Temporal progression in migratory status and sexual maturation in European silver eels during downstream migration |
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Authors: | Arjan P Palstra Ma Angeles Guerrero Gerard de Laak Jan P G Klein Breteler Guido E E J M van den Thillart |
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Institution: | (1) Molecular Cell Biology, Institute of Biology, Leiden University (IBL), Sylvius Laboratory, Leiden, The Netherlands;(2) Departament de Fisiologia, Facultat de Biologia, Universitat de Barcelona and Institut de Biomedicina de la Universitat de Barcelona (IBUB), Barcelona, Spain;(3) Institute for Marine Resources and Ecosystem Studies (IMARES) of the Wageningen University, P.O. Box 77, 4400 AB Yerseke, The Netherlands;(4) Sportvisserij Nederland, Bilthoven, The Netherlands;(5) VIVION BV, Utrecht, The Netherlands |
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Abstract: | The onset of downstream migration of European eels is accompanied by a cessation of feeding and the start of sexual maturation
which stresses the link between metabolism and sexual maturation, also suggesting an important role for exercise. Exercise
has been tested with eels in swim tunnels and was found to stimulate the onset of sexual maturation. In this study, we have
investigated the interplay between migration and maturation in the field during the downstream migration of female silver
eels. Temporal changes in migratory status and sexual maturation among silver eels of the upstream Rhine River system over
3 months of the migration season (August, September and October) were determined in biometrical parameters, plasma 17β-estradiol
and calcium levels, oocyte histology and gonadal fat levels. Furthermore, the ecological relevant parameters age as determined
by otolithometry and health aspects indicated by haematocrit, haemoglobin and swim-bladder parasite load were measured. Silver
eels were estimated to be 14 years old. A strong temporal progression in migratory stage was shown over the months of downstream
migration. Catches probably represented a mix of reproductive migrants and feeding migrants of which the ratio increased over
time. Furthermore, this study confirmed our hypothesis linking the migratory stage to early maturation as indicated by enlargement
of the eyes, oocyte growth and fat deposition in the oocytes, exactly the same changes as found induced by exercise but not
ruling out environmental influences. Migrants show extensive fat uptake by the oocytes, probably stimulated by the swimming
exercise. In addition, at least 83% of the silver eels in this spawning run may have suffered from negative effects of swim-bladder
parasites on their swimming performance. |
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