Based on a Roundtable
Co-hosted by the Habitat Advisory Board of the Great Lakes Fishery Commission
the Great Lakes Water Quality Board of the International Joint Commission
Nutritional Strategies for the Management of Aquaculture Wastes
by Dominique P. Bureau and C. Young Cho
Fish Nutrition Research Laboratory, Ontario Ministry of Natural Resources (OMNR) and Dept. of Animal and Poultry Science, University of Guelph, Guelph, Ontario, N1G 2W1
There is a growing consensus about the need to reduce waste production in aquaculture to minimize the negative impacts on the environment, comply with legislation and limit self-pollution. For many years now the Fish Nutrition Research Laboratory (OMNR/University of Guelph) has been at the forefront of the development of nutritional strategies for the management of aquaculture waste output and has organized three international symposia on the topic. Three types of strategies have been used by our laboratory and OMNR fish culture stations for the management of aquaculture waste output, namely feed formulation, feed requirement prediction models, and biological approaches to waste output estimation.
Fundamental to waste management strategy is a reduction of waste at the source, namely the diet. Pollution problems are often related to undigested carbohydrate, nitrogen, and phosphorus levels in effluent which stimulate eutrophication. Protein and lipid are, in general, well digested by fish and represent a minor component of solid waste. However, nitrogen excretion resulting from dietary protein oxidation is a major component of dissolved waste. Optimizing the protein/energy ratio in diets reduces nitrogen excretion. Application of the above principles has led to the development of high nutrient dense (HND) diets which are both highly digestible and nutrient/energy dense. The basic principles used in formulating HND diets are neither new nor complicated. Firstly, very digestible ingredients with low phosphorus to nitrogen ratio should be selected for feed formulation. Secondly, nutrients (mainly nitrogen and phosphorus) in the diet must be well balanced as to optimize their utilization by the animal and hence reduce dissolved waste outputs. Simply stated, the approach is to exclude poorly digested, low energy and low protein ingredients, such as grain by-products rich in starch and fiber, and to reduce reliance on high ash fish meals as protein sources. HND diets, high in both protein and fat while maintaining a protein to energy ratio of 20-22 g digestible protein/MJ digestible energy (84-92 g/Mcal) and a digestible energy level around 20 MJ (4.8 Mcal), are very desirable for effective management of waste reduction at the source.
Scientific Estimation of Feed Requirements
Figure 1 illustrates the importance of minimizing feed waste from an environmental point of view. Only a small proportion (approximately 15-25%) of a given amount of feed consumed by a fish will be excreted as solid waste. Feed that is not consumed by the animal, on another hand, will become 100% solid and suspended wastes. Figure 1 shows that as feed wastage increases from 0% to 30% (in this example when feed conversion ratio (FCR) (feed/gain) of 1.11:1 is obtained instead of 0.83:1), solid nitrogen waste outputs by the fish quadruples, total solid waste triples, and solid phosphorus waste is increased by about 60%. On top of these solid wastes add the dissolved wastes produced by the fish and this results in substantial, yet partly avoidable, waste loads.
Feed wastage depends mostly on feeding practices used and little on the feed itself. Most of the feeding charts available today are "desktop" modifications of feeding guides originally developed with semi-moist meat/dry meal based diets of the past. One must be cautious in applying these charts to modern diets which have higher energy and nutrient densities. Ultimately the animal itself should determine the quantity of energy and nutrients appropriate to satisfy its requirements, but this is often impossible. The scientific estimation of feed allowance and careful feeding of the fish may be the only sensible approaches to ensure environmentally- and economically-sustainable aquaculture production.
Sufficient data on nutritional energetics are now available to allow reasonably accurate feeding standards to be computed for different aquaculture conditions. Bioenergetic models were developed by our laboratory and a stand-alone multimedia program (Fish-PrFEQ) is being developed to facilitate computation of the models. This program predicts growth and energy, nitrogen and phosphorus retention, requirements and excretions to determine feeding standards, waste outputs, and effluent water quality.
Regardless of the feeding system or method used, accurate growth and feed requirement models can be very valuable management tools since they may allow us to forecast growth and objectively determine biologically achievable feed efficiency (based on feed composition, fish growth, composition of the growth). These estimates can be used as yardsticks to adjust feeding practices or equipment, compare results obtained, and help improve husbandry practices.
Estimating Waste Output
Directly monitoring and estimating waste in effluent is often an inaccurate and costly process. Biological Methods for the Prediction of Aquaculture Waste Outputs (BMPAWO) have been developed as simple and economical alternatives to limnological/chemical methods of estimating waste outputs. Waste output loading from aquaculture operations can be estimated using simple
Figure 1. Effect of feed wastage on solid waste
Feed: 44% Digestible protein (DP), 20 MJ/kg Digestible Energy (DE),
DP/DE = 22 g/MJ
7.6% Nitrogen (N), 1% Phosphorus (P)
Rainbow trout growing from 10 to 100 g in 410 days, water temperature 0.5 to 20°C
[Reproduced from "Aquaculture - The Environment 98" by Northern Aquaculture]
principles of nutrition. Ingested feedstuffs must be digested prior to utilization by the fish and the digested protein, lipid, and carbohydrate are the potentially available energy and nutrients for maintenance, growth, and reproduction of the animal. The remainder of the feed (undigested) is excreted in the feces as solid waste (SW), and the by-products of metabolism (ammonia, urea, phosphate, carbon dioxide, etc.) are excreted as dissolved wastes (DW) mostly by the gills and kidneys. The total aquaculture wastes (TW) associated with feeding and production are made up of SW and DW, together with apparent feed waste (AFW). Since direct estimation of AFW is almost impossible, best estimate can only be obtained by comparison with theoretical feed requirement calculated with bioenergetic models.
In summary, the nutritional strategies for the management of aquaculture waste (NSMAW) are the best approach to reduce the waste output from the source for sustainable aquaculture:
Comparative studies conducted in a number of fish culture stations have shown that BMPAWO are less expensive and yield more realistic and consistent results than chemical/limnological methods based on continuous sampling of the effluent. BMPAWO are also more flexible since waste outputs can be estimated in advance as well as for culture conditions where it would be very difficult to estimate waste outputs using limnological methods (e.g., cage culture).
Information on the procedures and models discussed here, the Fish-PrFEQ program and the three international symposia, as well as a list of references, can be obtained from our website www.uoguelph.ca/fishnutrition.
Bureau, D.P. and C.Y. Cho. 1998. Three key strategies for the management and reduction of aquaculture waste. Aquaculture - The Environment. Northern Aquaculture Supplement Publication. pp. 25-26.
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Cho, C.Y. and D.P. Bureau. 1997. Reduction of waste output from salmonid aquaculture through feeds and feedings. The Progressive Fish Culturist, 59: 155-160.
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