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

The gonadosomatic indices (I_G) of female conger eel Conger conger , aged between 2 and 11 years postmetamorphosis, ranged between 0·04 and 4·78 and were correlated with both age and body length. Microscopical examination of the gonads showed immature ovaries at two main stages of oocyte development, pre-vitellogenic oocytes for I_G < 1, and oocytes at an early vitellogenic 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 deeper oceanic waters. Radioimmunoassays (RIA) of sex steroids gave low serum levels of oestradiol and of 11-ketotestosterone, but higher levels of testosterone correlated with increase in I_G. Immunoenzymatic assay (ELISA) indicated low serum levels of vitellogenin (VtG), which were significantly correlated with I_G. The pre-vitellogenic and early vitellogenic stages observed 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 with silvering in Anguilla species, such as the increase in ocular index, and regression of the digestive tract, did not occur at the early vitellogenic stage in conger eels.