In the period from winter 1996 to autumn 1999, three feeding trials were conducted to study the possible role of mammalian blood meal in preventing lens opacities in Atlantic salmon. In addition to blood meal supplemented feed, experimental diets were formulated, aiming to mimic the total concentrations of selected elements present in blood meal (BM) enriched diets; the amino acid histidine (His) and the mineral iron (Fe). In addition, these diets were supplemented with Zn. The trials included different rearing conditions, sizes and strains of Atlantic salmon; one small scale feeding experiment with presmolt of Strain A in fresh water (Trial 1) and two semi large trials with sub-yearling (S0s) and yearling (S1s) smolts of Strains A and B in seawater (Trials 2 and 3). The trials included duplicate or triplicate feeding groups and lasted 6-12 months. Standard commercial diets not supplemented with BM were used as controls. High prevalence of cataract developed over time in all three studies, with end point ranges as follows; Trial 1: 40-52 %; Trial 2: 51-83 %; Trial 3: 63-100%. The cataracts normally first appeared in the anterior and/or posterior cortical region and later affected the perinuclear region. In all the experiments, there was a significant cataract preventative effect of both the BM supplemented and experimental diets compared to the respective control diets. To some extent, cataract incidence was reduced, but primarily cataractogenesis was less severe in these groups. A concurrent positive effect on growth of both added BM and experimental diets was observed in the two seawater trials, probably related to maintaining vision and feed uptake ability. In one of the trials, there were significant differences in cataract susceptibility between the two strains, measured both as frequency and severity of cataractous changes. Our data suggest that elevated concentrations of dietary His and/or Fe mitigate cataract formation. KEY WORDS

This study was conducted to investigate protein synthesis rates and metabolism of histidine (His)-derivatives in lenses of Atlantic salmon (Salmo salar L.) of different dietary His background during parr–smolt transformation. Two populations of Atlantic salmon parr of equal origin were established in freshwater (FW), 3 months prior to transfer to seawater (SW). The populations were fed either a control diet (CD) containing 8.9 g kg-1 His or the same diet added crystalline His to a total level of 14.2 g kg-1 (HD). On the basis of these two populations, 14C His force-feeding studies were performed; in FW 3 weeks prior to sea transfer and in SW 6 weeks after transfer. The studies were conducted by force-feeding the respective diets enriched with 14C labelled His, with subsequent measurements of incorporation of 14C His into lens free amino acid pool, as well as into lens proteins and other free His pool fractions. The latter included the major lens imidazole N-acetylhistidine (NAH). Lens concentrations of His and NAH were clearly influenced by dietary His history, both in parr and smolt. The lens His and NAH concentrations in the CD population were considerably lower in SW than in FW, while in the HD group the His level was equal and the NAH level 50% higher in SW than in FW. Fractional synthesis rate for NAH, KS (NAH), in FW was 8.2 and 4.2 µmol g-1 day-1 for fish in the CD and HD populations, respectively. The corresponding KS (NAH) values in SW were 5.1 and 33.0 µmol g-1 day-1. Our data show that free His is rapidly converted to NAH in the lens, and that NAH seems to have a very high turnover, especially in salmon reared in SW. Fractional synthesis rate for lens proteins, KS (PROTEIN), ranged between 1.8 and 17.3% day-1 (182 and 2791 µg g-1 day-1, respectively), and was generally higher in SW than in FW (P < 0.01). In SW, KS (PROTEIN) was highest in fish in the HD population (P < 0.05), whereas lens protein retention in the HD group was significantly lower than the CD group (P = 0.01). In a second model assuming that His from lens NAH is available for protein synthesis, calculated values of KS (PROTEIN) ranged between 0.17% day-1 (17.6 µg g-1 day-1) and 0.48% day-1 (70.2 µg g-1 day-1). Cataract scores recorded in the His populations at a later point (day 204), showed that the CD fish had significantly higher mean cataract scores than individuals in the HD population (P < 0.01), confirming that low levels of lens His and NAH are associated with cataract development

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The appetite-stimulating effect of krill hydrolysate (KH), free amino acids and specific water-soluble low molecular N-compounds was explored in a short feeding trial with Atlantic salmon. A 100 g kg À1 fish meal positive (MFM), a 30 g kg À1 fish meal negative control (LFM) and six more LFM diets were produced added known attractants for fish-KH at two levels: low (LAK) or high (HAK), AMP, choline chloride (CC); an amino acid mix resembling the free amino acid composition of KH (AA) or a mix of AMP, CC and AA (ACA). HAK, ACA and AA showed highest feed intake and significantly higher plasma phospholipids and cholesterol. The lower performing treatments (LFM, AMP) showed higher liver lipids and hepatosomatic index. CC induced reduced liver lipids and increased plasma phospholipids and cholesterol. Appetite regulating neuropeptide gene expression analysis (qPCR) was performed in fish fed LFM, HAK and AA. Pyy showed the highest postprandial expression in LFM, whereas the expression of the anorexigenic neuropeptides cart, pomca1, pomca2 and pomca2s was low. These apparently contradictory results may be explained by initial appetite stimulation by HAK and AA, resulting in higher feed intake, followed by satiation and appetite downregulation.

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Recently, we reported that growth of Atlantic salmon was reduced as dietary fishmeal (FM) was lowered from 25 % to 5 % in dual-substituted feeds compared to a control diet, formulated to represent the current upper levels of substitution of FM and fish oil. In the present study, the effects of different alternative protein blends and binders on growth of salmon fed dual-substituted feeds containing only 11 % FM, and with 60 % of dietary fish oil replaced by rapeseed oil were investigated. Salmon of initial weight 1.3 kg were grown to market size (> 3 kg) over a period of 19 weeks. Salmon fed the diets with reduced FM showed lower final weight, SGR and TGC, associated with reduced feed intake. There was a tendency for increased FCR in fish fed the diets containing reduced FM although this was not significant, and there was no effect on PER. There were no significant effects on digestibility of protein or fat but the two parameters varied reciprocally and there were clear trends of increased protein and lower fat digestibilities in fish fed diets with reduced FM. Although lipid and fatty acid compositions did not vary greatly between diets there were significant effects on fish tissue compositions. Thus, liver lipid was generally reduced in fish fed diets with lower FM, significantly so in two of the four treatments. The proportions of monoenes were significantly lower and those of polyunsaturated fatty acids (PUFA) significantly higher in flesh and liver of fish fed diets with reduced levels of FM. The increased proportions of PUFA were due to increased percentages of 20:4n-6, 20:5n-3, 22:5n-3 and, although not consistently significant, 22:6n-3. The mechanisms for these unexpected effects of diet on tissue lipids and fatty acids are discussed.