African catfish were cannulated in the dorsal aorta to study diurnal changes in blood metabolites. Cannulation of the branchial artery was tested but proved to be less successful. A clear diel fluctuation in the two major blood metabolites, free fatty acids (FFA) and glucose, was observed. Compared to the initial value at 8.30 a.m., the plasma FFA levels dropped by ca. 50% within 2 hours, after which the FFA concentration stayed relatively constant. Minimum values of 0.26±0.04 mM were reached at 12.30. The FFA concentration recovered to the initial value within the following 3 hours. The fluctuation in plasma glucose levels showed a comparable course but there was a phase-shift by 2 hours. The most astonishing finding of our study was the almost complete absence of glucose in the plasma of African catfish (0.05 ± 0.03 mM), a never reported phenomenon for any fish species. This study demonstrates the relatively low level of control of plasma glucose levels as compared to plasma FFA levels in African catfish.
Gonadal development and spawning behaviour of artificially-matured European eel (Anguilla anguilla L.) was studied. Treatment of males with Human Chorionic Gonadotropin (HCG; 1 IU g/week) resulted in a Gonado-Somatic Index (GSI) of 10.88 ± 3.39 and spermiation. Treatment of females with carp Pituitary suspension (cPs) (20 mg cPs/kg body weight per week) resulted in oogenesis with a GSI of 20.0 ± 11.3 (n = 7), and the number of eggs per female was 1874 * 103 ± 1116 * 103; (n = 7). Ovulation of the females was induced with 17α, 20β dihydroxyprogesteron (DHP) at 2 μg/g bodyweight. Eggs of European eel were found to be non-sticky and typically pelagic. Maximum speed of eggs rising to the surface in a water column was 2.24 ± 0.33 metres (m) per hour (h). To study behaviour in a qualitative way, two females were used together with three groups of three males. During a 283 minute (min) observation of the two females, we observed female-female interaction: 'lethargic behaviour' (33.6%) vs. 'cruising together' (66.4%). In the period when males and females were together (188 min), we observed 'approaching the head region of the female' (57.7%), 'touching the operculum' (39.4%), or 'approaching the urogenital area' (2.9%) by the males (total 725 seconds (s)). Sperm release in the presence of a female took 115 s of the total approaching time of 725 s (15.9%), while in the case of male-male interaction this was only 15 s of the total period of 116 s (12.9%). Induced spawning behaviour of eels was collective and simultaneous, corresponding to spawning in a group. This is the first time group spawning behaviour has ever been observed and recorded in eels.