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 together with long time series of data on temperature and zooplankton in Lofoten and abundance indices of Arcto-Norwegian cod. The importance of environmental factors at the spawning grounds on the year class strength lis discussed.High temperature is a necessary, but not sufficient condition for the production of strong year classes . Spawning of Calanus finmarchicus in Lofoten is strongly influenced by the temperature in the upper layer, causing large variations in the time of nauplii production. The peak spawning of the Arcto-Norwegian cod is fixed within a period of one week in late Marchlearly April and is very stable between years. The incubation period is influenced byo the temperature of the coastal watermasses which vary between 0.5 and 4.5 C, causing a maximum difference in peak hatching of more than two weeks. Young larvae are found to have the center of their distribution close to the spawning ground, where the best feeding conditions for the larvae als0 are found. However , the distribution of subsequent stages of eggs shows both spreading and transport. Only the most advanced larvae seem to be able to survive the drift through T'poorer" feeding areas on their way from the "retentionTt area to the Barents Sea. Gut analyses of larvae subjected to different prey densities indicate that the critical prey density for successful feeding must be on the order of 5-10 plankter s per liter. Lengthldry weight plot of larvae from 1982-1985 indicate the best growth conditions in 1983 followed by -84, -82, and -85. In 1983, 1984 and 1985 out standing year classes of Arcto-Norwegian cod were produced , while the year class' was of medium size in 1982.