Uptake, distribution and elimination of chemicals in fish - Which physiological parameters are the most relevant for toxicokinetics?

Bioconcentration and toxicity studies are regularly conducted for the risk assessment of chemicals. If such tests yield different results for different fish species, this can either be due to differences in toxicokinetics or to differences in toxicodynamics. Here we investigate which physiological parameters could cause major differences in the toxicokinetics in fish. To this end it is important to distinguish physiological parameters that affect the sorption capacity of the fish from those that affect kinetic processes. Variability in the lipid content of a fish is the most influential parameter for the sorption capacity of fish and therefore most relevant for the total concentration in fish under steady-state conditions when metabolism is not relevant. In terms of kinetics, ventilation rate, uptake efficiency from food and metabolism are the most influential factors. While ventilation rate can roughly be estimated from allometric scaling equations, little general information is available on the uptake efficiency from food. The metabolism rate constant appears to be the single most influential toxicokinetic factor. This information cannot be estimated but must be determined experimentally, preferably from in vitro experiments.