We have already presented you with trout spawningThis is a kind of nesting. But what happens to these tiny little fertilised eggs when they're left to their own devices?
Before hatching, the eggs enter an incubation period for the first stage of development. The length of this period depends on the temperature of the water; the eggs must reach a total temperature of 420°. What does this really mean? The principle is simple: you add up the temperatures of each day since the eggs were laid to obtain a total of 420° (in theory, of course). This means that egg incubation will be longer if the water is cold, and shorter if it's warm. Good oxygenation of the eggs is also crucial to their development. If the water is polluted, muddy or contains suspended matter, this compromises the life of the eggs by clogging them up and depriving them of oxygen.
Then comes the hatching. But the fry are not yet fully developed. Without fins or a mouth, they can neither swim nor feed. In order to be fed, they have a vitelline vesicle, a sac containing nutrient reserves, at the level of their neck, which keeps the small fry alive. Fragile and vulnerable, they hide at the bottom of the spawning ground, a veritable nursery, sheltered from the light in the gravel, until the yolk bladder resorbs and their development is complete.
When the vesicle is completely resorbed, the fry carefully emerge from their spawning grounds in search of food. This is when they emerge. On average, only around twenty of the hundreds of eggs laid reach this stage. At first, the fry remain in areas close to their spawning grounds, then they start to gain ground, actively defending their territory. Their favourite areas are shallow and slow-moving, which seems reasonable for a fragile fry just starting to grow. You will also find them in overhanging vegetation or under dead wood, strategic shelters for hiding from predators. It is only when they are one or two years old that some trout leave their river and migrate to the lake. The others remain for the rest of their lives in the river in which they were born. Specialists are still unable to explain why some trout migrate and others choose to stay in their rivers. Still a mystery, the trout will never cease to fascinate us.
Information taken from the FIBER information brochure and the article "Trout and their habitat" on the Trout and Rivers website.
Photo credit: ©Swiss fishing consultancy FIBER and ©Uwe Kils
