But it is from

the following facts that we may more reasonably
infer that fishes sleep. Very often it is possible to take a fish
off its guard so far as to catch hold of it or to give it a blow
unawares; and all the while that you are preparing to catch or
strike it, the fish is quite still but for a slight motion of the
tail. And it is quite obvious that the animal is sleeping, from its
movements if any disturbance be made during its repose; for it moves
just as you would expect in a creature suddenly awakened. Further,
owing to their being asleep, fish may be captured by torchlight. The
watchmen in the tunny-fishery often take advantage of the fish being
asleep to envelop them in a circle of nets; and it is quite obvious
that they were thus sleeping by their lying still and allowing the
glistening under-parts of their bodies to become visible, while the
capture is taking Place.