Gravity.

Fish live pretty dangerous lives. Trout and Bass especially due to angling, however fish in general just didn't get the long straw in this whole deal. I was thinking about how when we walk around on the earth gravity pushes us down, and yes, this does in some ways affect our ability to walk on flat ground. However, what if, instead of pulling down - gravity pulled across (for example, if gravity pulled from north to south). You know it would be easy to go south, but if you had to make your way north you'd have a hell of a time. Well fishies that live in rivers basically have that kind of deal. All they want to do is just chill there like the rest of creation, but the river is always trying to pull them downstream. So, just in order to sit in one place and rest, they have to exert all kinds of energy just to avoid being swept up in the current. I think that would be the pits.