They sleep

in the night-time more than during
the day; and so soundly at night that you may cast the net without
making them stir. Fish, as a general rule, sleep close to the
ground, or to the sand or to a stone at the bottom, or after
concealing themselves under a rock or the ground. Flat fish go to
sleep in the sand; and they can be distinguished by the outlines of
their shapes in the sand, and are caught in this position by being
speared with pronged instruments. The basse, the chrysophrys or
gilt-head, the mullet, and fish of the like sort are often caught in
the daytime by the prong owing to their having been surprised when
sleeping; for it is scarcely probable that fish could be pronged while
awake. Cartilaginous fish sleep at times so soundly that they may be
caught by hand. The dolphin and the whale, and all such as are
furnished with a blow-hole, sleep with the blow-hole over the
surface of the water, and breathe through the blow-hole while they
keep up a quiet flapping of their fins; indeed, some mariners assure
us that they have actually heard the dolphin snoring.