Young Arctic charr, Salvelinus alpinus (L.), mean weight 2.56 ± 0.02 g, were fed nine isoenergetic (˜16.6 MJ digestible energy (DE) kg−1) practical diets formulated to supply digestible crude protein (DCP) at 40g kg−1 increments from 230 to 550g kg−1, for 84 days. Mean weight gain (MWG) and specific growth rate (SGR) were determined every 14 days while carcass composition was determined at the start and end of the experiment. Growth responses attained the highest values in the fish fed the diet with 350 g kg−1 DCP. Carcass moisture gain, protein gain and apparent net lipid accumulation increased as DCP levels increased to a maximum at 350 g kg−1 after which there were no differences among treatments. Total carcass lipid and lipid gain decreased as dietary DCP increased up to 470 g kg−1 with no differences thereafter. Apparent net protein accretion decreased with increasing DCP levels up to 350 g kg−1 after which there were few differences among treaments. Protein requirements were estimated by fitting MWG and SGR data to broken line regression, quadratic and saturation kinetics models. Results from these analyses suggest that dietary DCP should be provided at between 340 and 392 g kg−1 (equivalent to ˜370 and 420g kg−1 crude protein) for optimal growth of young Arctic charr reared in similar conditions.

Sexually immature Arctic charr, Salvelinus alpinus (Linnaeus), were fed one of five isoenergetic practical diets of differing lipid:protein ratios (0.98, 0.67, 0.41, 0.26, 0.19) for an 84-day period to examine the influence of diet composition on growth, and growth hormone (GH) and thyroid hormone physiology. All five diets supported growth at approximately the same rate, but the diet with a lipid: protein ratio of 0.98 had the lowest weight gain and highest food conversion ratios. A GH enzymelinked immunosorbent assay (ELISA), developed for use with oncorhynchid fishes, was validated for use with Arctic charr. Plasma GH concentrations were significantly higher in fish fed the diet with a lipid:protein ratio of 0.98, and there were significant direct and inverse correlations between plasma GH levels and dietary lipid and protein content respectively. There were no significant differences in pre-and post-prandial plasma GH concentrations for any group. There were significant post-prandial elevations of plasma triiodothyronine (T 3 ) and thyroxine (T 4 ) for fish fed the lower lipid:protein ratio diets, but there were no differences related to the diets. The results are discussed in terms of GH as a factor in the regulation of lipid and protein homeostasis in fishes.

Embryo somatic tissues, non-somatic yolk-sac materials, and whole, individual fingerlings (age 0+) of Arctic charr, Salvelinus alpinus (L.), as well as a commercial trout diet, were analysed for a wide spectrum of amino acids. Analytical material consisted of prefeeding swim-up fry that were separated into discrete yolk sac and somatic embryo tissue samples. Amino acid concentrations in fry somatic tissue and whole fingerlings were generally very similar to each other, but were lower than those measured in yolk materials. Higher correlations were observed between the majority of specific amino acid concentrations in the trout diet when compared with fingerling data (r 2 ¼ 0.91) and fry somatic tissue data (r 2 ¼ 0.89), than when correlated with fry yolk sac material (r 2 ¼ 0.76). These results indicate that the essential amino acid profiles of fry somatic tissue and whole fingerlings are closer to that of a commercial feed than they are to the endogenous profiles found in the embryonic yolk sac material itself. The dietary ratios of individual essential amino acids were also compared with the total essential amino acid concentrations (A/E ratios) in whole fingerling tissues, and these ratios could be used to accurately estimate the apparent essential amino acid requirements of Arctic charr. The rationale for using carcass amino acid composition data to estimate the dietary essential amino acid requirements of Arctic charr is discussed. KEY WORDS

Ractopamine is a synthetic catecholamine analogue which is known to affect the growth performance of both terrestrial and aquatic livestock species when it is incorporated into their feed. To investigate the effect of ractopamine on fish, rainbow trout, Oncorhynchus mykiss (Walbaum), (initial weight = 385.2 ± 1.4 g) were fed six iso‐energetic (16.6 MJ kg−1) practical diets supplying two levels of ractopamine (0 and 10mgkg−1) at three levels of crude protein (CP; 25%, 35% and 45%) for 112 days. The mean weight gain (MWT), specific growth rate (SGR), condition factor (CF), viscerosomatic index (VSI), hepatosomatic index (HSI), muscle pigmentation intensity, and the proximate composition of eviscerated carcasses and viscera were measured at 28‐day intervals up to day 112. There were no significant (P > 0.05) main effects of ractopamine on any of the responses measured. Significant (P & < 0.05) main effects of dietary protein were observed for MWT, SGR, CF, HSI, pigmentation, eviscerated‐carcass ash, and visceral lipid and visceral ash content. Modest but significant (P < 0.05) interactions between ractopamine and protein were observed for HSI, pigmentation intensity and eviscerated carcass moisture content. The protein and lipid contents of both whole viscera and eviscerated carcasses were not affected (P > 0.05) by the inclusion of 10mgkg−1 ractopamine in the diet. These results indicate that ractopamine may not invoke the beneficial effects of enhancing protein accretion or reducing lipid content of rainbow trout when fed at 10mgkg−1 of diet at the protein levels tested.