Seasonal variation in steroid hormones and blood parameters in cage-farmed European sea bass (Dicentrarchus labrax L.)

Abstract

Summary For a period of 1 year, some blood parameters were evaluated on a monthly basis in a population of adult European sea bass (Dicentrarchus labrax L.) intensively reared in floating marine cages (Ionian Sea, Mediterranean). From April (13 months old) to July (16 months old) males (35–50%) and sexually indeterminate individuals were collected. From August to March (24 months old) only males were sampled. During this period the percentage of spermiated males was highest (100%) from November (20 months old) to January (22 months old). Plasma testosterone in males was inversely related to sunlight (h month−1) and was elevated between October and January, when males first achieved sexual maturity. Testosterone showed the highest value (0.49 ± 0.02 ng ml−1) in January and the lowest (0.09 ± 0.02 ng ml−1) in March. Haematocrit, red blood cell counts and haemoglobin concentration were elevated from November to March, being inversely related to sunlight. The two latter parameters were also inversely related to daily food intake. Haematocrit, red blood cell counts and haemoglobin concentration were highest in December [53 ± 1%, (5.36 ± 0.06) × 106 mm−3, 10.08 ± 0.14 g 100 ml−1, respectively] and lowest in June [35 ± 1%, (3.33 ± 0.05) × 106 mm−3, 6.47 ± 0.13 g 100 ml−1, respectively]. White blood cell counts were not correlated with sea water temperature, sunlight or daily food intake. They were highest in February [(8.45 ± 0.20) × 104 mm−3] and lowest in April [(6.07 ± 0.14) × 104 mm−3]. Total plasma protein concentration (4.88 ± 0.11–5.93 ± 0.10 g 100 ml−1) and mean cell volume (93.3 ± 0.9–105.5 ± 1.8 μm3) showed small fluctuations throughout the year. Sexual maturity appears to be a major factor that significantly affects haemopoiesis of D. labrax. This study contributes to the evaluation of normal levels of some blood parameters in European sea bass, which are helpful for the assessment of physiological status and health of this species.