To determine the optimum indispensable (I) amino acid (AA) balance in Atlantic salmon (Salmo salar L.) fry, a single protocol established for the pig was adapted. The balance was calculated from the reduction in N gain after replacing about 45 % of a single IAA by a mixture of dispensable AA in isonitrogenous diets. We confirmed that the mixture of AA simulating the AA pattern of cod-meal protein and gelatine (46:3, w/w) was used with the same efficiency as cod-meal protein and gelatine. From the deletion experiment an optimum balance between the IAA was derived. Expressed relative to lysine ¼ 100, the optimal balance was: arginine 76 (SE 0·2), histidine 28 (SE 2·2), methionine þ cystine 64 (SE 1·7), phenylalanine þ tyrosine 105 (SE 1·6), threonine 51 (SE 2·4), tryptophan 14 (SE 0·7), valine 59 (SE 1·7). No estimates were made for isoleucine and leucine. Expressed as g/16 g N, the optimal balance was: arginine 4·0 (SE 0·0), histidine 1·5 (SE 0·1), lysine 5·3 (SE 0·2), methionine þ cystine 3·4 (SE 0·1), phenylaline þ tyrosine 5·6 (SE 0·1), threonine 2·7 (SE 0·1), tryptophan 0·7 (SE 0·0), valine 3·1 (SE 0·1). This AA composition is close to that of the Atlantic salmon whole-body, but using it as an estimation of the IAA requirements may lead to an overestimation of the branched-chain AA requirements and an underestimation of aromatic and S-containing AA requirements. The results are discussed in accordance with the key assumptions associated with the model used (brokenline model, IAA efficiencies and maintenance requirements).

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The effect of two digestible protein levels (310 and 469 g/kg DM) on the relative lysine (Lys; g Lys/kg DM or g Lys/100 g protein) and the absolute Lys (g Lys intake/kg 0·75 per d) requirements was studied in rainbow trout fry using a dose -response trial. At each protein level, sixteen isoenergetic (22-23 MJ digestible energy/kg DM) diets were tested, involving a full range (2-70 g/kg DM) of sixteen Lys levels. Each diet was given to one group of sixty rainbow trout fry (mean initial body weight 0·78 g) reared at 158C for 31 feeding d. The Lys requirements were estimated based on the relationships between weight, protein, and Lys gains (g/kg 0·75 per d) and Lys concentration (g/kg DM or g/100 g protein) or Lys intake (g/kg 0·75 per d), using the broken-line model (BLM) and the non-linear four-parameter saturation kinetics model (SKM-4). Both the model and the response criterion chosen markedly impacted the relative Lys requirement. The relative Lys requirement for Lys gain of rainbow trout estimated with the BLM (and SKM-4 at 90 % of the maximum response) increased from 16·8 (19·6) g/kg DM at a low protein level to 23·4 (24·5) g/kg DM at a high protein level. However, the dietary protein content affected neither the absolute Lys requirement nor the relative Lys requirement expressed as g Lys/100 g protein nor the Lys requirement for maintenance (21 mg Lys/kg 0·75 per d).

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