This study investigates whether tissue free amino acid (FAA) pools in rainbow trout,Oncorhynchus mykiss (Walbaum), are altered following feeding and the relationships between the amount of food consumed and the FAA pool size. Trout were starved for 7 days to provide baseline data and then refed on day 8. Individual food intake was measured by radiography and the consumption of amino acids (AA) calculated from dietary protein consumption. Total FAA concentrations in the stomach, liver and white muscle were little changed at various times after the meal and this pattern was repeated for the majority of individual FAA. Overall, the most notable change was a reduction in essential FAA concentrations (principally in valine, leucine and isoleucine) in the white muscle following feeding. However, in the caeca total FAA, total essential FAA and a number of individual FAA were significantly elevated at 4, 9 and 15h following feeding. There were few significant correlations between dietary amino acid consumption and total tissue FAA and essential FAA concentration in the stomach, caecum and white muscle; correlations were stronger in the liver. In order to explain the relative constancy of total FAA concentrations in the tissues following food intake (which represents over 100% of the total FAA pool) a model is presented that quantifies the AA flux through the free pools and considers the role of protein turnover in regulating FAA pool size.

No abstract

Two methods for assessing the status of an individual rainbow trout Oncorhynchus mykiss within a group hierarchy, radiographic determination of individual food intake and analysis of brain serotonergic activity, were compared. The results showed that individual food intake, measured as the average share of the group meal, and brain serotonergic activity, measured as brain levels of 5- hydroxyindoleacetic acid (5-HIAA) or as 5-HIAA/5-HT (serotonin) ratios, were inversely correlated with each other, suggesting that both methods could be used as indicators of the position of the rainbow trout in a dominance hierarchy. In addition, specific growth rate correlated significantly with brain 5-HIAA/5-HT ratios. The results indicate that the increase in brain 5-HIAA/5-HT ratios in subordinate individuals is caused by an increased use (release) of 5-HT in these fish, and not by an increase in the level of tryptophan, the amino acid precursor of 5-HT. The relationships between social rank, food intake, growth, stress and brain serotonergic activity of fish in a social hierarchy are discussed.