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.

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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.

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In the diatom, Skeletonema costatum, as well as in a zooplankton sample, the polyunsaturated fatty acids of the triglycerides were found accumulated in the β-position of the glycerol. This fatty acid distribution pattern is typical for animal fats, in particular for fish oils. Together with the recent demonstration that fish and invertebrates retain in part the structure of ingested triglycerides, the findings of the present study show that the typical structure of marine triglycerides originates in phytoplankton and is to a large degree retained through the marine food chains.The fatty acid composition of several other lipid fractions of the plankton samples was also determined.

A muddy flavor and odor in commercial fish from Cedar Lake resulted in the closure of the commercial fishing season. Gas–liquid chromatography indicated the presence of geosmin and 2-methylisoborneol which are known to cause a muddy flavor in fish. The source of these compounds was not established but they are known to be produced by blue-green algae and actinomycetes.Key words: organoleptic properties, geosmin, 2-methylisoborneol, commercial species, freshwater, Cedar Lake, Manitoba; Stizostedion vitreum vitreum, Esox lucius, Coregonus artedii, Coregonus clupeaformis

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