Stock structure of Norwegian spring-spawning herring: historical background and recent apprehension

For centuries, Norwegian spring-spawning herring have been the basis for a winter fishery of adult fish along the west coast of Norway. Since regular biological investigations were begun in 1907, more than half a million herring have been studied for age, length, growth, maturity, and racial characters. The present paper reviews the development of the understanding of the structure of this herring stock and how the use of age-structured data eventually became so central for both assessment and research. During the 20th century, several rich year classes contributed to the fishery. This resulted in great fluctuations in stock abundance, but structural features such as age composition, growth, and maturation also changed. Recently, a long-term virtual population analysis (VPA) was made for this stock, and trends in structural features in relation to the long-term variation in stock abundance are discussed. It was found that, even though a very large time-series was studied, the direct stock-recruitment relationship was still unclear. While recruitment seems to be governed by external factors, growth and maturation are more related to stock abundance and year-class strength.