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

1. Variation in year-class strength of Norwegian spring spawning\nherring is analysed in relation to the structure and the size of the spawning\nstock, the location and the time of spawning and environmental conditions\nduring the egg stage and the early larval development with special\nreference to the 1959-1965 year-classes.\n2. Stock size estimates were obtained from tagging experiments, and\nthe number of eggs spawned in each year was calculated from fecundity\ndata in the literature. The highest figure for the stock size (7.3 million\ntons) was found in 1965. The stock size had its lowest figure in 1962\nwhen it was about 2.5 million tons.\n3. The location of spawning was determined from the distribution of\ncatches of spawning and spent herring and from the distribution of newly\nhatched larvae. During the period 1959-1965 the most important spawning\ntook place off Møre, i.e. on the shelf between Stad and Grip, and\nmainly in depths of less than 200 m. Spawning south of Bergen was\nnegligible except in 1959. During the entire period spawning took place\noff Halten and Sklinna, whereas spawning at Lofoten was more sporadic.\n4. The duration of the spawning season was determined from the\nmaturity composition of herring collected several times a week during\nthe spawning season. Off Møre spawning was late in 1962 (15-19 March)\nand early in 1965 (28 February-4 March). In 1963 spawning off Lofoten\noccurred about two weeks later than off More, and in 1964 three weeks\nlater, whereas in 1965 spawning took place about simultaneously in the\ntwo regions.\n5. The time of main hatching was estimated by adding an incubation\nperiod of generally 18-24 days (depending on the temperature) to the\ntime of main spawning. Approximate figures for the incubation periods\nwere obtained from the literature.\n6. The distribution of herring larvae during the first month after\nhatching was analysed for the period 1959-1965. The larvae were collected\nwith Clarke-Bumpus plankton samplers in oblique hauls during regular\nsurveys covering the coastal banks between Stad and Lofoten. Factors\naffecting larval abundance estimates are discussed, and a general description\nof the vertical distribution and diurnal migration of herring larvae\nis given. The geographical distribution of larvae was analysed in relation\nto the spawning grounds. The drift of larvae was inferred from subsurface\ndrogue experiments, wind observations and drift bottle experiments.\nLarval length was analysed in relation to the time of spawning. During\nthe first month after hatcl-ting the larvae were concentrated near the\ncoast, and almost no larvae were found outside the edge of the shelf. A\nmore concentrated distribution of larvae was observed in 1961, 1962 and\n1965 than in the other years considered, The larvae are transported\nnorthward in coastal water and the force and direction of the wind play\nan important role in determining the distribution pattern of larvae.\n7. The results indicated that within the range of observed stock sizes\na relationship may exist between the parent stock and abundance of the\nresulting year-classes when favourable conditions occurred. However, it\nis likely that the year-class strength was determined by a complex of other\nfactors which completely ruled out the effect of parent stock size in most\nof the years. Conditions assumed to be advantageous for the development\nof strong year-classes were inferred from the present and other investigations.\nStrong year-classes seemed to occur when a combination of the\nfollowing conditions existed :\n1) widespread distribution of spawning;\n2) long duration of the spawning period;\n3) a rapid dispersion of larvae from the spawning grounds.\nIn 1959, 1960, 1963 and 1964, spawning was extensive and prolonged and\nwith a rapid northward dispersion of larvae from the different spawning\ngrounds. In 1962 the opposite conditions existed. Spawning was extensive\nin 1965 and prolonged in 1961, but the transport conditions during and\nafter the yolk sac stage resulted in a concentrated distribution pattern.\n8. The coincidence in time between the occurrence of suitable food\nand hatching of herring larvae is assumed to be the most important\nenvironmental factor controlling year-class strength during the early\nlarval development. The gradual northward displacement of the main\nspawning centre during the last decades probably has increased the\nimportance of the timing factor, since only two definitely rich year-classes\noccurred during the period 1947-1965, namely those of 1950 and 1959.