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The main goal of aquaculture is to efficiently convert feed into fish and shellfish. Inherent to this practice is the generation of waste; however, from a waste management viewpoint, aquaculture differs in important respects from terrestrial animal farming. Measurement of actual feed intake is nearly impossible; consequently, unconsumed feed contributes a relatively large proportion of total waste output in most operations. Moreover, containment of wastes is difficult, and wastes are rapidly dispersed into the surrounding waters. Monitoring and estimating quantitative waste outputs in effluent directly is an inaccurate and costly process. Other methods of estimating the waste output from aquaculture operations should be used. Because most aquaculture wastes are dietary in origin, efforts to reduce waste should focus on nutrition and feeding, including the formulation of special diets, development of feeding systems, and improvement of the efficiency of nutrient utilization. Modern low‐pollution or highly digestible, nutrient‐dense (HND) diet formulations yield outputs of less than 150 kg solid waste and 3 kg phosphorus per metric ton of salmonid fish produced. However, in addition to the use of HND diets, reduction of wastes in salmonid culture requires revised feeding standards based on average digestible energy requirement per kilogram of fish produced and a target feed efficiency (gain: feed) of much greater than 1.0. Experimental data with HND diets, feeding standards based on energy requirements, and biological procedures for quantifying waste output in the effluent are reviewed.

This study examined the hypothesis that a diet containing excess leucine may promote protein deposition in the body of rainbow trout. Diets were formulated with wheat germ meal and crystalline amino acids as major nitrogen sources. In Experiment 1, diets containing 1.1, 1.5, 2.2, 2.7, 3.5, 4.5, 6.0 and 6.5% leucine in wheat germ meal-crystalline amino acid diets were fed to fingerling rainbow trout. Diets containing up to 6.5% leucine did not inhibit weight gain or food intake. Body protein concentration tended to decrease as dietary leucine increased. In Experiment 2, fish were fed similar diets containing 3.3, 6.2, 9.2 and 13.4% leucine. After 10-11 wk of feeding, gross lesions including scoliosis, deformed opercula, scale deformities, scale loss, spongiosis of epidermal cells and scale regeneration were observed in 20% of the fish fed diets containing 13.4% leucine. High dietary leucine did not depress plasma valine or isoleucine concentrations. Therefore, the gross lesions could be attributed to a toxic effect of excess dietary leucine. Polyamine concentrations, which were used as a metabolic indicator for growth, were not significantly different in the tissues of fish receiving different treatments, thus supporting the hypothesis that increasing dietary leucine did not increase body protein deposition.

Young rainbow trout were given diets containing graded levels of methionine for 16 wk. Analysis of the weight gain and food efficiency data showed the methionine requirement to be not more than 0.76% of the diet (1.9% of dietary protein). Activities of regulatory enzymes of the transulfuration pathway, methionine adenosyltransferase and cystathionine synthase in trout liver were not altered by changes in methionine intake. Concentrations of free serine in liver and plasma of the trout were high at low levels of methionine intake but fell as dietary methionine increased. This implied decreased flux through cystathionine synthase at low methionine intakes. Large increases in liver and plasma taurine occurred at high methionine intakes, implying enhanced transulfuration activity. Liver ornithine decarboxylase activity was reduced at the lowest level of dietary methionine used but the activity of S-adenosylmethionine decarboxylase was unchanged. Eye lenses of the trout given these diets were examined by a scanning lens monitor. Analysis of focal length variability with this equipment demonstrated that, if abnormality of the lens is to be avoided, a higher concentration of dietary methionine (0.96% or 0.6% methionine + 0.36% cystine) is needed than that required to maximize growth.

Twenty-eight rainbow trout Oncorhynchus mykiss and six feed samples were obtained from two cage culture operations in Ontario and analyzed by proximate analysis. This information was then used to estimate the waste outputs of rainbow trout cage culture operations. Calculations of nitrogen (N), lipid, phosphorus (P), and gross energy (GE) gains by the fish were based on the theoretical growth curve predicted by a thermal-unit growth coefficient model, and the N, lipid, P, and GE contents of the fish were estimated by regression analyses. The feed required to achieve the predicted growth and energy gain was calculated based on digestible energy requirements that were determined by modifying published Fish-PrFEQ bioenergetics models (assuming a 5% feed wastage). The theoretical digestible energy requirement of rainbow trout growing from 10 to 1,000 g reared in cages was estimated at 22.7 MJ/kg weight gain. Under these conditions, rainbow trout fed the various feed sampled were predicted to have feed conversion ratios (FCR ϭ weight of food fed/weight gained) varying between 1.14 and 1.29. Total solid wastes were estimated to be between 240 and 318 kg/ metric ton of fish produced, total N wastes between 47 and 71 kg, and total P wastes between 7.5 and 15.2 kg, the lowest values being representative of the two feeds most widely used by fish farms in Ontario.

Most species in aquaculture are new to cultivation and so behave like wild animals. They are products of evolution, with adaptations to specific habitat conditions. In the wild, food is not available uniformly throughout the day or the year, or in space, and rarely exceeds the fishes needs. Competition is energetically expensive, reducing growth efficiency. Consequently, feeding activity patterns have evolved, implying internal appetite rhythms, which optimise food intake under these various constraints. Salmonids can adapt quickly to short term variation in food availability, but show seasonal genetically determined anorexia. Rational feeding regimes in culture should take all such features into account. When appetite is high naturally, food should be presented so that it is economically indefensible - where every individual can eat, and where fighting does not pay. At periods of anorexia it will be prudent to offer no food. Manufacturers' feed tables are usually regimes devised to meet the bioenergetic needs of fishes, as they are understood in a physico-chemical sense. While useful first approximations, they do not take into account these evolutionary features of the fishes, and can lead to waste. Methods of presentation are described which allow the fish to determine when food shall be available, and in ways which, by diminishing the advantages of social dominance, ensure relatively even opportunities to feed for all individuals in the population. Allowing the fish to set the time-table reduces the likelihood of waste.

The impetus for accurate information on the nutrient requirements of fish derives very largely from the development, in many parts of the world, of an aquaculture industry that is dependent on artificial feeds. At the same time such information can provide the basis for comparative nutrition, whereby features of the nutrition of cold-blooded, water-breathing and mainly, carnivorous vertebrates which differ from the pattern largely common to omnivorous mammals are identified. Since this topic was last addressed (Cowey, 1988) the aquaculture industry has continued to grow, fuelled both by the continuing overexploitation of the marine environment, resulting in declining yields, and by the high quality of aquaculture products.The first serious studies on nutritional requirements of fish were made in the 1950s. Since then much has been learned concerning fish husbandry, water quality, pellet quality (water stability) of fish diets and so on. Consequently, general methodological standards in nutritional experiments have improved greatly and some of the early values for nutrient requirements need to be revised, usually downward.