The researches of Professor Agassiz prove that the localisation of
species of fish in these waters is peculiarly distinct and permanent,
their migrations being very limited--consisting chiefly in removing from
shallow to deeper waters, and from these to shallow again, at those
seasons when the range of the shore in the same water-basin is affected
by the rise and fall of the river. Thus, the fishes found at the bottom
of a lake covering, perhaps, a square mile in extent when the waters are
lowest, will appear near the shores of the same lake when, at the season
of high-water, it extends over a much wider area. In the same way,
fishes which gather near the mouth of a rivulet at the time of
low-water, will be found as high as its origin at the period of
high-water; and those which inhabit the larger igarapes on the sides of
the Amazon, when they are swollen by the rise of the river, may be found
in the Amazon itself when the stream is low. There is not a single fish
known to ascend from the sea to the higher courses of the Amazon at
certain seasons, and to return regularly to the ocean.
The striking limitation of species within different areas does not,
however, exclude the presence of certain kinds of fish simultaneously
throughout the whole Amazonian basin. The piraracu, for instance, is
found everywhere from Peru to Para; and so are a few other species more
or less extensively distributed over what may be considered distinct
ichthyological fauna. But these wide-spread species are not migratory.
They have normally and permanently a wide range--just as some
terrestrial animals have an almost cosmopolitan character--while others
are circumscribed within comparatively narrow limits.
Surprising indeed is the variety of species of fishes contained in the
Amazonian basin. Professor Agassiz, during his expedition, collected
nearly two thousand, "for the most part," as he observes--and which is
still more surprising--"circumscribed within different limits, from
Tabalinga to Para, where the waters differ neither in temperature, nor
in the nature of their bed, nor in the vegetation along their borders.
There are met with, from distance to distance, assemblages of fishes
completely distinct from each other."
Still more curious, perhaps, is the intensity with which life is
manifested in these waters. All the rivers in Europe, from the Tagus to
the Volga, do not nourish a hundred and fifty species of fresh-water
fishes; and yet in a little lake near Manaos, called Lago Hyanuary, the
surface of which hardly covers four or five hundred square yards, more
than two hundred distinct species were discovered, the greater part of
which have not been observed elsewhere.