Changes in vertebral-column characters and in swimming abilities of the walleye pollock Theragra chalcogramma (Pallas) under the pressure of fishing activities

Abstract

After 30 years of intensive walleye pollock fishing in the Sea of Japan and the Sea of Okhotsk, the average number of vertebrae has increased significantly in the caudal section of the spinal column. The maximum frequency of occurrence of “multivertebrae” phenotype (33 caudal vertebrae) has increased from 3–5 to 76–78%. The number and frequency of occurrence of the abdominal vertebrae did not change. Experimental testing of walleye pollock on its ability to resist the flow justifies our suggestion that the number of vertebrae in the caudal section is an important adaptive feature. We argue that the changes in phenotypic structure of the studied population are the result of intensive fishing.