SOURCES OF VARIATION IN POST-CHERNOBYL RADIOCESIUM IN FISH FROM 2 CUMBRIAN LAKES (NORTH-WEST ENGLAND)

1. The objective was to evaluate sources of variation in Cs-137 in brown trout (Salmo trutta) and perch (Perca fluviatilis) from Devoke Water (samples from April 1986, September 1986-October 1988), and trout, perch and pike (Esox lucius) from Loweswater (samples from May 1986, August 1986-March 1988). 2. Overall geometric means for Cs-137 in trout (540 Bq kg-1) and perch (1192 Bq kg-1) in Devoke Water were significantly higher than those in Loweswater (trout 404, perch 989 Bq kg-1); means for perch were significantly higher than means for trout in both lakes, and the mean for pike (568 Bq kg-1) in Loweswater lay between the values for trout and perch. 3. Cs-137 content increased with fish weight for trout from both lakes, perch from Devoke Water but not perch or pike from Loweswater; Cs-137 values for perch were generally higher than those for trout in Devoke Water; the power-function relationship between Cs-137 and fish weight invalidates the use of uncorrected means for temporal comparisons. 4. The stomach contents of trout and perch from Devoke Water indicated a diverse diet; fish were in good condition according to the length-weight relationship and there was no indication that diet differences had any significant effect on the Cs-137 content of individual fish. 5. When Cs-137 values were scaled to a standard weight of trout to remove the marked effect of fish size, monthly geometric means for Cs-137 increased from April/May 1986 to a maximum in October 1986 in Devoke Water and February 1987 in Loweswater, then decreased exponentially at a slow rate in Devoke Water and a fast rate in Loweswater, and finally increased slightly in Loweswater only. Geometric means for perch and pike followed a pattern similar to that for trout, except that maxima occurred in February/March 1987 for perch in both lakes and in June 1987 for pike in Loweswater. 6. Exponential rates of decrease for geometric mean values of Cs-137 were used to estimate ecological half-lives of Cs-137 in the fish, and these were significantly different between lakes and also between species in Devoke Water but not Loweswater (Devoke Water trout 825 days, perch 2390 days; Loweswater trout 103 days, perch 94 days, pike 112 days). Differences between species may be due to different metabolic rates (gastric evacuation rates are compared as an example) and differences between lakes due to variations in their limnology and the nature of their catchments.