Bacterial infections in fish are usually treated using medicated food pellets which
are added directly to pens or cages (Samuelsen et al.1992a,b; Hektoen et al. 1995 ).
When infected, cultured fish show reduced appetite and thus feed intake. Conse-
quently, a large proportion of medicated feed is not eaten and this passes through
the cages and is available for distribution to other compartments. Furthermore, the
bioavailability of many antibacterial agents is relatively low and drugs may also
enter the environment, via faeces and urine (Bjo ̈rklund and Bylund 1991 ; Hustvedt
et al. 1991 ). In recent years, improved husbandry practises have reduced the amount
of waste feed generated and more recently authorised medicines have greater
bioavailability (F>95%). Nevertheless, deposition of drugs from uneaten feed or
faeces on or in under-cage sediment can be a major route of environmental
contamination for pharmaceuticals used in aquaculture (Jacobsen and Berglind
1988 ; Bjo ̈rklund et al. 1991 ). Once present on or in sediment, compounds may
also leach back into the water column. During periods of treatment, some of the
drugs entering the environment in waste feed and faeces are also taken up by
exploitative wild fish, shellfish and crustaceans (Bjo ̈rklund et al. 1990 ; Samuelsen
et al.1992b; Capone et al. 1996 ).
Where topical applications of chemotherapeutics are made, fish are usually
crowded into a small volume of water for treatment. Concentrated drugs are
added directly to the water of open net-pens or ponds, net-pens enclosed by a
tarpaulin or tanks. Waste effluent is then either released into the surrounding water
column or subjected to local wastewater treatment and recycling (filters, settlement
basins and ponds) (Montforts 1999 ). Sludge recovered from waste water recycling
activities may be applied directly onto land or sold as fertiliser (Montforts 1999 ).
3 Fate and Behaviour
Once a veterinary medicine is released into the environment, its behaviour will be
determined by its underlying physical properties (including water solubility, lipo-
philicity, volatility and sorption potential). In the following sections information on
the fate and transport of veterinary medicines in the environment is reviewed.
3.1 Sorption in Soil
The degree to which veterinary medicines may adsorb to particulates varies widely.
Consequently, the mobility of different veterinary medicinal products also varies
widely (e.g. Fig. 2 ). Available data indicate that sulfonamide antibiotics and organ-
ophosphate parasiticides will be mobile in the environment whereas tetracycline,
macrolide and fluoroquinolone antibiotics will exhibit low mobility. The sorption
behaviour of individual veterinary medicines can also vary widely in different soil
types and unlike many other classes of soil contaminant (e.g. hydrocarbons and
Veterinary Medicines and the Environment 295