Why do fish sleep

1000 answers Do fish sleep in the river?

Sleep fish, e.g. trout, in the river - and if so, why aren't they drifting downstream towards the sea?

So the simple answer is, yes the trout sleep, and when they sleep they go to quiet places in the river, shelters where the current is almost zero. This can be in small niches, for example, or near the bottom of the river, where the current is much less than near the surface. That is one explanation. That is why it is so important that when rivers are renatured, care is taken to ensure that such quiet shelters remain; the fish can rest there without being washed away.

But there is apparently another mechanism that the fish scientist Robert Arlinghaus from the Leipniz Institute for Freshwater Ecology drew my attention to. Indeed, it has been found, at least in the case of zebrafish, that they not only sleep at night, but also have short phases of sleep during the day, which often only last a few seconds, and are then replaced by phases with very high activity. That is, their sleep is a microsleep and so brief that the fish cannot be drifted far. In addition, it has been found in zebrafish that the pectoral fins are also active during sleep - at least active enough that the fish can hold their position. This is less well researched in the case of trout, but the zebrafish is used very often in science to research general mechanisms in fish. In this respect it can be assumed that it will be similar with the trout.

And how do the fish orient themselves in general: How do you manage, for example, to stay more or less exactly in place, and not to land at the source or the mouth at some point, but always swim so fast that the current does just compensated?

Orientation: It is difficult for us to imagine that because we would have exactly these difficulties in the water. But the fish have completely different sense organs. They can orientate themselves very well visually, and then they have their own sensory organ, the so-called lateral line organ. That is because it is like a channel that runs along the side of the fish and with which you can record extremely fine flow differences. They also orientate themselves on that. And as far as orientation over long distances is concerned, it was discovered a little more than ten years ago in trout that they also have sensory cells that react to magnetic fields. This means that for the long distances that some trout cover - from the stream to the sea and back again - they may also orient themselves to the earth's magnetic field.