We have a Koi pond here at school, with a larger catchment that holds
about 20 fish, ranging in size from a pound or so up to a couple of 8
pounders. These are pretty big fish, and being a public institution,
they are well accustomed to people. They also get fed all day by
passerbys, although its discouraged, its hard to prevent.

They are definately trained; whenever someone comes to the spot where
they are most easily fed, they gather around and start 'carping'. This
involves swimming around with their mouths open, skimming the surface
and 'inhaling' anything that's floating. They are so used to being fed
that you can pet them as they come closer, and they don't swim off.

Usually, when there is a piece of bread or some fishfood in the water,
you can see that they sight it, swim up to it from about 45degrees, but
lose sight of it in the last few inches. They will 'vacuum' the water
around an area until they get the food, then slip back down with it.
Often, when you toss in a piece of bread, one fish will come closer
which will trigger a frenzy. ALL the fish will come around and for a
moment it will look like a piranha attack or something. The two biggest
ones are most belligerent and bullldoze the others out of the way to
establish the best feeding positions, right in front of you.

The last several months have been the cool, dry season, and the fish
behavior has been very consistent. However, this weekend the drizzly
season has begun, and the fish behavior has changed considerably. I
walked by the pond this morning and saw that someone had tossed a large
handful of bread chunks in. Instead of attacking them with gusto, as
usual, they were ignoring them; swimming below them and not
congregating at all. I thought at first that they didn't see the bread,
but then I saw one of the smaller fish rise up to 'attack' a piece of
bread. It struck out violently, with a loud and quick slurping sound,
then dove quickly to the bottom. The rise on the surface was precisely
what I usually see in a 'refusal', but these fish would have no reason
to refuse the bread, as it is their standard diet and this was the real
deal. Then I noticed that it was only the smaller fish striking, and
usually missing. Occasionally, one would manage to hit the bread, and
quickly drag it down where it would eat it without being bothered by
the other fish. Throughout all of this, the larger fish were ignoring
the bread.

My conclusions:
1) Since every variable was constant (bread, fish, feeding time,
location, amount) except for weather, I can only attribute their change
of behavior to the weather. So it appears that, in Koi ponds with carp
at least, fish feed much less vigorously during overcast weather.

2) Carp can see fine right up until the last few inches, when they lose
sight of the food. In 'slower feeding' situations, they will suck up a
large amount of water to get the food, but in 'fast feeding'
situations, they will strike then retreat quickly. That might affect
how long you should wait before you set the hook.

3) Feeding behavior during overcast weather isn't really contagious.
Whereas the fish will cue off each other and congregate in sunny
weather, a single fish feeding in overcast weather had little or no
effect on the rest.

4) Big fish are much less prone to feed during overcast weather.

Of course, these observations are anecdotal, and can only be said to
apply to these particular fish in this particular pond. But its very
educational to watch these guys.

--riverman