Effekter av kalkning på fisk i sjöar: resultat av 48 års nätprovfisken

Biological effects of acid rain appeared in the 1970s and 1980s in lakes and streams in Sweden and other countries. Fish monitoring in the 1970s and 1980s revealed the effects of acidification on fish in Swedish lakes, such as recruitment failure, reduced abundance, and loss of fish species. During the same period, liming experiments started in many of the lakes. Recruitment of acid-sensitive fish species resumed after liming in some lakes sampled with gillnets both before and after liming. Liming started in many lakes without previous inventory of the fish fauna, but many of them were later monitored at least once. Some lakes were later included in the national programme for integrated studies of effects of liming (IKEU), where sampling and age determination of fish are performed more frequently than in other limed lakes.We asked the county administrative boards to provide liming and water chemistry data, linked to samples of fish in the national database of gillnet surveys. For 1,343 lakes the year of first liming was known (median year 1986). We also found 698 non-limed lakes and, by pH and alkalinity, 432 were classified as either acidic (69 lakes), neutral (256 lakes) or calcareous (107 lakes). Roach (Rutilus rutilus) was the most common biological target for liming. The next most common fish targets were brown trout (Salmo trutta) and Arctic char (Salvelinus alpinus); perch (Perca fluviatilis), pikeperch (Sander lucioperca), smelt (Osmerus osmerus), whitefish (Coregonus lavaretus) and vendace (Coregonus albula) sometimes appeared among listed targets for liming.We found support for earlier reports on increased species richness, abundance, and biomass of fish after liming. In lakes sampled before and after liming, the targeted roach, Arctic char and brown trout were in several cases only caught after liming had occurred. The current gillnet standard captures small and young fish more efficiently than gillnet types used before and for a few years after the first liming, confounding interpretation of the long-term development of the fish fauna. In fish samples captured with the current standard gillnet the ecological status (assessed by two multi-metric fish indices) was higher in limed than in acidic lakes, bust the most acidspecific index was still lower in limed than in neutral lakes. We also examined fish metrics in relation to pH three years before fish sampling. Analyses included abundance and biomass, multi-metric fish indices, and metrics based on size and age related to recruitment of roach and perch. Fish metrics with known response to pH in non-limed lakes were generally insignificantly related to pH in the limed lakes.We also highlight other fish and liming related questions that could not be answered with existing data. These questions relate to differences in biological response between directly limed lakes and lakes with only upstream liming, excess versus normal liming, to subsurface acid episodes, and to fish re-stocking or natural recovery.