Dietary vitamin requirements of cultured young fish, with emphasis on quantitative estimates for salmonids

Abstract

The majority of currently accepted estimates of vitamin requirements for salmon are astonishingly high. These estimates are, for the most part, based on the use of the classic H440 purified diet. This diet provided the seminal results pertaining to water-soluble vitamin deficiency signs in salmonids, especially Pacific salmon. The H440 diet, however, is highly water-soluble and thus inappropriate for precise estimation of water-soluble vitamin requirements. Unfortunately, this is now generally overlooked. Improved purified diets (such as that developed in Guelph, Ontario, during the 1970s) are high-fat, steampelleted formulations which minimize leaching losses and which support good growth rates in rainbow trout ( Oncorhynchus mykiss). This species is particularly suitable for studies of nutrient requirements because of its voracious feeding behaviour in captivity. Weight gain is arguably the best single index of vitamin requirement on which to base diet formulation practice. On the basis of this criterion, the dietary requirements for most water-soluble vitamins do not differ substantially when comparing the trout and diverse warm-blooded species. Interspecies similarity also is apparent regardless of the way in which vitamin requirements are expressed (i.e. on a dietary weight or energy basis), regardless of strain-specific growth potential, and (possibly except for vitamin E) regardless of water temperature. From the standpoint of phylogeny, therefore, results obtained using rainbow trout should be considered generally applicable to other salmonid species. Exceptions to this generalization may include vitamin B 12 as well as the vitamin-like nutrients, choline and myo-inositol. Vitamin C is a special case for which a precise estimate of minimal requirement has awaited discovery of a stable, biologically available form. Attention is now focused on ascorbate phosphate in this regard. More precise information is needed in relation to the minimal dietary requirements of salmonids for the fat-soluble vitamins, as well as for vitamin C and the vitamin-like compounds choline and myo-inositol. Ultimately, it may prove useful to define a maximum dietary requirement for nutrients such as vitamin K, vitamin B 12, choline and myo-inositol for which significant and variable non-dietary sources may exist. Minimal dietary requirements are defined by means of diets containing large surfeits of all components except for the nutrient which is under investigation. It will be important to determine whether a diet containing all the vitamins at their minimal requirement level (defined as indicated) is, in fact, complete for salmonids or other species.