TRIALS OF TWO-LEVEL NESTS FOR INCUBATION OF BROWN TROUT (SALMO TRUTTA L.) EGGS IN THE ULMOSENJOKI RIVER (LAKE LADOGA CATCHMENT)
The outcome of a trial application of a non-hatchery method for the reproduction of brown trout, Salmo trutta L., in the Ulmosenjoki River (Lake Ladoga catchment) is presented. Specially designed incubation nests were planted on the bottom of a rapid section of the river. Each device is a streamlined flattened vessel with two layers of incubation plates with egg slots inside it. There is a water inlet in the bottom of the device to secure continuous supply of fresh water to the eggs; short tubes through which larvae can leave the nest are situated laterally. The nest allows for incubating impregnated brown trout eggs through the winter, yielding in spring viable larvae, which will then disperse around the rapid. The trials revealed both advantages (increased capacity for the incubated eggs) and structural shortcomings (poor flow at the lower incubation plate, hindering the exit of larvae). Overall, the nest’s hatching yield was 92-95 %, but no more than 70 % of the larvae exited the nest. Given a minor upgrade, the device can be used for restoration of brown trout stock in small rivers, where hatchery-based reproduction is for various reasons impractical.