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The image shows the difference in the lower jaw of male and female

 

 

Females become sexually mature from their third year, when they are more than twenty centimetres long. The males mature one year earlier and at that time they are barely twenty centimetres. Occasionally younger brown trout too have been glimpsed passing the camera. .

 

The basic colour of the brown trout is quite variable – the back is darker, the sides covered with a bright border with red (there are more of them above the lateral line) and black dots. The males are as a rule darker-coloured, in places the body seems  to have  “sooty” patches.  In the spawning period a slightly marked cartilaginous hook develops on the lower jaw. The belly is white but a yellow colour can be seen too. On the sides of some female fish silvery blue may be visible. The spawning nest is dug by the female and so determining the sex in the camera image is not very complicated.