Energy Losses in the Waste Products of Brown Trout (Salmo trutta L.)

The total energy content of the food intake of a fish is either lost in the faeces and the excretory products or used for metabolism, growth and any reproductive products released during the period concerned. Winberg (1956) first proposed that about 15% of the total energy intake is lost in the faeces and about 3% in the excretory products. He suggested that a slightly higher value of 20% probably represents the total energy losses in the waste products and this value has been used by many workers. Few workers have measured energy losses in salmonids. Brocksen, Davis & Warren (1968) obtained a value of 14'5%o for cutthroat trout Salmo clarki (Richardson) at 10? C, and and Brocksen & Bugge (1974) obtained values ranging from 28-2% at 5? C to 15-2% at 20? C for rainbow trout S. gairdneri Richardson. None of these values include energy losses in the excretory products: ammonia and urea. Ammonia is the chief excretory product in freshwater teleosts and is excreted primarily from their gills. Kleerekoper & Mogensen (1959) found that 98%0 of amino-derived nitrogen was excreted as ammonia in brown trout S. trutta L. and brook trout Salvelinus fontinalis (Mitchill). Except for the work of Burrows (1964) on chinook salmon Oncorhynchus tschawytcha (Walbaum), only small quantities of urea have been recorded in the excreta of teleosts (Black 1957; Brett 1962; Forster & Goldstein 1969). Other excretory products appearing in very small quantities in the urine are uric acid, creatine, creatinine, amines and amino acids. The present paper is one of a series dealing with feeding and growth in brown trout (Elliott 1972, 1975a, b, c, d, 1976). Its chief purpose is to examine energy losses in the faeces and excretory products of brown trout and to develop equations to estimate these losses.