Modeling the drift of European (Anguilla anguilla) and American (Anguilla rostrata) eel larvae during the year of spawning

The distribution of the leptocephalus larvae of European (Anguilla anguilla) and American (Anguilla rostrata) eels collected during recent Sargasso Sea surveys was used to model larval drift. The drift trajectories of individual larva were back-calculated to the estimated time of spawning, using current data from two global oceanographic assimilation models. The results of both models give the same overall result; widespread spawning extended in time from December to March. The drift was also calculated forwards for approximately 1 year. The forward drift modelling showed that most leptocephali remained in the area south of the Subtropical Frontal Zone. One conclusion is that the majority of leptocephali remain trapped and possibly die in the retention area. A small proportion of leptocephali are entrained into the Gulf Stream system. An implication is that the spawning success maybe highly sensitive to oceanographic and climatic factors that alter the dispersion of leptocephali out from the retention area. An alternative interpretation is that the surveys were made too late after the peak spawning period and that the core spawning area was missed. La repartition de leptocephales d'anguilles europeennes (Anguilla anguilla) et americaines (Anguilla rostrata) preleves durant de recents releves dans la mer des Sargasses a ete utilisee pour modeliser la derive des larves. Les trajectoires de derive de leptocephales individuels ont ete retrocalculees jusqu'au moment estime du frai en utilisant des donnees sur les courants issues de deux modeles d'assimilation de donnees oceanographiques planetaires. Les resultats des deux modeles donnent le meme resultat global, soit un frai repandu dans l'espace et reparti dans le temps de decembre a mars. La derive a egalement ete calculee prospectivement pour environ 1 annee. La modelisation prospective de la derive demontre que la plupart des leptocephales sont demeures dans la region au sud de la zone frontale subtropicale. Il est conclu que la majorite des leptocephales demeurent pieges et meurent possiblement dans la zone de retention. Une petite portion des leptocephales sont entraines dans le systeme du Gulf Stream. Cela sous-entend que le succes de reproduction pourrait etre tres sensible a des facteurs oceanographiques et climatiques qui modifient la dispersion des leptocephales vers l'exterieur de la zone de retention. Une autre interpretation serait que les releves ont ete faits trop longtemps apres la pointe du frai et qu'ils ont rate le creur de la zone de frai. [Traduit par la Redaction]
Introduction European eels (Anguilla anguilla) and American eels (Anguilla rostrata) have been known to spawn in the Sargasso Sea, since small specimens of the leptocephalus larvae of both species were [...]