Biochemical parameters as a measure of food availability and growth in immature rainbow trout (Oncorhynchus mykiss).

1. Rainbow trout held in brackish water (15 parts per thousand) were starved or fed different amounts of food. 2. A significant correlation was found between the growth rates of the different animals and the feed rates. 3. The RNA:DNA ratio in the white epaxial muscle is lowest in starved fish and increases in proportion to the feed rate and individual specific growth rate. The correlations are significant at the P less than 0.01 level. 4. Liver metabolism varies according to food availability. 5. The protein synthesis capacity of the liver (RNA:DNA ratio) and liver somatic index increase as the feeding rate increases. It also correlates significantly with the specific growth rates of the different animals. 6. The intermediary metabolism of the central metabolic organ, the liver, varies in the same way. 7. The activities of the NADPH producing liver enzymes glucose-6-phosphate dehydrogenase (G6PDH), 6-phosphogluconate dehydrogenase (6PGDH), isocitrate dehydrogenase (IDH) and malic enzyme (ME) increase as the feed rate (and therefore the specific growth rate) increases. 8. G6PDH and IDH activity in the kidney is influenced to a much lower degree by food intake. 9. Summarizing, it can be stated that biochemical parameters can be used to describe comprehensively the metabolic status and growth of rainbow trout.