The origin of four-year-old chum salmon caught in the Bering Sea in July, 1981 was estimated from their gonad weight and scale characters. There were three different maturity groups in the gonad weight frequency distributions for both female and male chum salmon. The maturing group with lower gonad weight (MT2) had more circuli and narrower width of the first year zone of the scale than those of the immature group (MT1) and the maturing group with higher gonad weight (MT3). A comparison of the scale characters of these three groups with those of chum salmon caught in the coastal regions showed that the MT1 group was similar to U.S.S.R. and Japanese chum salmon, the MT2 group was similar to Japanese chum salmon, and the MT3 group was similar to U.S.S.R. chum salmon. This suggests that different maturity groups in the high seas catch correspond to stocks from different geographical regions.Chum salmon are the most widely distributed and also the second most abundant species of all Pacific salmon.1) Their distribution and stock composition in offshore waters have been in vestigated on the basis of tag recovery data and scale characters.2-5)However, these studies were mainly based on data and materials obtained before 1970. Since that time, the production of chum salmon by Japanese hatcheries has increased from 4 million (average from 1950 to 1970) to 16 million fish per year (average from 1971 to 1986). Also, there has been a large decrease in salmon fishing effort on the high seas, especially in the number of Japanese fishing vessels.6)In spite of these many recent changes in stock abundance and fishing effort in the North Pacific, stock identification studies have not been carried out thoroughly for all species. Insufficient stock identification research is partly due to the labori ous nature of scale measurement analysis which is perhaps the most widely used technique for stock identification .7) Thus, easy methods for stock identification are needed .The purpose of this paper is to evaluate the feasibility of identifying stocks of chum salmon from their maturity and scale characters . We also compared the preliminary results using this new method with results of previous studies .
Materials and MethodsA total of 860 scale samples was obtained from four-year-old chum salmon caught by Japanese research vessels and motherships in the Bering Sea in July, 1981. The primary sampling gear was gillnets, and the secondary gear was longlines. Fish were processed by recording fork length, body weight, sex, and gonad weight.Scales were removed from the part of the body just below the insertion of the dorsal fin and 2 or 3 scale rows above the lateral line. Scale impressions were made by methods similar to those of Clutter and Whitesel.8) Methods of scale examination of Tanaka et al. were followed.5) The images of scale impressions were projected with a magnifi cation of 100x onto sheets of graph paper divided into one millimeter squares.The scale was orient ed so that the longest axis of the image lay along the lines of th...
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