It depends on turbulence, though. If it's a wide uniform flow, there are no vortices to exploit, so the intuitive assumption holds. You need the vortices to wor

It depends on turbulence, though. If it's a wide uniform flow, there are no vortices to exploit, so the intuitive assumption holds. You need the vortices to work against, and certain configurations will have negative impact, so a fish has to pay attention enough to surge or turn when needed. Throw a dead fish in a turbulent river and it's 100% going to end up far downstream, even though it might take a few upstream jaunts on the way.