Thus, the actual VOLTAGE operating on our fish will depend upon how long the
fish is (or how thick, if he/she is swimming at right angles to the current),
not on where the fish is located along that path. The fish will have a very
very low resistance, a lot lower than the water. (unless it is Sea water, and
in that case, all bets are off! :-P ) Again, by Ohm's Law, the very low
resistance fish will now carry the lion's share of the current through
his/her hapless body. Now, if we remember just how tiny the electrical
impulses are, that the fish's brain is sending down the nerves to stimulate
the muscles of the fish so it can swim, we can see that our current may be
quite a nuisance for the fish.
An interesting corollation to this is seen when sampling a stream with a
backpack generator powered "shocker". Larger fish such as trout are stunned
for a much longer period of time than minnows which, depending on how high
one has the shocker cranked, usually just hit the electric field, are
stunned, float to the surface and swim away as if nothing happened.