Differential utilization of energy during wintering and spawning migration in Norwegian spring-spawning herring

The energy utilization in non-feeding Norwegian spring-spawning herring Clupea harengus was examined during periods of wintering in a fjord system in northern Norway and southward spawning migration (800 km distance) to the main spawning area off western Norway. At the onset of wintering the gonadosomatic index (I-G) was higher and the amount of mesenteric fat was lower in male than in female herring, whereas the subsequent increase in I-G and decrease in mesenteric fat occurred at a higher rate in females than in males. However, no differences were recorded between sexes in fat content (%) of whole fish at any time of the year. The absolute energy loss increased, whereas the relative energy loss decreased. with fish length. Furthermore, the absolute and relative length specific energy losses were two to three and three to four times: higher, respectively, during spawning migration than during wintering. The distinct energy loss was attributed mainly to ht metabolism, whereas the storage protein only was transferred from the soma to the gonads and remained relatively constant in whole herring throughout the wintering and spawning migration. (C) 1999 The Fisheries Society of the British Isles.