Modelling age-dependent natural mortality of juvenile Norwegian spring-spawning herring (Clupea harengus L.) in the Barents Sea

Abstract

Understanding the patterns of mortality in the juvenile stages is essential for improving current stock assessment procedures. A crucial aspect is at which stage in the life cycle of fish is year-class strength determined. 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 with variable mortality during the juvenile stages. A statistical model, including age and year-class effects, was built, and fitted to 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. The results indicate that mortality is strongly age-dependent, and that most of it occurs during the first year of life. There is also a large inter-cohort variability in juvenile survival. Survival from the start of the O-group stage to age 3 years may have varied from very low to around 30% for the cohorts investigated.