Precise target strength measurement and morphological observation of juvenile walleye pollock (Theragra chalcogramma)

This study examines a relationship between target strength (TS) and fork length (FL) for walleye pollock at the juvenile stage. Dozens of juvenile walleye pollock were caught by a set net nearby spawning area in Pacific coast of southern Hokkaido. These juveniles were kept for a year in a tank and measured periodically with growth. Samples were anesthetized and treated carefully not to expose to air till the end of TS measurements. To measure TS directivity pattern in dorsal aspect, the tilt angle of fish was controlled from minus to plus 50 degrees (head down to head up) by 1 degree steps. The soft X-ray images of swimbladder were recorded before and after acoustic measurement. All over this study, 42 fish were used for TS measurement with X-ray observation. Measured directivity patterns of TS were coincided with theoretical ones calculated from swimbladder and body shape using Kirchhoff-Ray Mode (KRM) backscatter model. The obtained relationship averaged TS and FL is following;
=20logFL-67.8 (R=0.98). On the other hand, FL vs swimbladder length ratio changed about 10 cm in FL, and turning of relationship between maximum TS and FL was observed.