Weight gain and feed consumption results showed that rapeseed protein concentrate (RPC) and rapeseed meal (RM) can be substituted for soybean meal (SM) and perhaps partially for herring meal (HM) in rainbow trout (Salmo gairdneri) control (SM–HM) diet and thus have economic benefits. Rapeseed flour (RF) was a poor source of proteins due to its greater effect on thyroid function. Rapeseed proteins have little effect on liver weight, body moisture, liver and body lipid, liver and body sterol content, visual and histological appearance of liver, heart and visceral tissues, or on the flavor, but caused yellow pigmentation of the skin. The levels of individual fatty acids in the diets directly affected the levels of individual fatty acids in the liver and body, while the type and level of dietary rapeseed proteins appeared to have no effect. The levels of fatty acids, 20:1 and 22:1 (includes erucic acid), were lower in the body and even lower in the liver compared with the diets. All test diets, except RPC–HM diet, caused marked thyroid hyperplasia, believed to be due to glucosinolates (goitrogens). However, some compensation for this goitrogenic action must occur because plasma T4 levels indicated a hypothyroid state for only five test diets (highest RPC-containing diet; highest RM-containing diet; all three RF-containing diets). Key words: dietary rapeseed, glucosinolates, feed–gain ratios, growth, lipids, fatty acids, plasma T4, thyroid histology, flavor, weight gain
Eleven axenic or unialgal cultures of blue-green algae, 10 producing geosmin (trans-1, 10-dimethyl-trans-9-decalol) and 1 producing 2-methylisoborneol (2-exo-hydroxy-2-methyl-bornane) were isolated from saline lakes in southwestern Manitoba. These compounds are responsible for the muddy flavor in fish and water in some lakes. Algae producing geosmin were Oscillatoria cf. prolifica (Greville) Gomont, O. tenuis Agardh, O. cf. cortiana Meneghini, O. cf. variabilis Rao, O. agardhii Gomont, O. cf. splendida Greville, O. sp., Symploca cf. muscorum (Agardh) Gomont, Lyngbya cf. aestuarii (Mertens) Liebman, and L. sp. Lyngbya cf. cryptovaginata Schkorbatov is the first alga reported to produce 2-methylisoborneol.Actinomycetes, which are known to produce these muddy flavor compounds, were not detected in these lakes.
Geosmin has been isolated and identified, using gas-chromatographic, mass-spectrometric and organoleptic analysis, as the principal compound responsible for the muddy flavor in fish from various saline lakes in western Canada. A method for quantitative estimation of geosmin has been described and evaluated. Placing of muddy-flavored rainbow trout (Salmo gairdneri) in clean water resulted in reduction of flavor to the borderline level between 3 and 5 days, and of geosmin concentration from 1.1 to 0.3 μg/100 g of flesh in 14 days. The taste threshold level of geosmin was estimated to be 0.6 μg/100 g flesh.
Growth. body composition and feed utilization were evaluated for Arctic charr fed a control diet and nine practical diets formulated to contain three protein levels (34.44 or 54%) at each of three lipid levels (10, 15 or 20%). Fish were raised for 24 weeks at 12" C. Increasing either dietary protein and/or lipid resulted in improved weight gain, feed efficiency and energy retained. Protein efficiency ratio and protein retained were directly related to dietary lipid and inversely related to dietary protein. A sparing effect of lipid on protein was evident where, at each protein level, Arctic charr utilized protein more efficiently with each increase in dietary lipid. Levels of 34% protein and/or 10% lipid were well below the requirements for Arctic charr (initial weight, 4.6 g). Within the range tested, the dietary combination O f 54% protein with 20% lipid maximized weight gain and feed efficiency and resulted in fish with low liver and body lipid contents. However, if lowest feed cost per kg gain is an important factor, the 44% protein-20% lipid diet would be less expensive to use, although it results in a minor reduction in growth potential.
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