A critical consideration of the metamorphosis zone when identifying daily rings in otoliths of European eel, Anguilla anguilla (L.)

Otoliths of glass eels and larvae collected from the Rio Minho (Portugal/Spain) as well as from the Iberian continental slope from the Bay of Biscay were analyzed by scanning electron microscopy. Preliminary data are presented on the total radius of the otolith and the width of the zone exhibiting a diffuse structure which, in the literature, is suggested to be the zone of metamorphosis. It was found that the radius increased from the development stage I through stage II to the glass eel stage (Vb). The width of the diffuse zone also exhibited an increase. Calculations of the dimensions of the diffuse zone of specimens older than stage I revealed that the area of metamorphosis amounts to about 28–60% of the total diffuse zone. From these results it is evident that part of the diffuse zone must have formed during a larval phase of retarded growth, during which no formation of daily growth rings takes place. Only the outer portion of the diffuse zone can be accounted for by the metamorphosis. For these reasons, an exact age determination by counting daily rings seems impossible. A determination of the oceanic life of the eel recruits is difficult for other reasons too: all earlier and recent studies have indicated that Anguilla leptocephali and their metamorphosis stages do not occur on the continental shelf, which could add an additionally high amount of time needed until arrival at the coasts.