Factors influencing year-class strength of Norwegian spring spawning herring (Clupea harengus Linné)

1. Variation in year-class strength of Norwegian spring spawning herring is analysed in relation to the structure and the size of the spawning stock, the location and the time of spawning and environmental conditions during the egg stage and the early larval development with special reference to the 1959-1965 year-classes. 2. Stock size estimates were obtained from tagging experiments, and the number of eggs spawned in each year was calculated from fecundity data in the literature. The highest figure for the stock size (7.3 million tons) was found in 1965. The stock size had its lowest figure in 1962 when it was about 2.5 million tons. 3. The location of spawning was determined from the distribution of catches of spawning and spent herring and from the distribution of newly hatched larvae. During the period 1959-1965 the most important spawning took place off Møre, i.e. on the shelf between Stad and Grip, and mainly in depths of less than 200 m. Spawning south of Bergen was negligible except in 1959. During the entire period spawning took place off Halten and Sklinna, whereas spawning at Lofoten was more sporadic. 4. The duration of the spawning season was determined from the maturity composition of herring collected several times a week during the spawning season. Off Møre spawning was late in 1962 (15-19 March) and early in 1965 (28 February-4 March). In 1963 spawning off Lofoten occurred about two weeks later than off More, and in 1964 three weeks later, whereas in 1965 spawning took place about simultaneously in the two regions. 5. The time of main hatching was estimated by adding an incubation period of generally 18-24 days (depending on the temperature) to the time of main spawning. Approximate figures for the incubation periods were obtained from the literature. 6. The distribution of herring larvae during the first month after hatching was analysed for the period 1959-1965. The larvae were collected with Clarke-Bumpus plankton samplers in oblique hauls during regular surveys covering the coastal banks between Stad and Lofoten. Factors affecting larval abundance estimates are discussed, and a general description of the vertical distribution and diurnal migration of herring larvae is given. The geographical distribution of larvae was analysed in relation to the spawning grounds. The drift of larvae was inferred from subsurface drogue experiments, wind observations and drift bottle experiments. Larval length was analysed in relation to the time of spawning. During the first month after hatcl-ting the larvae were concentrated near the coast, and almost no larvae were found outside the edge of the shelf. A more concentrated distribution of larvae was observed in 1961, 1962 and 1965 than in the other years considered, The larvae are transported northward in coastal water and the force and direction of the wind play an important role in determining the distribution pattern of larvae. 7. The results indicated that within the range of observed stock sizes a relationship may exist between the parent stock and abundance of the resulting year-classes when favourable conditions occurred. However, it is likely that the year-class strength was determined by a complex of other factors which completely ruled out the effect of parent stock size in most of the years. Conditions assumed to be advantageous for the development of strong year-classes were inferred from the present and other investigations. Strong year-classes seemed to occur when a combination of the following conditions existed : 1) widespread distribution of spawning; 2) long duration of the spawning period; 3) a rapid dispersion of larvae from the spawning grounds. In 1959, 1960, 1963 and 1964, spawning was extensive and prolonged and with a rapid northward dispersion of larvae from the different spawning grounds. In 1962 the opposite conditions existed. Spawning was extensive in 1965 and prolonged in 1961, but the transport conditions during and after the yolk sac stage resulted in a concentrated distribution pattern. 8. The coincidence in time between the occurrence of suitable food and hatching of herring larvae is assumed to be the most important environmental factor controlling year-class strength during the early larval development. The gradual northward displacement of the main spawning centre during the last decades probably has increased the importance of the timing factor, since only two definitely rich year-classes occurred during the period 1947-1965, namely those of 1950 and 1959