Variable natural mortality rate of juvenile Norwegian spring-spawning herring (Clupea harengus L.) in the Barents Sea

Abstract

Understanding the patterns of mortality rate in the juvenile stages of exploited fish populations is essential for improving current stock assessment procedures. Most of the research on 'the recruitment problem' has focused on the larval stages, but recent work indicates that a large part of the variability in year-class strength may be associated to variable mortality rates during the juvenile stages. The data on abundance of juvenile Norwegian spring-spawning herring in the Barents Sea collected by the Bergen Institute of Marine Research between 1983 and 1993 was analysed to test this hypothesis. The results give strong support to the hypothesis. Survival from the start of the 0-group stage to age 3 years may have varied from 0 to 30% for the cohorts investigated. Mortality rate is shown to be strongly age-dependent, and between 75% and 99% of the total juvenile mortality occurs during the first year of life.