EFFECT OF DIETARY AMPROLIUM ON TISSUE THIAMIN AND CAYUGA-SYNDROME IN ATLANTIC SALMON

Broodfish of Atlantic salmon Salmo salar were fed either thiamin-defic ient diets fortified with 0.05% and 0.1% amprolium chloride (Amp, a th iamin antagonist) or thiamin-sufficient diets (0% Amp and ASD2-30, a U .S. Fish and Wildlife Service open-formula feed) for 6 months before s pawning. We observed Cayuga syndrome (CS) only in the fry produced fro m broodfish fed the amprolium diets. The percent fry mortality due to CS was less than 1% in the 0.05% Amp group and was more than 3% in the 0.1% Amp group. Free thiamin was the predominant form of thiamin in t he eggs of all broodfish; concentrations were two orders of magnitude greater than that of thiamin pyrophosphate (TPP) and thiamin monophosp hate (TMP). Free thiamin was greatest in eggs of broodfish fed the thi amin-sufficient diets. Total thiamin and TMP in red blood cells (RBCs) of broodfish fed the thiamin-deficient diets were reduced to the init ial levels by the end of the study. Levels of free thiamin in RBCs wer e similar in all treatment groups at spawning, as were levels of TPP. Amprolium feeding significantly reduced total thiamin levels in heart, liver, and muscle but not in kidney. Except for thiamin found in eggs , TPP was the predominant form of thiamin in all the other tissues, wh ereas free thiamin was the least predominant. The data support the hyp othesis that thiamin deficiency may cause Cayuga syndrome in Atlantic salmon sac fry. The low incidence of the syndrome in this study may be due to the short duration of the feeding trial or to the low levels o f amprolium added to the diets.