Reproductive activity of a twice-annually spawning strain of rainbow trout.

Reproductive activity of rainbow trout breeding twice a year was individually studied using 94 females of 2.5 years old for a year from just after their first ovulation in winter. Fish were divided into 2 groups. Group I consisted of 44 females from which 3 ml of blood were repeatedly sampled. Group II consisted of 50 females from which a piece of ovary was repeatedly sampled with 3 ml of blood. Individual information on the reproductive activity was obtained from 77 females in the summer breeding season and from 62 females in the next winter breeding season. Ovulation was observed from April 24 through July 28 and from November 30 through January 26 in the summer and winter breeding season, respectively. Percent of ovulated fish was 62.3% (48/77) and 95.2% (59/62) in the summer and winter breeding seasons, respectively. Early spawners in summer ovulated smaller eggs and amount of ovulated eggs were not observed in the winter breeding season. Individual tracing of body weight and body length clearly revealed antagonistic relationship between gonadal maturation and growth. Growth gradually stopped with the advance of vitellogenesis. Twice-spawners showed lower percent increase in body weight and larger amount of ovulated eggs than those of once-spawners for a year.