Relations between seasonal migrations and fat content in Norwegian spring spawning herring (Clupea harengus L.)
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The relationship between seasonal migrations and fat content in Norwegian spring spawning herring (whole herring and fillets) was examined using data for the period 1994-1996. During the feeding period (April-August) in the Norwegian Sea the herring stored a considerable amount of fat as energy reserves. When the stock entered the wintering area in Vestfjorden in autumn (August-September), the fat content was approximately 21-22%. From the time of arrival in Vestfjorden to the end of spawning (March-April) the herring did not feed. During this period the fat store was used for metabolism and gonad development. The decrease in fat content during the wintering period (September-December) was only around 2%. However, the energy expenditure during spawning migration was quite extensive. The fat content was reduced by approximately 10% during the spawning migration and spawning period (January/April). The fat content was higher in the body fillets (muscles and skin) than in the whole herring during the wintering period and spawning migration, and lower after spawning and during feeding season. The fat content was approximately 3% lower in fillets without skin compared to fillets containing skin. A positive correlation was found between the fish weight and fat content in herring from the spawning area off Møre, while no correlation was found in the wintering area. This indicates that the energy expenditure during spawning migration is higher in small herring compared to larger individuals. There was no consistent difference in fat content between male and female herring.