Reproductive biology of the conger eel from the south coast of Brittany, France and comparison with the European eel

The gonadosomatic indices (I-G) of female conger eel Conger conger, aged be tween 2 and 11 years postmetamorphosis, ranged between 0(.)04 and 4(.)78 an d were correlated with both age and body length. Microscopical examination of the gonads showed immature ovaries at two main stages of oocyte developm ent, pre-vitellogenic oocytes for I-G<1, and oocytes at an early vitellogen ic stage (lipid vesicle stage) for I-G>1. The immaturity of the conger eels sampled in Concarneau Bay indicates that this species probably spawns in d eeper oceanic waters. Radioimmunoassays (RIA) of sex steroids gave low seru m levels of oestradiol and of 11-ketotestosterone, but higher levels of tes tosterone correlated with increase in I-G. Immunoenzymatic assay (ELISA) in dicated low serum levels of vitellogenin (VtG), which were significantly co rrelated with I-G. The pre-vitellogenic and early vitellogenic stages obser ved in the coastal C conger were similar to the oocyte stages found in the European eel Anguilla anguilla, at the yellow and silver phases of its life cycle respectively. However, other morpho-functional changes, associated w ith silvering in Anguilla species, such as the increase in ocular index, an d regression of the digestive tract, did not occur at the early vitellogeni c stage in conger eels. (C) 2001 The Fisheries Society of the British Isles