completed rematuration within 3 to 4 months, including an initial stay of 2 to 3 weeks at 24°C, and could stay mature indefinitely.