The effect of fishing on the walleye pollock phenotype composition

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The study of phenotypic characteristics (vertebral column) gives us the valuable data for comprehension of population structure and genetic changes in fish. M. Iwata has studied in details the variability of vertebral column characters in 42 samples of the walleye pollock collected from the Pacific Ocean, the Okhotsk Sea and the Sea of Japan in 1968–1970. M. Iwata has found from 17 to 20 vertebrae in the abdominal section and from 29 to 33 in the caudal section of vertebral column in the walleye pollock specimens. We have col-lected more than 60 samples of walleye pollock from the trawls, Danish seines and set nets catches in 1999–2001 at different commercial fishing areas. It was found that after 30 years of intensive walleye fishing the average number of vertebrae in the caudal section significantly increased. In our samples we have never had a chance to see a specimen with 29 vertebrae in the abdominal section. We have supposed that the short-tail fishes have disappeared under the pressure of selective trawl fishing. In support of this hypothesis, a special series of original experiments has been prepared and worked out in a flow pool on the board of a fishing vessel with the flow velocity equal to the speed of trawling. As a result, each sample of the walleye pollock was separated into two parts differing in their swimming performance. After studying the vertebral column structure in these two groups we have determined that the number of vertebrae in the caudal section was less for fish unable to resist the flow (trawling). Hence, the structure of walleye pollock vertebral column may be considered as a mark of population differences under the influence of fishing process. The re-sults of our investigations have proved a hypothesis of selective fishing caused elimination of the short-tail fishes changing the structure of the walleye pollock population as a whole.