Möjliga åtgärder för att öka lågvattenföringen och förutsättningarna för havsöring (Salmo trutta) i Gråskaåns avrinningsområde

During the 19th-century many lakes in Sweden were lowered because the need of agriculture land was high. The population was increasing so the government decided to lend money to farmers that wanted to expand their land. Today we are experiencing the environmental results of these actions. Hydrological effects like changed flowing directions and the damping effects of the lakes have been taken away. The management of the forest is also affecting the water system. In the catchment of Gråskaån several lakes has been lowered and a lot of land has been drained. The result is a quick outflow of water to the sea, when it's raining or when the snow is melting. Measurements in the field and also simulations with the HBV light model shows a quick decrease in flow when it's receiving water, during the summer months anyway. Computer simulations shows that during the 1980's the flow was higher, that is probably because of larger amount of precipitation during the 80's. The winters were also colder and that gives a larger amount of snow accumulation and more melting water in the spring. The low water flows are also affecting the fishes. The county administrative board of Stockholm wants to have a stock with salmon trout in Gråskaån. Therefore there have been projects going on, since the year of 1976, with putting out salmon trout in the river. In May 2003 were the last time for that, and test fishing in October the same year showed that there was no trout left in the river which is probably because of the low water flow during the summer months. The fish are responding to low water flows with heading downstream, and in this case, the fish probably swims out to Edeboviken. During the flow measurement the stream velocity varied between 37 cm/s and 1 cm/s. The salmon trout needs at least 15 cm/s to survive. So the big variation in water flow causes big problems. The salmon trout was reproducing in the stream during the 1980 but not anymore and that is probably because of the larger amount of precipitation during that time compared to now. So in order to keep the salmon trout in the river some counter measures has to be made. Countermeasures like damming of lakes to ensure a water flow during summer when the stream sometimes is completely dry, is one of the things that can be done. Also countermeasures like the removal of plants and construction of god playing grounds for the salmon trout has to be done in order to keep the fish in the stream