Distribution, abundance and mortality of young and adolescent Norwegian spring spawning herring (Clupea harengus Linné) in relation to subsequent year-class strength
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1. The distribution of young and adolescent herring in coastal and offshore waters of northern Norway has been investigated for the 1959- 1965 year-classes by combined acoustic surveys and fishing experiments. 2. In the autumn (August-October) 0-group herring occur pelagically in the upper 50 m of water together with the 0-group of several other species, such as cod, haddock, redfish, capelin and others. This complicates the charting and identification of the sound scatterers, but detailed studies of the echo recordings and frequent sampling in the scattering layer made it possible to determine the distribution of 0-group herring. 3. In 1959, 1960, 1964 and to some extent in 1963 and 1965, 0-group herring had an oceanic distribution, and only a minor part of the 0-group population entered the fjords of northern Norway. In 1961 and 1962 the distribution was more restricted to the coastal areas, and a greater proportion of the total 0-group population was present in the fjords. Herring entering the fjords at the 0-group stage in the autumn emigrate during the I-group stage from March to May. 4. Herring in the northern and eastern Barents Sea had slower growth rates and consequently a longer phase of migration to the spawning areas than did those distributed farther south and west. Herring from nursery grounds in the Barents Sea, therefore, are considerably more reduced in numbers before they reach the spawning grounds off Møre than those which have spent their 0-group stage in the southern nursery areas in Norwegian coastal waters. The growth rate, therefore, may influence the migration pattern and the duration of the adolescent phase. Growth on the other hand is determined by the distribution during the 0-group stage, and the 0-group distribution consequently plays an important role in recruitment mechanism. 5. Abundance estimates of 0-group herring were obtained from three independent sources, i.e. acoustic surveys, catch and effort statistics and tagging experiments. The 1959 year-class, but also that of 1960, were numerous during the 0-group stage and remained abundant throughout the adolescent phase and in the adult stock. Year-classes of low abundance at the 0-group stage, such as the 1962 year-class, remained weak throughout the subsequent stages. The 1963 year-class, which was fairly abundant as 0-group, did not show up in significant numbers in the adult stock in 1968 and 1969. The 1964 and 1965 year-classes have not been studied in their adult phase because they had not attained maturity when the material was compiled. The year-classes of 1963 and 1964 showed about the same relative strengths during the adolescent phase as at the 0-group stage. 6. Mortality estimates obtained from catch and effort data and tagging experiments indicate that the fishing mortality of 0- and I-group herring in Norwegian fjords was relatively high, but because natural mortality was much higher, the exploitation rate was relatively low. Taking into account that the fjord population is only part of the total 0-group population, it is concluded that the fishing mortality generated by the 0- and I-group fishery in Norwegian fjords was too small to be the primary cause of the failure of recruitment to the adult stock. 7. A considerable increase in the exploitation of fat-herring, i.e. I- to IV-group herring took place from 1965 to 1968. The increase in exploitation during the adolescent phase of the 1963 and 1964 year-classes compared with those of the 1959-1961, and a long duration of the migration of the former year-classes from the nursery areas to the spawning grounds may explain why the year-classes of 1963 and 1964 tended to be relatively weak in the adult stock. 8. With the relatively low level of the spawning stock size an intensified fishery on the Norwegian herring should be avoided. An extension of the small- and fat-herring fisheries into the open sea may result in an appreciable reduction in the subsequent abundance in the adult stock, and a reduction of the fishery on young and adolescent herring should be considered to improve the abundance of adult herring.
SeriesFiskeridirektoratets skrifter, Serie Havundersøkelser
vol 15 no 4