THE EFFECT OF LIGHT EXPOSURE ON BUOYANCY OF HALIBUT EGGS

Effects of light-exposure on eggs of the Atlantic halibut (Hippoglossu s hippoglossus) were investigated. Egg buoyancy, yolk sac osmolality a nd perivitelline space (PVS) in light and dark-exposed eggs were follo wed during 3-12 days after fertilization. In light-exposed eggs, the d ensity increased to a maximum at day 6 while the density in dark expos ed eggs significantly decreased between day 4 and 10. There was no sig nificant difference between treatments at day 12. The pattern of yolk osmolality reflected these changes in density. Embryonic volume, calcu lated from estimates of total volume and PVS, was found to decrease ra pidly at days 3-4 after fertilization in the light exposed group, wher eas the control group during the same period showed no change. After t his period, the embryonic volume showed a parallel decrease in both gr oups. The increased egg density is caused by both the loss of water fr om the embryonic compartment and by increased yolk osmolality. These r esults are discussed in relation to changes in vertical distribution i n both natural and culture systems. (C) 1995 The Fisheries Society of the British Isles