Distribution, abundance and mortality of young and adolescent Norwegian spring spawning herring (Clupea harengus Linné) in relation to subsequent year-class strength

1. The distribution of young and adolescent herring in coastal and\noffshore waters of northern Norway has been investigated for the 1959-\n1965 year-classes by combined acoustic surveys and fishing experiments.\n2. In the autumn (August-October) 0-group herring occur pelagically\nin the upper 50 m of water together with the 0-group of several other\nspecies, such as cod, haddock, redfish, capelin and others. This complicates\nthe charting and identification of the sound scatterers, but detailed\nstudies of the echo recordings and frequent sampling in the scattering\nlayer made it possible to determine the distribution of 0-group herring.\n3. In 1959, 1960, 1964 and to some extent in 1963 and 1965, 0-group\nherring had an oceanic distribution, and only a minor part of the 0-group\npopulation entered the fjords of northern Norway. In 1961 and 1962 the\ndistribution was more restricted to the coastal areas, and a greater proportion\nof the total 0-group population was present in the fjords. Herring\nentering the fjords at the 0-group stage in the autumn emigrate during the\nI-group stage from March to May.\n4. Herring in the northern and eastern Barents Sea had slower growth\nrates and consequently a longer phase of migration to the spawning areas\nthan did those distributed farther south and west. Herring from nursery\ngrounds in the Barents Sea, therefore, are considerably more reduced in\nnumbers before they reach the spawning grounds off Møre than those\nwhich have spent their 0-group stage in the southern nursery areas in\nNorwegian coastal waters. The growth rate, therefore, may influence the\nmigration pattern and the duration of the adolescent phase. Growth on\nthe other hand is determined by the distribution during the 0-group stage,\nand the 0-group distribution consequently plays an important role in\nrecruitment mechanism.\n5. Abundance estimates of 0-group herring were obtained from three\nindependent sources, i.e. acoustic surveys, catch and effort statistics and\ntagging experiments. The 1959 year-class, but also that of 1960, were\nnumerous during the 0-group stage and remained abundant throughout\nthe adolescent phase and in the adult stock. Year-classes of low abundance\nat the 0-group stage, such as the 1962 year-class, remained weak throughout\nthe subsequent stages. The 1963 year-class, which was fairly abundant\nas 0-group, did not show up in significant numbers in the adult stock in\n1968 and 1969. The 1964 and 1965 year-classes have not been studied\nin their adult phase because they had not attained maturity when the\nmaterial was compiled. The year-classes of 1963 and 1964 showed about\nthe same relative strengths during the adolescent phase as at the 0-group\nstage.\n6. Mortality estimates obtained from catch and effort data and\ntagging experiments indicate that the fishing mortality of 0- and I-group\nherring in Norwegian fjords was relatively high, but because natural\nmortality was much higher, the exploitation rate was relatively low.\nTaking into account that the fjord population is only part of the total\n0-group population, it is concluded that the fishing mortality generated\nby the 0- and I-group fishery in Norwegian fjords was too small to be the\nprimary cause of the failure of recruitment to the adult stock.\n7. A considerable increase in the exploitation of fat-herring, i.e. I- to\nIV-group herring took place from 1965 to 1968. The increase in exploitation\nduring the adolescent phase of the 1963 and 1964 year-classes compared\nwith those of the 1959-1961, and a long duration of the migration\nof the former year-classes from the nursery areas to the spawning grounds\nmay explain why the year-classes of 1963 and 1964 tended to be relatively\nweak in the adult stock.\n8. With the relatively low level of the spawning stock size an intensified\nfishery on the Norwegian herring should be avoided. An extension\nof the small- and fat-herring fisheries into the open sea may result in an\nappreciable reduction in the subsequent abundance in the adult stock,\nand a reduction of the fishery on young and adolescent herring should be\nconsidered to improve the abundance of adult herring.