Maturity changes in Norwegian spring-spawning herring before, during, and after a major population collapse

The Norwegian spring-spawning herring (Clupea harengus) population collapsed to the state of commercial extinction in the late 1960s; the stock remained at extremely low levels for over two decades, but has recovered fully since the 1980s. It is known that the collapse coincided with strong changes in maturation characteristics. However, the long-term patterns of maturation in this stock have remained largely undescribed. We applied discriminant and neural network analysis to historical scale data to predict age at maturation for individual herring of the year-classes 1930-1992. Based on these data, this paper (1) presents new, revised maturity ogives for these year-classes with a temporal resolution of 1 year and (2) describes the long-term variability in age at 50% maturity, length at 50% maturity, and length-at-age. This new information shows that during the period of low stock abundance, age at 50% maturity was considerably reduced and length at 50% maturity moderately increased as compared to the situation both before and after the collapse. These variations coincided with changes in length-at-age, suggesting that the maturity changes to a large extent reflect variations in body growth related with stock abundance.