Causes of trends and fluctuations in the Arcto-Norwegian stock
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- Symposium proceedings 
Published information is used to review trends and fluctuations in landings and stock characteristics/variables (abundance, growth, maturation, distribution, and migrations) in relation to changes in environmental and anthropogenic factors as well as in interactions with other species. The stock declined from about 4-5 million tonnes in the 1950s to less than 1 million tonnes in the 1980s owing to exploitation. For more than 30 years the annual fishing mortality rate was well above any calculated safe level, so that the spawning stock was at times reduced to levels at which recruitment was impaired. Age of maturity fell as the stock declined. Recruitment is positively related to temperature. A temperature-regulated mechanism of interaction between cod larvae and their prey, and driven by variations in inflows of Atlantic waters to the area, determines larvae survival and probably also the abundance and size of the 0-group at age 5-6 months. Predation from birds and marine mammals as well as cannibalism are shown to cause considerable interannual variations in the mortality of juveniles and young cod, and thus variations in the number of fish recruiting to the fisheries at ages 3- 5 years. Large short-term variations in the growth of cod caused by varying availability of prey (capelin) have been observed; growth is also positively related to the temperature within the distribution area of the stock.
SerieICES Marine Science Symposia