A review of diet formulation strategies and feeding systems to reduce excretory and feed wastes in aquaculture

Significant efforts should be devoted to reducingwaste outputs from aquaculture operations in orderto lower the environmental impacts of aquaculturein many parts of the world. Since most aquaculturewastes are ultimately from dietary origin, reductionof waste outputs should first be through improve-ments of diet formulation and feeding strategies.The first step in the production of feeds producingless solid waste is to eliminate poorly digestibleingredients (such as whole grain or grain by-products used as binders and fillers in the feedformulae) and to use highly digestible ingredientswith good binding properties. Further reduction ofsolid waste can then be achieved through carefulselection of the ingredients to improve apparentdigestibility and the nutrient balance of the feed.Nitrogen waste outputs can be reduced through thereduction of the digestible protein to digestibleenergy (DP/DE) ratio of the diet. Phosphorus wasteoutputs can be reduced through careful selection ofthe ingredients and optimization of the digestiblephosphorus content of the diet to meet therequirement of the fish but avoid greatly exceedingthis required level. Finally, feeding practices thatminimize feed wastage should be adopted since feedwastage can have a very significant impact onwaste outputs from fish culture operations.Keywords: aquaculture, waste, feed, feedingIntroductionOvert the past decade, the environmental impacts ofaquaculture operations have become a matter ofconcern for the public, the various levels ofgovernment and the aquaculture producers them-selves. Minimizing environmental impacts is there-fore a key factor in insuring long-term sustainabilityof the aquaculture industry.Containment and collection of wastes, both solidand dissolved, is very difficult and costly inaquaculture, as the wastes are rapidly dispersedinto the surrounding waters. The release of solidwastes, phosphorus (P) and nitrogen (N) fromaquaculture operations may have significant eutro-phication effects on receiving waterbodies andassociated ecosystems (Persson 1991). P wasteoutputs are a greater concern in freshwater sinceP is generally the most limiting factor for plant(algae) in that environment. N waste outputs aregenerally a greater concern in the marine environ-ment for the same reason (Persson 1991). Solidwastes (faecal material and wasted feed) settling tothe sediment can have an impact on the benthicecosystem of inland and marine waters (Gowen,Weston & Ervik 1991). Degradation of organicwaste by bacteria and other organisms leads to theconsumption of oxygen (O