No abstract

The ages (n = 11 178) and length frequency distributions (n = 173 300) of Pacific saury Cololabis saira caught from September to November in 1989-2000 were examined. Most specimens were determined as either age 0 or 1 based on the number of annual rings in the otolith. The knob lengths (length from the tip of the lower jaw to the posterior end of the muscular knob on the caudal peduncle) at which 50% of the fish were age 1 ranged 277.8-304.7 mm. Analysis of length frequency distribution was also useful to divide the knob lengths of the two groups. The modal lengths of age 1 fish ranged 303.9-325.9 mm.

Growth of Pacific saury Cololabis saira (Brevoort) inhabiting the western North Pacific was studied based on counting daily growth increments with a scanning electron microscope (SEM). The small-size group (knob length: 200-240 mm), medium-size group (knob length: 240-280 mm) and large size group (knob length: >280 mm) showed increments of 256•}26.0 (S.D.), 405•}76.1 and 566•}48.2, respectively. Based on daily growth increment formation, we determined that the ages of the 3 groups were about 8, 13, and 19 months. Small and large-size fish were considered to have been hatched from autumn to winter, and medium-size fish from spring to summer. The oldest specimen of Pacific saury examined in this study was about 23 months of age; thus, the lifespan of Pacific saury in this area seems to be at least 1.5 years.

Since there have been practically no surveys of the eggs of Pacific saury (Cololabis saira) in the western North Pacific (WNP), its spawning ground (SG) distribution has been poorly resolved, based mainly on the larval distribution. This means of estimating SG distribution is imprecise because saury eggs drift for more than a week before they hatch, in a region with intense western boundary currents and their extensions. To improve our understanding of the immature saury, a large number of larvae (body length <25 mm) collected in the WNP during 1993-96 were numerically backtracked to take into account the advection by geostrophic and wind-forced Ekman currents, and the SG locations and ambient sea surface temperatures (SSTs) for the eggs and larvae on the backtracking trajectories were estimated. The resulting seasonal distributions of SGs indicated that both the locations and the intensities of spawning change from season to season. Moreover,