Sometimes I go to a secluded lake and want to take some table size Catfish home. However, the lake is particularly muddy and dirty, the water is actually brown. Is there a problem with bleeding fish in it?
If the water is sufficiently clean that you are willing to eat the fish, it's clean enough to use to bleed the fish. Dirt as such isn't poisonous. Clear water isn't necessarily safe.
Two ways come to mind:
Use a pail of river water. This will at least keep most of the local critters nibbling on it.
Wrap in wet burlap, and set up a can to drip on it. The evaporation of the water on the burlap will gain you another 5-10 degrees cooling (dpending on humidity)
I would follow the same rules as drinking water.
The water was not standing, i.e.
- it comes from a stream that is rather fast and
- the stream is big enough that it is not just a connection of puddles or ponds where the water rinses from one to the other.
There are no sources of contamination upstream. As you mention, thinkable sources are
- agricultural areas where fertilizers or herbicides could have been used
- meadows, where cattle was present
- (quite unlikely) the water runs through some geologic formation where harmful stuff gets washed out.
If your not confident enough to drink it I wouldn't be confident enough to place any food into it.