Relationship between growth and selected liver enzyme activities of individual rainbow trout

Interpretation of enzymatic data requires consideration of the food intake of each animal studied. Food intake and body mass gain are closely correlated in rapidly growing animals. Direct measurement of food intake by individual fish within a school is nearly impossible. We examined the relationship between growth and liver enzyme activity as a means of inferring the food intake of individual fish within a school. Trout, identified by passive integrated transponder implants, were fed either 0, 0.3, 1, or 2% body mass/d to produce a wide range of growth rates. The activities of five enzymes, predominantly localized in liver, were measured. Results showed that, although the magnitude of response differed, increases in total liver activities of all five enzymes measured were linearly related to growth. Hexokinase (EC 2.7.1.1) increased at a rate below, and beta-D-glucose:NAD(P)+1-oxidoreductase (EC 1.1.1.47) increased at a rate equivalent to, observed increases in total liver mass. Malic enzyme (EC 1.1.1.40), glucose-6-phosphate dehydrogenase (EC 1.1.1.49) and 6-phosphogluconate dehydrogenase (EC 1.1.1.44) showed preferential increases in activity as food intake increased. Correlation of enzyme activities measured in fish fed restricted rations with either growth or nominal feeding rate showed that growth of individual fish was more closely related to liver enzyme activities than nominal feeding rate.