THE EFFECT OF LIGHT EXPOSURE ON BUOYANCY OF HALIBUT EGGS

Effects of light-exposure on eggs of the Atlantic halibut (Hippoglossus hippoglossus) were investigated. Egg buoyancy, yolk sac osmolality and perivitelline space (PVS) in light and dark-exposed eggs were followed during 3-12 days after fertilization. In light-exposed eggs, the density increased to a maximum at day 6 while the density in dark exposed eggs significantly decreased between day 4 and 10. There was no significant difference between treatments at day 12. The pattern of yolk osmolality reflected these changes in density. Embryonic volume, calculated from estimates of total volume and PVS, was found to decrease rapidly at days 3-4 after fertilization in the light exposed group, whereas the control group during the same period showed no change. After this period, the embryonic volume showed a parallel decrease in both groups. The increased egg density is caused by both the loss of water from the embryonic compartment and by increased yolk osmolality. These results are discussed in relation to changes in vertical distribution in both natural and culture systems. (C) 1995 The Fisheries Society of the British Isles