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Well even without the fish moving itself it wouldn't be a perpetual motion machine because the running water would be an input to the system. Agree though that

Well even without the fish moving itself it wouldn't be a perpetual motion machine because the running water would be an input to the system. Agree though that a living, untethered fish would need to expend energy to stay balanced and in the right spot.

JoeAltmaier 4y ago
Reframe the system to the moving water. The fish is then swimming upstream. All frames of reference are valid. So yes, perpetual motion.
- plutonorm 4y ago
  what about sailing up wind?
- PeterisP 4y ago
  Reframe the system to the moving water and put a watermill or a hydroelectric power plant in there in there - perpetual motion and extracting energy. A flowing river is not a closed system.