Study of variations of anatomical and endocrine parameters in sedentary and downstream migrating female European eels (Anguilla anguilla) : Application to the characterization of the silver stage.

Silvering process in eels is a complex transformation from the sedentary ph ase of juvenile growth (yellow stage) to the downstream migratory phase (si lver stage), which is the initial step of the reproductive oceanic migratio n. The present comparative study was performed on batches of sedentary or m igratory female eels, body length > 45 cm, caught in natural environment. W e looked for criteria of silver stage by combining, for the first time, end ocrine parameters to anatomical ones. Anatomical parameters included extern al characteristics (body weight, body length, ocular index, OI) and interna l ones (gonadosomatic index, GSI, digestive tract-somatic index, DTSI). End ocrine parameters included hormones implicated in metamorphosis (thyroid ho rmones, T-3 and T-4), growth (growth hormone, GH) and reproduction (gonadot ropic hormone, GtH). Sedentary eels show a progressive development of ovaries, correlated to bod y growth. GSI further increases in downstream migratory eels (x 2.85), a th reshold value greater than or equal to 1,4% characterizing the silver stage . The enlargement of the eyes also is a progressive phenomenon, initiated a t the yellow stage in correlation to the GSI. As well as for GSI, OI furthe r increases in migratory eels (x 2), a threshold value greater than or equa l to 8 being proposed as an external criterion of silvering. Regression of the digestive tract is not initiated at the yellow stage and occurs at silv ering, with a threshold value less than or equal to 1.5% characteristic of the silver stage. Among the endocrine parameters studied, only pituitary GtH content allows t o characterize silvering. GtH content steadily increased at the yellow stag e, in correlation to the GSI, then strongly raised (x 25) in migratory eels , with a threshold value greater than or equal to 15 ng/hypophyse character istic of the silver stage. Pituitary GH content increases at the yellow sta ge, in correlation with body length. GH content drops at silvering (x 0.5), but the large overlapping of individual values does not allow to use this criterion. Circulating levels of thyroid hormone, T-4, increase at silverin g (x 1.38) but with a large overlapping of individual values. T-3 levels do not vary at silvering. In conclusion, silvering includes multiple transformations, which are chara cteristic of metamorphosis (increase in T-4) as well as of initiation of pu berty (increases in GSI and GtH), fasting (decrease in DTSI), and stop of g rowth (decrease in GH). Some of these anatomical (OI, GSI, DTSI) and endocr ine (GtH) parameters, which exhibit very few overlapping of individual valu es between yellow and silver eels, may be used as criteria of silvering.