Kochi Prefactural Fisheries Experimental Station, Kochi 785-0167, Japan
Age, growth, maturation and life of bullet tuna Auxis rochei collected in Kochi prefectural coastal waters from March 1993 to March 1998, were estimated from fork length, body weight, gonad weight, and the largest oocyte diameter of 124,485 individuals. The relationship between growth and age based on the monthly change of the fork length composition are: 0.5-year-old=25cm, 1-year-old=29cm, 1.5-year-old=32cm, 2-year-old=35cm. Two groups of bullet tuna were recognized in this research area: One is the summer-birth group of which spawning occurs in Japanese coastal waters, and the other is the winter-birth group of which spawning occurs in winter in southern tropical waters. The latter group is supposed not to return the tropical waters. Maturation age of the summer-birth group is 1-year-old, whereas that of the winter-birth group is 1.5-year-old, and 1st spawning occurs in summer around Japanese coastal waters. Most fishes of the summer-birth group finish their life after spawning in 2-year-olds, while most fishes of the winter-birth group die after spawning in 1.5-year-olds.