The use of Terramycin (TM-100F), a feed premix containing the broad spectrum 
antibacterial agent oxytetracycline (220 g/kg), is being extended to control 
columnaris infections caused by Flavobacterium columnare in cool- and warmwater 
fish. The study objective was to determine the safety of oxytetracycline 
administered in feed to walleye Stizostedion vitreum, yellow perch Perca 
flavescens, and hybrid striped bass Morone saxatilis x M. chrysops. The 
presently approved oxytetracycline treatment regimen is 55 to 82.5 mg/kg 
bodyweight (BW)-1d-1 for 10 d. Nominal oxytetracycline dose rates evaluated were 
0 (control), 82.5, 248, or 413 mgkg BW-1d-1 offered for 10 (yellow perch and 
hybrid striped bass) or 20 consecutive days (walleye). Parameters evaluated 
included daily mortality, behavioral observations (appetite, distribution, and 
flight/fright response), water chemistry observations, initial and terminal 
weight measurements, and gross and microscopic pathology. Yellow perch and 
hybrid striped bass consistently consumed 50 % to 100% of the diet offered 
during dosing. Although walleye feed consumption was occasionally less than 50% 
of the diet offered, feed or fecal material was present in the gastrointestinal 
tract of all necropsied walleye except for one control fish. There were no 
oxytetracycline-related mortalities or clinically observable changes in behavior 
for any of the species tested. Oxytetracycline-related changes were limited to 
growth and histopathology data. The single oxytetracycline-related growth effect 
was that hybrid striped bass offered a nominal dose of 413 mgkg BW-1d-1 were 
significantly smaller than untreated controls at the terminal sampling. 
Oxytetracycline-related histopathological findings were limited to walleye, and 
these findings included decreased hematopoietic/lymphopoietic (H&L) tissue in 
the anterior kidneys, diffuse hyperplasia of the gill filament epithelium, and a 
decreased prevalence of fish with eosinophilic droplets in their renal tubular 
epithelial cells. Although the incidence of decrease H&L tissue tended to 
increase in proportion to the oxytetracycline dose, this finding was only 
statistically significant for fish that received a nominal dose of 413 mgkg BW-
1d-1. All three microscopic findings were of low severity, and their mechanism 
with regard to oxytetracycline administration remains undetermined. Given the 
pathogenicity of the types of bacteria controlled by oxytetracycline treatment, 
the long history of its use in major aquaculture species, and the relatively 
minor treatment-related effects in walleye and hybrid striped bass, the relative 
risks of oxytetracycline treatment are probably outweighed by its disease 
control benefits.