Like a planet in space, a rainbow trout egg sparks and crackles as biological processes begin a miraculous transformation, the same that progresses silently in the inscrutable waters of New Zealand’s wild rivers every day. But even in clean rivers, the odds are stacked against this small vessel of life—only one in a thousand eggs will hatch and survive until adulthood.
Rainbow trout eggs are approximately five millimetres in diameter and weigh about one third of a gram when they are laid into a depression in the gravel river bed, or spawning redd, by the female trout. On hatching the fry unfurl to about 10 mm long and remain hidden in the gravel feeding off the yolk sac for three weeks. They emerge into a world where everything wants to eat them and have to find shelter from both the fierce flow of the river and their predators. The survivors quickly find shelter in shallow backwaters and start looking for food. From here on their lives are dominated by the requirement for shelter, and their growth rate determined by food availability and water temperature. The faster they grow, the safer they are, and because they are cold-blooded and a cold water fish, they grow best in the chill waters of spring. As water temperature increases the fish use more of the energy derived from food for the metabolic needs of their body and have less left over for growth. Eventually, when the temperature gets too high, growth stops altogether and they struggle to survive unless they can seek out cool water in deep lakes or spring-fed inflows.