Fate and distribution studies of some drugs used in aquaculture

Residue concentrations of drugs that are administered to fish by bath immersion are related primarily to passage of the drugs across the gills. The elimination of these chemicals by fish can be mediated by biotransformation, but the route of elimination depends on physical characteristics of the chemicals or on their biotransformation products. Uptake of the anesthetics tricaine methanesulfonate, benzocaine, Piscaine, and quinaldine is rapid because they are lipophilic. Loss of their residues also is rapid after the fish are removed from anesthetic solutions because the gradient of concentration favors passage back across the gills. Among therapeutants, uptake and loss of malachite green residues in fish follow the same general pattern as the anesthetics, although at much slower rates; the residues accumulate in the eggs of gravid female salmon after treatment and are detectable in eggs and newly hatched fry. In fish treated with formalin, residues of formaldehyde cannot be detected by currently available analytical methodology. Sulfonamides are metabolized in fish by acetylation and conjugation; however, the free form of the drug appears to be eliminated more rapidly than the acetylated form.