TIIE BREEDING OF THE NORTH ATLANTIC FRESHWATKR-EELS
149
are very striking in appearance and thereforc reported by the fishermen,
so we can safely conclude that they are cstremely rare, considering the
thousands of tons of silver eel caught in the Baltic area every year.
Vladykov (1958) assumes that the bronze eel stage in North America
corresponds to that of the silver eel for the European species. I would
suggest rather that it is more advanced, but no decision can be made
without (t study of thc cgg-diameter in relation to the change of colour
before and during the migration. I linve seen ten live bronze eels,
received through the kind assistance of Dr. Vladykov, and they certainly were more advanced than the European silver eel. The eggdiameters were from 0.20 to 0.35 mm in four specimens, which died
within 27 days after their arrival from Canada, while those of the
European silver eel very rarely exceed 0.20 mm.
Tucker (1959s, p. 496) mentions that the eel shows positive reactions
to higher temperature and salinity and continues : “ there can be only
one common inherited pattern of behaviour, yet this has hitherto been
assumed adequate to direct the return journeys of eels leaving points
all over Europe and North Africa and succcssfully traversing a great
variety of conditions to a common oceanic torminal.”
We may agree with Tucker that there can only be one inherited
pattern of behaviour ; but it may very well be much more complicated
than we think.
As has been long known, the elvers (glass eels) work their way towards
brackish or fresh water and against the currents ; this may be observed
anywhere in the world where Anguilla occurs. Later on, when the
yellow eel has turned into tho silver eel, they leavct the brackish water
or frmh water and go for ocr~atiic salinitinfi. 1:ut not rtlwrryrr rro : Natiirn
has mado tho grc:rtt e x pcrirri(mt of t(:d,iriK thc: ocl towtlrdw highcr
salinities than those of tho ocean. We may cite Heldt and Heldt (1928,
p. 7): “ En ce qui concerne le lac de Tunis, les conditions hydrologiques sont tout autres. Aucun apport d’cau douce no se fait dans ce
lac oh, par contre, l’kvaporation concentre cn sels une masee d’eau
peu profonde etalbe sur une grande euperficie.” But lake Tunis
produces every year up to 110 tons of d v e r eel (Gandolfi-Hornyold,
1930, p. 6), and Heldt and Heldt (1930) have shown how the elver8
enter (p. 7): ‘ I La plus frappante est que, normalcment, lcs civelh
marchent dans le sen9 du courant, soit qu’il cntre, soit qu’il R O ~ .
Elles Be laissent porter par h i . 11 eUt rare qu’ellcs no prhentertt d f : b O { I C
au courant que leur vigueur pourrait aidment remontcr. NOUN n’avon~
noti5 le fait qu’8 de rares occasions et particulihrement lors do no8
premibres observations, ce qui nous fit prendre d’abord I’exception
pour la rhgle gdnkrale.
149
are very striking in appearance and thereforc reported by the fishermen,
so we can safely conclude that they are cstremely rare, considering the
thousands of tons of silver eel caught in the Baltic area every year.
Vladykov (1958) assumes that the bronze eel stage in North America
corresponds to that of the silver eel for the European species. I would
suggest rather that it is more advanced, but no decision can be made
without (t study of thc cgg-diameter in relation to the change of colour
before and during the migration. I linve seen ten live bronze eels,
received through the kind assistance of Dr. Vladykov, and they certainly were more advanced than the European silver eel. The eggdiameters were from 0.20 to 0.35 mm in four specimens, which died
within 27 days after their arrival from Canada, while those of the
European silver eel very rarely exceed 0.20 mm.
Tucker (1959s, p. 496) mentions that the eel shows positive reactions
to higher temperature and salinity and continues : “ there can be only
one common inherited pattern of behaviour, yet this has hitherto been
assumed adequate to direct the return journeys of eels leaving points
all over Europe and North Africa and succcssfully traversing a great
variety of conditions to a common oceanic torminal.”
We may agree with Tucker that there can only be one inherited
pattern of behaviour ; but it may very well be much more complicated
than we think.
As has been long known, the elvers (glass eels) work their way towards
brackish or fresh water and against the currents ; this may be observed
anywhere in the world where Anguilla occurs. Later on, when the
yellow eel has turned into tho silver eel, they leavct the brackish water
or frmh water and go for ocr~atiic salinitinfi. 1:ut not rtlwrryrr rro : Natiirn
has mado tho grc:rtt e x pcrirri(mt of t(:d,iriK thc: ocl towtlrdw highcr
salinities than those of tho ocean. We may cite Heldt and Heldt (1928,
p. 7): “ En ce qui concerne le lac de Tunis, les conditions hydrologiques sont tout autres. Aucun apport d’cau douce no se fait dans ce
lac oh, par contre, l’kvaporation concentre cn sels une masee d’eau
peu profonde etalbe sur une grande euperficie.” But lake Tunis
produces every year up to 110 tons of d v e r eel (Gandolfi-Hornyold,
1930, p. 6), and Heldt and Heldt (1930) have shown how the elver8
enter (p. 7): ‘ I La plus frappante est que, normalcment, lcs civelh
marchent dans le sen9 du courant, soit qu’il cntre, soit qu’il R O ~ .
Elles Be laissent porter par h i . 11 eUt rare qu’ellcs no prhentertt d f : b O { I C
au courant que leur vigueur pourrait aidment remontcr. NOUN n’avon~
noti5 le fait qu’8 de rares occasions et particulihrement lors do no8
premibres observations, ce qui nous fit prendre d’abord I’exception
pour la rhgle gdnkrale.