chapter 3 - RiuNet

GENERAL INTRODUCTION
who postulated that “the eels come from what we call the entrails of the
earth”. A few centuries later, some discoveries of eel larvae at different
wild locations allowed new hypothesis to be made and finally, thanks to the
marine expeditions of the early 20 th century conducted by Johannes
Schmidt, the central mystery of the eel breeding location was elucidated.
Schmidt recorded data of over 10.000 European eel larvae over a period of
25 years, and through a spatio-temporal study of the larvae distribution he
discovered the breeding location of this specie: the Sargasso Sea (Schmidt,
1923). The hypothesis that all European eel migrate to the Sargasso Sea for
reproduction is called the panmixia theory, and until recently has been
broadly accepted. However, based on field observations, morphological
traits and molecular studies there are some indications that Schmidt’s claim
of the complete homogeneity of the European eel population and a unique
spawning location may be an overstatement (van Ginneken and Maes,
2005).
Nowadays it is known that mature adults leave the continental waters
depending on the lunar phase and atmospheric conditions (Tesch, 2003).
They swim southward between 5000-6000 km using the Atlantic currents
and arrive at the Sargasso Sea 6-7 months later, where they spawn and die.
Eel larvae, called leptocephali, start the 8-9 month return journey to the
eastern Atlantic coast transported along the Gulf Stream and actively
swimming (Lecomte-Finiger, 1994). Near the coast, the leptocephali
metamorphose into glass eels, swim upstream rivers and grow to partial
maturity. Once in their final habitat, eels can live a long time, depending on
their sex: 5-8 years for the males and 8-20 years for the females. Finally,
pubertal eels return to the sea for the reproductive migration (Figure 1).
a
b
c
Figure 1. Pictures of European eel metamorphosis: a) leptocephali; b) glass eel;
and c) adult eel.
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