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The Dietary Requirement of Young Rainbow Trout (Oncorhynchus mykiss) for Folic Acid^1,2,3,

Department of Nutritional Sciences, University of Guelph, Guelph N1G 2W1, Ontario, Canada

We sought to determine the dietary folic acid requirement of young rainbow trout using growth indices supported by measurements of tissue folate concentrations. The investigation was conducted with purified diets that had, by assay, basal folic acid levels of 0.08 and 0.16 mg/kg in the first and second, respectively, of two experiments. Each experiment was started with fry (initial mean weight, 1.4 and 2.8 g/fish in Experiments 1 and 2, respectively) and was conducted at a water temperature of 15°C. Experiment 1 lasted 18 wk and Experiment 2 lasted 16 wk. Recovery tests (of 8 wk duration, performed on fish fed the unsupplemented diet) and pair-feeding showed that the unsupplemented diet led to a folate-specific deficiency condition in which the main hematological abnormality was the appearance of misshapen nuclei in a small proportion (2.3%) of erythrocytes. Dietary requirements were shown not to exceed 0.3 and 0.6 mg folic acid/kg (17 and 33 µg/MJ digestible energy) for optimizing survival and growth indices, respectively. We conclude that the dietary folate requirement of the trout is comparable to that of other vertebrates for the purpose of achieving maximal weight gain.

KEY WORDS: • folic acid • rainbow trout • requirement • erythrocyte • pathology

¹ Presented in part at the 21st Fish Feed and Nutrition Workshop, October 7–10, 1992, Davis, CA [Woodward, B. & Cowey, C. B. (1992) Dietary folic acid requirements of young rainbow trout. Abstracts, p. 28].

² Supported by the Ontario Ministry of Agriculture and Food, the Natural Sciences and Engineering Research Council of Canada, and the Department of Fisheries and Oceans Canada.

³ The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 USC section 1734 solely to indicate this fact.

⁴ To whom correspondence should be addressed.

Manuscript received 8 March 1993. Revision accepted 26 May 1993.