Asköviken was discovered as a bird locality in the 1930s. One of the leading bird-watchers in Västerås at that time was Hans Avelin. He had noticed that the black-headed gulls that were to be seen in the city disappeared every evening in a south-westerly direction. Mounting his bike, he followed the birds and found that they nested around the hill of Kvarnberget in Asköviken. The year was 1933, and since then bird-watchers have visited the bay and its surroundings and been impressed by their bird-life.
Asköviken became a nature reserve in 1985, followed in 1991 by the southern part of the area, Tidö. In 2003 the two of them were put together to form the Asköviken-Tidö nature reserve.
The wet meadows serve as foraging and nesting grounds for a range of bird species and are among the most important types of environment in the Asköviken area. Up to about the middle of the 19th century the wet meadows around the bay were mowed, meaning that the vegetation was harvested and used as winter fodder. As patterns of agriculture changed, however, mowing became unprofitable, and the wet meadows came to be used as pasturage instead. During the 1950s and '60s many of the farms reduced their grazing around Asköviken and the wet meadows again showed signs of overgrowth. Tall grasses dominated the vegetation and various species of osiers began to invade the wet meadows. The overgrowth caused many species of birds that were dependent on upkeep to abandon Asköviken.
Improvements in Asköviken during the 1980s and '90s
During the 1980s a start was made on extensive work intended to restore the wet meadows in Asköviken. By soil milling, tussock reduction and burning, the wet meadows were restored to a state such that they were able to support grazing again.
As a consequence, many of the bird species that had earlier left Asköviken began to find their way back to the area.
Great efforts were also put into making Asköviken more accessible for visitors by providing new rambling pathways, information boards and bird towers. Finance for the measures being taken in Asköviken came in part from the Asköviken Fund WWF, which was set up towards the end of the 1980s. The Fund is a joint project between the World Wide Fund for Nature WWF, the County Administrative Board of Västmanland and the City council of Västerås.
At the beginning of the 1990s a Nature School operation was commenced in Asköviken. In 1998, with the backing of numerous sponsors, the operation was given premises of its own, located outside the nature reserve north-west of Asköviken. The basic idea behind the Nature School is to arouse in children and young people an interest in and a feeling for nature through the medium of practical studies. Year by year the operations of the Nature School bring some 6,000 pupils of various ages to Asköviken.
The regulation of Lake Mälaren has changed Asköviken
Under natural conditions, the water level in a lake displays wide variations over the year. During the spring thaw the level may go so high that low-lying shore areas are inundated, whereas during dry summers the level may fall so low that shallow bays are almost dried out. Today, Mälaren is heavily regulated, and the variations in water level are therefore very small. The difference between the highest and lowest level during one year is about 40 cm, as compared with 1.5 metres before the lake was regulated.
It is the variations in water level that normally set limits to the spread of reeds in a lake. The fluctuations give the reedbed a hard time, particularly along the fringes towards open water. Reeds that have been frozen in the ice are lifted when the water level rises during the spring thaw, and dead plant material that has collected in the reedbed is washed away. The effect is to form reedbeds with indented edges where the reeds are rejuvenated in a natural manner. In a regulated lake system this limiting factor is rendered inoperative.
Owing to the severe regulation of Lake Mälaren the reedbeds in Asköviken have altered in appearance over recent decades. At the end of the 1950s the reedbed was varied, with reeds of different ages and indented edges. By today the reedbed has spread out and has almost lost its indented edge towards open water, which is unfavourable to many of the wetland birds that live in Asköviken.
Eutrophication is a serious threat
The influx of nutrients to Asköviken increased during the latter part of the 19th century, and like many other bird lakes in Sweden Asköviken is today showing signs of eutrophication. A large proportion of the nutrients comes in by way of the Asköbäcken, a stream which debouches in the north-western part of the bay. The nutrients come above all from agricultural land in the catchment area of Asköviken and from private drain lines discharging into the Asköbäcken. The eutrophication has many negative effects, among other things hastening the overgrowth of the bay.
