Modelling ecological half-lives for radiocaesium in Norwegian brown trout populations

1. Models of ecological half-life may be valuable and cost-effective predic tive tools for authorities setting restrictions on human consumption of fre shwater fish after environmental releases of radioactivity. This work aimed to validate such a model for radioactive caesium (Cs-134 and Cs-137) in br own trout Salmo trutta populations. Data were drawn from lakes with a wide variability in abiotic and biotic factors and initial caesium load. 2. In Norway, the highest fallout (more than 150 kBq m(-2) of Cs-137) from the Chernobyl accident occurred in Oppland county, in south central Norway. Radioactivity was measured in more than 1800 samples of brown trout in nea rly 100 localities in this region during 1986-95. 3. The back-calculated maximum initial radioactivity on 1 January 1987 show ed a strong regional variability (range 443-13 370; average 3855 Bq kg(-1)) . Large variation in initial radioactivity was also recorded on a local sca le (within 50 km). 4. The ecological half-life model for caesium in brown trout populations fo r 1987-94/95 gave a close fit to real data from all localities with suffici ent 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). 5. The overall variability in radioactivity levels over time was almost ent irely related to the initial load and, with few exceptions, 88% of the chan ges in radioactivity was explained by the simple regression model. Modest v ariability in ecological half-life was not correlated with initial activity , and no clear effects of water quality or season could be detected. For mo st lakes, levels of radioactivity in brown trout appeared to be predictable , with high accuracy after a fallout event, without extensive information o n population ecology and water quality. However, more detailed work may be required to assess patterns within individual lakes.