Schmidt, L.J., Gaikowski, M.P., and W. H. Gingerich. 2004. 
An environmental assessment of the proposed use of 
oxytetracycline-medicated feed in freshwater aquaculture. 
Submitted to the Center for Veterinary Medicine, October, 2004. 78pp.

An Environmental Assessment of the Proposed Use of Oxytetracycline-Medicated Feed in 
Freshwater Aquaculture

This environmental assessment (EA) provided an examination of the possible environmental 
effects of oxytetracycline (OTC) discharge into freshwater ecosystems after use as a feed-additive 
therapeutant in aquaculture. The EA presented (1) a summary of the scientific literature relevant to present 
uses, potential impacts, and environmental fate and effects of OTC; (2) a risk characterization for intensive 
aquaculture use on the basis of combined data from the publicly available scientific literature and OTC use 
and discharge data from Investigational New Animal Drug (INAD) permit users (INAD 9332); and (3) 
tables, figures, and appendixes that included both qualitative and semi-quantitative fate models, hatchery 
schematics, projected hatchery use data, and water-solubilized Environmental Introduction Concentrations 
(EICs) of OTC in hatchery discharge water. An expanded approval is presently being sought for the use of 
OTC medicated feed to control mortality associated with several systemic diseases. 

We described a qualitative environmental model in which OTC would be discharged after 
aquaculture use. Based on this general model, a semi-quantitative environmental discharge model was 
developed using the Water Quality Analysis Simulation Program, version 6.0 (WASP-6). Once dissolved in 
water, OTC exists in ionic form. The OTC octanol-water partition coefficient indicates that its potential for 
bio-accumulation is minimal. Oxytetracycline is not metabolized by fish; more than 90% of OTC 
consumed is excreted in an unmetabolized state. Oxytetracycline may also enter the water column as 
leachate from medicated feed. Oxytetracycline becomes associated with dissolved organic materials (DOM, 
e.g., humic materials) as well. Oxytetracycline bound to solids or associated with organic matter is largely 
not biologically available. Such sorption is an important aspect of OTC fate in freshwater, especially with 
respect to the biological availability of OTC. Oxytetracycline chelates with divalent cations such as 
magnesium and calcium. Chelated OTC is generally not biologically available. Leaching into water, either 
from suspended solids or sediment, is unlikely to result in significant toxic concentrations even in the 1-cm 
layer immediately above sediment deposits. Oxytetracycline may be stable in undisturbed sediments for 3-6 
months or more. The environmental fate of OTC is thus complex. However, all of the factors presently 
identified seem to reduce the concentration of water-solubilized OTC released from hatcheries into public 
receiving waters. After discharge, water-solubilized OTC is expected to be diluted by receiving waters to 
concentrations well below those that can be reliably detected by current analytical instrumentation. 
       
Freshwater fish and invertebrates are relatively tolerant of OTC. The most sensitive aquatic 
receptors, other than aquatic bacteria, are algae. Oxytetracycline does not appear to be harmful to sewage 
treatment bacteria at exposure levels predicted from aquaculture effluents. Risk for acute toxicity involved 
calculation of a risk quotient (RQ) for three critical aquatic receptors of interest (ROI): algae, invertebrates, 
and fish. The RQ is the predicted environmental concentration (PEC) divided by the predicted no-effect 
concentration (PNEC). The PNEC is the LC50 or EC50 result from a standard American Society for Testing 
and Materials (ASTM) or Office of Economic Cooperation and Development (OECD) test for the species 
involved divided by an appropriate application factor. An RQ of <1 indicates presumption of no risk to the 
ROI. We used a standard index referred to as the Hazard Quotient (HQ) for chronic risk assessment. The 
HQ was calculated and assessed in a similar manner as the RQ.
       
The PEC for water-solubilized OTC at discharge was derived from a number of sources, including 
actual data and also indications from the modified WASP-6 model. The resulting values from the various 
sources loosely converged, yielding OTC concentrations of 65 ppb or less (usually much less). The 65 ppb 
value was used for the risk assessment. The resulting RQs and HQs indicated no acute or chronic risk to 
any of the three aquatic ROI, except possibly for algae, which are capable of rapid repopulation in natural 
surface waters. In this EA, we demonstrated that under use practices at most hatcheries, OTC utilized in 
medicated feed as a therapeutant at intensive freshwater aquaculture operations constitutes no acute or 
chronic threat from associated water-solubilized OTC discharge to the environment, the populations of 
organisms residing there, or to public health and safety.