Daily and seasonal variations in haematological and blood biochemical parameters in the tench, Tinca tinca Linnaeus, 1758

The purpose of the present study was to characterize daily variations in haematology and blood biochemistry in healthy male and female tench (Tinca tinca) in the four seasons of the year. Red blood cells, haematocrit and haemoglobin presented a similar daily profile in spring and summer, with high values during the scotophase. Total and differential types of leucocytes showed significant daily fluctuations, with a different pattern depending on the season and/or the type of leucocytes studied. A daily rhythm in plasma glucose and triglycerides was detected in winter, but significant changes in cholesterol concentrations were only detected in females during summer. The lowest mean level for triglycerides and cholesterol was observed in winter. The daily profile of plasma total proteins was similar in the four seasons, without a significant daily oscillation. All these daily and seasonal changes in haematology and blood biochemistry indicate that the time of the day and the season must be considered when such parameters are used for assessing the physiological status of the fish, and as biomarkers for disease and/or pollutant exposure.