"instinct" what exactly is it?.... Has it been scientifically explained? ...
A pattern of behavior, they do without knowing why. We know why but they don't.
Also, a pattern that is not learned after birth but is programmed in, innately. Chicks imprinting on the first big moving thing they see is an example.
I've seen some "explanations" that I didn't really understand but which seemed to me to work for very simple behaviours.
For example, caterpillars that always climb
up branches are in fact climbing towards the light. Boffins like to mess with them by putting light bulbs at the bottom of grass stems and watching them climb down. (with lights at either end and a toggle switch, they have hours of fun...)
Anyway it turns out that if one of the caterpillar's eyes is receiving more light than the other, this sends a signal which inhibits (slows) the movement of legs on that side of the caterpillar's body. Just like with a tank, one side moving slower than the other means it turns, in this case, toward the light.
It is an open question, just how far this sort of behaviour can go in terms of complexity. For these fish, some mechanism to make them "swim into the current" seems quite possible.
