Shoal-fish, as a rule, are caught in
greater quantities as they leave the Euxine, and at that season they
are in the best condition. At the time of their entrance they are
caught in very plump condition close to shore, but those are in
comparatively poor condition that are caught farther out to sea.
Very often, when the coly-mackerel and the mackerel are met by a south
wind in their exit, there are better catches to the southward than
in the neighbourhood of Byzantium. So much then for the phenomenon
of migration of fishes.
Now the same phenomenon is observed in fishes as in
terrestrial animals in regard to hibernation: in other words, during
winter fishes take to concealing themselves in out of the way
places, and quit their places of concealment in the warmer season.
But, by the way, animals go into concealment by way of refuge
against extreme heat, as well as against extreme cold. Sometimes an
entire genus will thus seek concealment; in other cases some species
will do so and others will not. For instance, the shell-fish seek
concealment without exception, as is seen in the case of those
dwelling in the sea, the purple murex, the ceryx, and all such like;
but though in the case of the detached species the phenomenon is
obvious-for they hide themselves, as is seen in the scallop, or they
are provided with an operculum on the free surface, as in the case
of land snails-in the case of the non-detached the concealment is
not so clearly observed.
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