Blood-chemistry indicators of nutritional status in juvenile salmonids

Three experiments were performed to determine the effects of food deprivation (21–35 days at 10–12° C) on selected blood-chemistry variables in juvenile salmonids. The experimental groups were laboratory-reared rainbow trout Oncorhynchus mykiss (RBT), laboratory-reared Chinook salmon Oncorhynchus tshawytscha (LCS) and hatchery-reared, actively migrating Chinook salmon (MCS). After fasting, RBT and LCS were refed for 10 days. In fasted fishes, plasma alkaline phosphatase (ALP) activities decreased relatively rapidly, differing significantly ( P ≤ 0·05) from activities in fed control fishes after 1–2 weeks. Total protein (for all groups) and total cholesterol (for RBT and MCS) decreased relative to fed controls after 2–3 weeks, and total calcium (for RBT and MCS) after 3–4 weeks. Plasma ALP activities recovered after 10 days refeeding, but total protein, cholesterol and calcium concentrations did not. Triglyceride concentrations were highly variable and responded significantly to fasting only in the trial with RBT. Concentrations of some variables decreased (glucose and phosphorus) or increased (total magnesium) relative to control values during the initial days or weeks of fasting and then stabilized at new levels. Activities of four metabolic enzymes of intracellular origin (alanine aminotransferase, aspartate aminotransferase, lactate dehydrogenase and creatine kinase) did not differ significantly in the plasma of fed and fasted fishes.