They use pictures.... and probably hold-harmless contracts. ;-) ~ jan

Or, better. Microchipping is what they're doing now days.

OK they might hold pictures but then think of the whole logistics of
catching an individual fish without draining the pond
Gill

They seine the pond, and this is a yearly process for harvest. Koi, the
bigger they get the calmer and easier they are to handle. Apparently they
get use to it. If you ever have a chance to take in a koi show... it
becomes quite obvious which fish have been shown often. They're the ones
cruising around the holding tanks like nothing new is happening. The
novices are hiding as best they can, usually under the panning bowls.

I was stressed out the first time I had to catch one of my biggest koi. I
had to sell as she had just gotten too big for my modest ponds. I was so
surprised how easily she went in the net (koi sock), no thrashing, just a
gentle giant. She went to a pond 4 times bigger than what she was in, and
within 3-4 months took my breath away at how much she had grown. I'd had
her 11 years.

Later that year I had to start catching the smaller 4-6 inch koi, what
pitas they were to catch. They could jump the entire pond length, often
hitting me in the process. Thankfully no hits to the face, koi have been
known to cause a few bloody noses. ~ jan