Every fall trout migrate up Denmark's many shallow streams and creeks. Their goal is, for the females, to lay their eggs in the gravel banks in the bottom of the stream. The trouts have had their young years in the freshwater, but has grown large in the sea for several years, by eating small fish and crustaceans - that's why they're also called sea trout. Now they swim up in exactly the same stream where they were once born. There is a long way from the sea and up through the river system, where many dangers as otters and herons are lurking. But the trouts have only one thing in mind: to reproduce themselves even if it might costs them their lives. Some trouts will swim many miles up the river and further up the quite small tributaries, which are almost invisible to humans. When they finally reaches the spawning grounds in their home water, a fight among themselves are fought, and the males struggle to get the most beautiful females. The female fish uses her tail fin to dig a hole in the gravel bank. That's where she will put her small eggs, and when the spawning takes place, the male places himself beside the female to fertilize her eggs. Once the seatrout enters freshwater it stops feeding, so they have to draw energy from their fat reserves they've built up during their time in the sea. Seatrout can grow up to about 90cm long. Therefore, the small creeks can become really alive, when a shoal of fish reach their spawning grounds.