SMART FISH ???

I have kept Koi fish for a number of years they live in three
interconnected ponds via 8 inch pipes...Never thought about how a fish
thinks or its reasoning capacity.
About a week ago someone's pet ferret decided to take up residence under
my back porch. He caught one of my larger Koi's [just under 3 lbs.]
From the looks of things the fish gave a good battle. the little
animal just ate head parts. I saw the animal and blocked his access..
All of this took place in the middle pond from the scales and and mess
left bank side..
For a week not one Koi would venture into that pond , Only today I saw
one or two go in , do a couple of laps and out fast.
Are they smart or what ???