Substitution of fish meal in feeds
Substitution of fish meal (FM) in aquafeeds has become a prime need owing to two factors. One is the limited availability of FM, aggravated by the expansion of aquaculture and the widening gap between the demand and supply of fish meal, as shown by extrapolating from the present level of utilization of FM in the aquafeed industry and the plateauing of capture fisheries production. The other factor is the environment pollution owing to increases in N and P loading caused mainly by the inclusion of FM in fish diets. Consequently in the evolution of feeds causing reduced nutrient (N and P) and organic loading in environmental waters, part of the strategy is the substitution of FM by other protein ingredients, especially from plant sources. Soybean (SB) seems to be the main contender as a non-FM protein source. Success in partial substitution of FM protein by soybean protein has been reported in several teleosts (Tacon, 1993), but there are only a few reports on complete replacement by soybean derivatives (Wilson, 1992).
While the replacement (up to 50 per cent) of FM by soy flour having 52 per cent crude protein (DM), as well as complete replacement of FM by casein alone and casein and soy flour (50 : 50), reduced the growth rate of rainbow trout, replacement (33 to 100 per cent) of FM by soy protein concentrate, having 72 per cent crude protein (DM), did not affect the growth, nutrient utilization and physical and sensory quality of meat in rainbow trout (Kaushik et al., 1995). These studies also point out the need for strict tests for the effect of anti-nutritional factors in the diet while using soy sources for dietary protein (Tacon, 1993), and their possible neutralization (Kaushik et al., 1995).
Some useful information on fish meal substitution in carp, tilapia and catfish and the reduction in phosphorus loading has been obtained (Jhan, et al., 2001; El-Saidy and Gaber, 2002; Gaylord et al., 2002), but there is need for more information on this important area of fish feed formulation. Nutrition research should concentrate on the substitution of FM by developing alternative protein sources for fish feeds, which not only support rapid growth but, through reduced N and P and organic matter loading into the environment, would not increase pollution.
Since most of the aquaculture wastes, which cause serious environmental pollution, are dietary in origin, reduction in waste output has to be achieved through improved diets and feeding strategies. For this, a reduction in dietoriginated solid waste, nitrogen and phosphorus have to be brought about. To reduce solid waste, poorly digestible ingredients such as whole grain or grain by-products used as fillers and binders in the feed will have to be eliminated, and highly digestible ingredients, which could also serve as good binders, will have to be used instead. Nitrogen waste could be reduced by the selection of ingredients with a low digestible protein to digestible energy (DP/DE) value, and excreted phosphorus can be reduced by the selection of suitable ingredients and optimizing the digestible nitrogen in the diet to the requirement of the farmed organism.
In addition, improved feeding practices which would reduce excretory wastes should be adopted, to avoid excess waste output from aquaculture installations (Cho and Bureau, 2001).