Variation of Square Mesh Codend Selectivity for Walleye Pollock Theragra chalcogramma with Respect to Difference in Body Shape.

Square mesh codend selectivity experiments for walleye pollock Theragra chalcogramma were carried out along the Pacific coast of the Kuril Islands and off Miyagi, Tohoku. Selection curves varied by differences in body shape. Catches taken off Miyagi tended to retain smaller individuals when compared with catches using the same codend mesh size off the Kuril Islands. A detailed investigation of the total length-body girth relationship showed that there were significant differences in the maximum body girth of walleye pollock for fish of the same length. It was also observed that when the catch of fish in the selection range was relatively small, the selection curve tended to be sharp. The selection master curve was expressed in terms of the retention ratio of the codend to the body girth (G) and mesh perimeter (P) ratio (G/P). From these findings, it is suggested that selectivity research should include investigations of changes in the target body shape in the fishing grounds.