Age-composition of adult yellowtail Seriola quinqueradiata in Toyama Bay [Japan]

The fork length, body weight and gonad weight of the adult yellowtail, Seriola quinqueradiata, landed in Toyama Prefecture from 1996 to 2003 were investigated. The age of the fish was also estimated from examination of the vertebrae. The results showed that the relationship between body weight (BW;g) and fork length (FL;cm) could be expressed by using the formula; BW=0.00092X FL3.65. Most of the fish were landed in December and January, and the gonad index (GI) of these fish, which was calculated from the gonad weight (g) and FL, was less than 1.4, but fish landed in May and June frequently had a GI much higher than 1.4. It was estimated that fish with fork lengths from 72.0 to 78.0 cm were 2 years old, those from 81.0 to 87.5 cm were 3 years old and those from 91.0 to 96.0 cm were 4 years old. The age-compositions of the fish were calculated according to the fishing season that lasted from October to June. The percentage of 2-and 3-year-old fish to total number of fish landed in Toyama Prefecture ranged from 97.2 to 99.7 %, and the percentage of those 4 years and older was less than 2.8 %. The age compositions of the fish were significantly different from the compositions recorded in the I960's, i.e., the ratio of fish 4 years and older had been dominant relative to the total landings of adult yellowtail.