The effect of biological and physical factors on the survival of Arcto-Norwegian cod and the influence on recruitment variability

Abstract

Data from egg and larval field investigations h the Lofoten area are used\ntogether with long time series of data on temperature and zooplankton in\nLofoten and abundance indices of Arcto-Norwegian cod. The importance of\nenvironmental factors at the spawning grounds on the year class strength lis\ndiscussed.High temperature is a necessary, but not sufficient condition for the\nproduction of strong year classes . Spawning of Calanus finmarchicus in\nLofoten is strongly influenced by the temperature in the upper layer, causing\nlarge variations in the time of nauplii production. The peak spawning of the Arcto-Norwegian cod is fixed within a period of one\nweek in late Marchlearly April and is very stable between years. The\nincubation period is influenced byo the temperature of the coastal watermasses\nwhich vary between 0.5 and 4.5 C, causing a maximum difference in peak\nhatching of more than two weeks. Young larvae are found to have the center of their distribution close to the\nspawning ground, where the best feeding conditions for the larvae als0 are\nfound. However , the distribution of subsequent stages of eggs shows both\nspreading and transport. Only the most advanced larvae seem to be able to\nsurvive the drift through T'poorer" feeding areas on their way from the\n"retentionTt area to the Barents Sea. Gut analyses of larvae subjected to different prey densities indicate that the\ncritical prey density for successful feeding must be on the order of 5-10\nplankter s per liter. Lengthldry weight plot of larvae from 1982-1985 indicate the best growth\nconditions in 1983 followed by -84, -82, and -85. In 1983, 1984 and 1985\nout standing year classes of Arcto-Norwegian cod were produced , while the\nyear class' was of medium size in 1982.