Tracking of the hormonally induced maturation of female eels by computed tomography

Female European eels were kept in artificial seawater for a trial period of 14 weeks. Three fish were injected intra-abdominally with carp pituitary suspension (twice a week) and human chorionic gonadotropin (every 2nd week), with the aim to induce artificial maturation. Three further fish were not treated (control). Fish were not fed during the trial. The treated fish were scanned by computed tomography (CT) every second week (the controls only at the start and at the end of the trial) to follow changes in body composition. Notable decreases were shown in total body pixel number (body volume), total body fat content, total fillet volume and fillet fat content during the experiment. Changes were more pronounced in the treated group than in the control. The abdominal volume strongly increased in the responding fish throughout the trial. The ovary volume increased measurably, while its fat content increased only until the 8th week, after which a decrease was measured. Tissue volumetric estimations of the ovary were also supported by histological results. A so-called volumetric gonadosomatic index (gonad volume/total body volume x 100) was developed for the quantitative characterisation of eel maturation.