Vertical distribution of pelagic fish eggs in relation to species, spawning behaviour and wind conditions

Abstract

1. The mean reduction of egg diameter from March to April is from 1.45 to 1.39 mm and 1.44 to 1.38 mm for artificially and naturally spawned eggs of Arcto-Norwegian cod, respectively. This means a reduction of volume of about 11%. 2. The neutral buoyancy of cod eggs ranges from 29.5-33.0°/oo salinity. Eggs of captive fish show a tendency of higher specific gravity. 3. The neutral buoyancy of the cod eggs is not correlated to egg size, but to the weight of the eggshell. 4. Computed ascending velocities in the natural environment of Lofoten range from 0.2 mm s⁻¹ to 1.7 mm s¹ , and show an approximately Gaussian distribution with a mean/ascending speed of 1 mm s¹ 5. Since the spawning of Arcto-Norwegian cod is strictly connected to the thermocline at temperatures of 4-6°C, spawning can occur both pelagically and close to the bottom. 6. Norway pout and other gadoids are also spawning in Lofoten, and occasionally their eggs are found in majority. Their vertical distribution have maxima in deeper water, indicating a higher specific gravity than for cod eggs. 7. During calm wind conditions the concentration of cod eggs increases rapidly towards the sea surface. Large variation in the upper centimeters of the sea is found during such conditionsm Increasing wind rapidly mixes down the eggs in the upper meters. As the wind speed maintains, larger parts of the water column are influenced by the wind mixing. At south-westerly wind of 10 ms⁻1 and at 24 hours duration more than 50% of the eggs are found below 30 m depth.