This is not a recommendation, but, I invited the local kids over for a swim, then had a couple of workers wadding around repairing the banks and in two or three days all had been caught.
The kids hollered a little at first.
My came from some water plants that were put in the pond several days before the kids went swimming.
They may be attracted to the heat?
But, Google "leech traps", they didn't work for me but they might for you.
