Early in summer, at dates varying with the distance of the rivers from
the open Atlantic, crowds of young eels or elvers come up-stream.
Sometimes the procession or eel-fare includes thousands of individuals,
each about the length of our first finger, and as thick as a stout
knitting needle. They obey an inborn impulse to swim against the stream,
seeking automatically to have both sides of their body equally
stimulated by the current. So they go straight ahead. The obligation
works only during the day, for when the sun goes down behind the hills
the elvers snuggle under stones or beneath the bank and rest till dawn.
In the course of time they reach the quiet upper reaches of the river or
go up rivulets and drainpipes to the isolated ponds. Their impulse to go
on must be very imperious, for they may wriggle up the wet moss by the
side of a waterfall or even make a short excursion in a damp meadow.
In the quiet-flowing stretches of the river or in the ponds they feed
and grow for years and years. They account for a good many young fishes.
Eventually, after five or six years in the case of the males, six to
eight years in the case of the females, the well-grown fishes, perhaps a
foot and a half to two feet long, are seized by a novel restlessness.
They are beginning to be mature. They put on a silvery jacket and become
large of eye, and they return to the sea. In getting away from the pond
it may be necessary to wriggle through the damp meadow-grass before
reaching the river. They travel by night and rather excitedly. The
Arctic Ocean is too cold for them and the North Sea too shallow. They
must go far out to sea, to where the old margin of the once larger
continent of Europe slopes down to the great abysses, from the Hebrides
southwards. Eels seem to spawn in the deep dark water; but the just
liberated eggs have not yet been found. The young fry rises to near the
surface and becomes a knife-blade-like larva, transparent all but its
eye. It lives for many months in this state, growing to be about three
inches long, rising and sinking in the water, and swimming gently.
These open-sea young eels are known as Leptocephali, a name given to
them before their real nature was proved. They gradually become shorter,
and the shape changes from knife-blade-like to cylindrical. During this
change they fast, and the weight of their delicate body decreases. They
turn into glass-eels, about 2-1/2 inches long, like a knitting-needle in
girth. They begin to move towards the distant shores and rivers, and
they may be a year and a half old before they reach their destination
and go up-stream as elvers. Those that ascend the rivers of the Eastern
Baltic must have journeyed three thousand miles. It is certain that no
eel ever matures or spawns in fresh water. It is practically certain
that all the young eels ascending the rivers of North Europe have come
in from the Atlantic, some of them perhaps from the Azores or further
out still. It is interesting to inquire how the young eels circumvent
the Falls of the Rhine and get into Lake Constance, or how their kindred
on the other side of the Atlantic overcome the obstacle of Niagara; but
it is more important to lay emphasis on the variety of habitats which
this fish is trying--the deep waters, the open sea, the shore, the
river, the pond, and even, it may be, a little taste of solid earth. It
seems highly probable that the common eel is a deep-water marine fish
which has learned to colonise the freshwaters. It has been adventurous
and it has succeeded. The only shadow on the story of achievement is
that there seems to be no return from the spawning. There is little
doubt that death is the nemesis of their reproduction. In any case, no
adult eel ever comes back from the deep sea. We are minded of Goethe's
hard saying: "Death is Nature's expert advice to get plenty of life."
§ 4