RADIOACTIVE CESIUM IN FISH IN SWEDISH LAKES 1986-1988 - GENERAL PATTERN RELATED TO FALLOUT AND LAKE CHARACTERISTICS

The aim of this work is to provide a picture of how the concentration (= activity) of Cs-137 varied geographically in Sweden during 1986, 1987 and 1988 in different fish species (especially pike, perch, char and trout), to study how different lake properties (water chemistry and morphometry) influence the Cs-137 uptake in fish, to try to develop a load diagram for lakes when Cs-137 in fish is used as an effect parameter, and to study how the caesium concentration varies seasonally and with fish size. A register was compiled, containing a broad set of data from 644 Swedish lakes. The median Cs-137 concentration in fish increased between 1986 and 1987 by between 13% (trout) and 240% (pike). The increase between 1987 and 1988 has stagnated for most species, but not for pike where the concentration increased 82% in 'the median lake'. The in-lake, between-lake and temporal variations are large and differ from species to species. About 14 000 lakes in Sweden had fish ('100 g perch') with Cs-137 concentrations above 1500 Bq/kg (w/w) (the Swedish guideline) during the autumn of 1987. Apart from the radioactive fallout, the water retention time is the individual factor of greatest importance for the variability of Cs-137 concentration in fish between these lakes - longer water retention time gives higher Cs-137 concentrations in fish. Lake water hardness and/or conductivity also influence the uptake of Cs-137 in fish - lakes with high values of these parameters tend to have lower concentrations of Cs-137 in fish.