Modelling ecological half-lives for radiocaesium in Norwegian brown trout populations.
Models of ecological half-life may be valuable and cost-effective predictive tools for authorities setting restrictions on human consumption of freshwater fish after environmental releases of radioactivity. This work aimed to validate such a model for radioactive caesium (134Cs and 137Cs) in brown trout Salmo trutta populations. Data were drawn from lakes with a wide variability in abiotic and biotic factors and initial caesium load. In Norway, the highest fallout (more than 150 kBq m-2 of 137Cs) from the Chernobyl accident occurred in Oppland county, in south central Norway. Radioactivity was measured in more than 1800 samples of brown trout in nearly 100 localities in this region during 1986-95. The back-calculated maximum initial radioactivity on 1 January 1987 showed a strong regional variability (range 443-13 370; average 3855 Bq/kg). Large variation in initial radioactivity was also recorded on a local scale (within 50 km). The ecological half-life model for caesium in brown trout populations for 1987-94/95 gave a close fit to real data from all localities with sufficient time series. Predicted half-lives ranged from 1.2 to 4.2 years (average 2.5) but 95% confidence limits were narrow (2.7 and 2.3 years). The overall variability in radioactivity levels over time was almost entirely related to the initial load and, with few exceptions, 88% of the change in radioactivity was explained by the simple regression model. Modest variability in ecological half-life was not correlated with initial activity, and no clear effects of water quality or season could be detected. For most lakes, levels of radioactivity in brown trout appeared to be predictable, with high accuracy after a fallout event, without extensive information on population ecology and water quality. However, more detailed work may be required to assess patterns within individual lakes.