However, Schram et al. (2012) found that blood plasma nitrate increased almost linearly with the concentration of the culture water in juvenile African catfish even though the ratio of plasma nitrate to waterborne nitrate was relatively low, i.e. 0.15–0.25.There fore, the theory of passive uptake of nitrate in freshwater fish is reasonable; but nonetheless uptake of nitrate does occur, suggesting that prolonged exposure times that are common within low exchange WRAS might lead to longer term toxic effects.