Effects of liming on crayfish and fish in Sweden

From 1976 to 1982 about 450 liming projects were carried out in Sweden to attempt to protect lakes and other surface waters for fisheries, recreational and nature conservancy purposes. Most follow-up studies were restricted to analysis of pH, alkalinity, water colour and magnesium and calcium concentrations but, in 158 of the projects, special studies were carried out such as gill-net surveys, electrofishing or crayfish fishing. Lakes were classified as those with pH less than 5.5 before and after liming (unsuccessful treatment), those in which pH was changed from less than 5.5 to more than 5.5 (successful treatment), and those in which pH was greater than 5.5 before and after treatment (mildly acidified waters). An increase in the catch of crayfish was recorded in 7 of the 17 lakes which were treated, but this was significant in only one lake. Thus, factors other than pH might be of importance for the development of crayfish populations after liming. In the 47 lakes and 7 rivers tested, fish numbers tended to increase after liming, and streams which had a stable pH of above 5.5 supported the same numbers of juvenile salmonids as unacidified streams.