Comparison of the physical properties of real and artificial salmonid eggs and of their performance when drifting in an experimental stream channel

The paper compares three batches of brown trout (Salmo trutta Linnaeus) eggs, two batches of Atlantic salmon (Salmo salar Linnaeus) eggs and the artificial trout eggs described by Ottaway (1981), in terms of their measurable physical characteristics. Attention was given to the effects of temperature and of the stage of development of the real eggs. Comparisons were also made between brown trout eggs and artificial eggs of the rate of fall in a water column, of the value and speed of attainment of terminal velocity when falling through water and of the pattern of settlement within an experimental stream channel.The main measurable characteristics of brown trout eggs (after water hardening) and of artificial eggs (values in parentheses) were: diameter 0.52–0.54 mm (0.54), fresh weight 0.078–0.086 g (0.09), density 1.071–1.075 g ml^–1 (1.066), and volume 0.073–0.081 ml (0.085). There was no evidence of any major change in these values as egg development proceeded. Salmon eggs had a similar density to trout and artificial eggs but had 30 to 70% greater weight and volume. The water-hardening of eggs caused their dry matter content to fall from 37–39% to 31–34% and there were corresponding increases in volume and fresh weight.The rate of fall of trout eggs and artificial eggs through water when timed from rest at the water surface to a depth of 125 cm was similar for both types of egg at 10 °C. This rate rose with increasing temperature at the same low rate of c. 0.02 cm s^–1 °C^–1 for both types of egg. Both types of egg had a terminal velocity of c. 8.8 cm s^–1 and both achieved terminal velocity in less than 1.5 s after release.The patterns of settlement of the two types of eggs in an experimental channel were similar.