Temporal progression in migratory status and sexual maturation in European silver eels during downstream migration

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.

Reproductive physiological characteristics of tropical Celebes eels Anguilla celebesensis in relation to downstream migration and ovarian development

“…Of the 64 migrating A. celebesensis , 36% and 61% of the eels had early vitellogenic‐ and midvitellogenic‐stage oocytes, respectively, which are much more advanced stages compared with other anguillid species that have started their spawning migrations. Many previous studies reported that temperate eels, A. anguilla (Palstra et al ., ), A. rostrata (Cottrill et al ., ), A. japonica (Sudo et al ., ) and A. australis (Lokman et al ., ; Todd, ), start their downstream migrations at the oil‐droplet or early vitellogenic stage. Only in the temperate New Zealand longfin eel Anguilla dieffenbachii (Gray 1842) it was reported that migrating eels had midvitellogenic‐stage oocytes (Lokman et al ., ; Todd, ).…”

“…Thus, the dynamics of the reproductive hormones corresponded reasonably well with the developmental stages of the oocytes and represented the physiological background of oogenesis in A. celebesensis that has remarkably developed oocytes just before downstream migration. The dynamics of reproductive hormones associated with gonadal development have been investigated in the more extensively studied temperate eels in natural conditions (e.g., A. japonica, A. anguilla and A. australis ;Han et al, 2003aHan et al, , 2003bSudo et al, 2011;Aroua et al, 2005;Nguyen et al, 2019), but unfortunately, the knowledge is limited only to the early development stages, because these species start their spawning migrations at the oil-droplet or early vitellogenic stage (Palstra et al, 2011: Sudo et al, 2011 and the later oogenesis processes occur during the oceanic spawning migration. For A. dieffenbachii, which starts downstream migration at the midvitellogenic stage, there has only been a comparative study of sex steroid levels between migrating and non-migrating eels (Lokman et al, 1998), so there is no knowledge about the continuous hormone dynamics associated with oocyte development in natural condition.…”

Morphological and physiological changes of female tropical eels, Anguilla celebesensis and Anguilla marmorata, in relation to downstream migration

“…Various methods have been used to categorize temperate yellow and silver eel stages, but no silvering index has been established yet for tropical species. In the present study, Okamura et al 's (2007) silvering index of A. japonica was used to categorize the silvering stages of the eels in this study because of the lack of a silvering index for tropical anguillids, and because the key characters used for A. japonica also are present in A. anguilla (Palstra et al , 2011) as well as the two species of tropical eels studied. Based on eye index, A. anguilla with an I E >6·5 have been classified as silver eels (Pankhurst, 1982 b ), however, this cut off point was not necessarily suitable for other anguillids (Cottrill et al , 2002; Lokman et al , 1998; Lockman & Young, 1998).…”

Selection of best-performing reference gene products for investigating transcriptional regulation across silvering in the European eel (Anguilla anguilla)

“…These changes correlate with increasing gonadosomatic index and LH levels in the pituitary gland, leading to consider silvering as the onset of puberty rather a true metamorphosis 43 . A close relation between sexual maturation and metabolism has also been suggested, since the relevant endocrine modifications occurring during silvering are also accompanied by a significant metabolic shift between yellow and silver stages 3 44 45 . Indeed, in yellow eels energy stores rely mainly on glycogen 46 while lipids become the main energy source in silver eels; moreover, silver eels do not eat during migration and energetic metabolism may be differently regulated with respect to yellow eels.…”