Migrations and stock size of herring in the northern North Sea in 1966; analyses of Norwegian tagging experiments
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8000 herring were tagged with internal steel tags in the summer of 1966; 4000 were released in the north-eastern North Sea and 4000 east of Shetland. On the basis of returned tags during the first year after tagging, Hara1dsvik (1967 and 1969a) determined migrations and stock size. In the present paper the data are re-examined taking into consideration the returns from later years. A high degree of intermingling of herring between the northeastern North Sea and the Shetland area was found. 30-35% of the autumn spawning herring tagged in the Egersund Bank area were in the following years found at Shetland. Vice versa, 20-40% of those tagged east of Shetland were in later years found in the north-eastern North Sea. The data on tag returns in 1967 and 1968 have been used to estimate the stock size of herring applying a modified, simple Petersen method. The number of autumn spawners, 3 years and older, in the northern North Sea in the summer of 1966 was estimated to be 12-17 x 10^9, corresponding to 2.3-3.3 million tons. The errors adhering to this estimate are discussed, and it is concluded that the stock in the northern North Sea is overestimated, while the estimate probably is somewhat below the total size of the adult autumn spawning stock in the North Sea as a whole, excluding the Skagerrak.