Seasonal Evolution and Individual Differences in Silvering Eels from Different Locations

Abstract

Silvering is a requirement for downstream migration and reproduction. It marks the end of the growth phase and the onset of sexual maturation. This true metamorphosis involves a number of different physiological functions (osmoregulatory, reproductive), which prepare the eel for the long return trip to the Sargasso Sea. Unlike smoltifica-tion in salmonids, silvering of eels is largely unpredictable. It occurs at various ages (females: 4–20 years; males 2–15 years) and sizes (body length of females: 50–100 cm; males: 35–46 cm) (Tesch 2003). It is most common when studying eels, to separate individuals into two groups, yellow (resident) and silver (presumably migrant), and to compare the physiological profiles between the two. Basic knowledge was obtained in this way and we will first review what is known about these two stages. Because of the difficulty of getting individuals while they are in the process of metamorphosing, little is known about the dynamics of the silvering process. Here we present new information on the triggers, duration and succession of events up to the silver migratory stage. The transitional stages—between the yellow (resident) stage and the silver (migrating) stage—were characterized through exhaustive sampling of both resident and migratory eels. New aspects concerning the endocrinology of the silvering process will be given based on seasonal sampling of eels undergoing metamorphosis. These results will also bring new insights on the triggers and timing of silvering.