A couple of redundant analyses indicate crude uncertainties in reproducibility of at least .07 ppm for lead. The source of the heavy metals is upstream from Spokane in the Coeur d'Alene mining district of northern Idaho, one of the richest mining districts in the US and also one of the most heavily contaminated. Metals are both dissolved in the river water and found as minute particles, and can enter the food web at various stages. Mean lead content of filets are highest for rainbow trout and lowest for mountain whitefish, with suckers in between.
Standard deviations are high compared to the mean value, indicating a lot of variability.
Size of the fish might be used as a proxy for age to test this assumption. Lead can cause health problems, especially in children.