Maturation characteristics in Norwegian spring-spawning herring before, during, and after a major population collapse

The immense Norwegian spring-spawning herring (Clupea harengus) population collapsed to the state of commercial extinction in the late 1960s, probably largely due to overfishing; the stock has fully recovered since the 1980s. It is known that the collapse strongly affected maturation characteristics. However, the long-term patterns of maturation in this stock have remained largely undescribed. Using discriminant analysis and artificial neural networks, the age at maturation for individual fish can be predicted from routine scale measurements. We applied these methods to historical data on herring scales available from 1935 up to the present, in order to (1) describe the long-term variability in both age and length at 50% maturity, and (2) compute new, revised maturity ogives for the stock over the same period with temporal resolution of one year. This new information, with improved temporal resolution, enables comparisons of maturation characteristics before, during, and after the population collapse. As a further step, in order to increase our understanding of the potential impact of commercial fisheries on the genetic composition of fish stocks, we aim to disentangle genetic from phenotypic aspects of maturation.
KEYWORDS: Herring; Sild; Stock development; Bestandsutvikling

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