Effects of sampling on blood parameters in the rainbow trout, Salmo gairdneri Richardson

The effects of sampling and short‐term storage of blood samples on the blood parameters of rainbow trout were evaluated by comparing values obtained from resting fish sampled via cannulae and from fish sampled by cardiac puncture. Sampling stress causes an increase in haematocrit value and a decrease in mean cellular haemoglobin concentration, indicating red cell swelling. These changes are aggravated by consecutive storage of samples. Also, blood ionic balance is affected. The K+ concentration of plasma increases and plasma Cl− concentration decreases owing to sampling. During storage the plasma K+ concentration decreases far below resting levels in samples taken by cardiac puncture, whereas plasma Cl− level returns to pre‐stress levels. Owing to the sampling‐ and storage‐induced changes in blood parameters, care must be taken to ensure that similar sampling procedure is employed throughout to allow reliable comparisons between groups to be made.