Impact of silvering stage, age, body size and condition on reproductive potential of the European eel

Deteriorating reproductive capacities are a possible cause for the decline in eel populations. However, since no sexually maturing adults have ever been found in the open ocean, there is no possible observation of this life-phase in the natural environment. To test whether eels display variations in their reproductive capacity, we investigated the response of the European eel Anguilla anguilla to artificially induced sexual maturation. The objective was to describe the variability in the maturation response of eels in terms of gonad weight and vitellogenin levels and to try and relate this variability to individual characteristics (length, condition, age and silvering stage). We carried out 3 sets of experiments in which sexual maturation was induced in female silver European eels from different locations using weekly carp pituitary extract (CPE) injections. External parameters of eels were measured every week, and several individuals were sacrificed at monthly intervals for blood and organ sampling. Although gonad weight in most treated eels increased by as much as 50 % of total body weight, results showed that individual responses were highly variable. Part of this variability could be explained by individual characteristics of eels. Individuals which initially were the most advanced in the silvering process and had a high condition factor showed the best maturation response. Age of eels was correlated with energy stores, and it was shown that eels benefit from delaying their migration and reproduction. Furthermore, highest gonad development was obtained for eels measuring over 700 mm. body length.